Poaceae

The poaceae (Poaceae) or grasses are a family of herbaceous plants, or very rarely woody, belonging to the order Poales of the monocotyledons. With more than 820 genera and nearly 12,100 described species, grasses are the fourth family with the highest species richness after Compositae, orchids, and legumes; but, definitely, it is the first in world economic importance. In fact, most of the diet of human beings comes from grasses, both directly —cereal grains and their derivatives, such as flours and oils— or indirectly —meat, milk, and eggs that come from cattle and poultry that eat grasses or grains. It is a cosmopolitan family, which has conquered most of the planet's ecological niches, from desert areas to saltwater ecosystems, and from depressed and flooded areas to the highest mountain systems. This incomparable capacity for adaptation is supported by an enormous morphological, physiological and reproductive diversity and by several mutualistic associations with other organisms, which make grasses a fascinating family, not only for their economic importance, but also for their biological relevance.

Among the most prominent species are sugar cane, wheat, rice, corn, sorghum, barley, oats, rye and bamboo.

Phenology

Typical gram line diagram

Growth habit

In general they are herbs, although they can be woody —like tropical bamboos—, tussock, rhizomatous, or stoloniferous. Due to the duration of their life cycle they can be annual, biennial or perennial. Annual grasses, as is logical to suppose, reproduce only once during their life cycle —the case of wheat or oats, for example. Perennial species, on the other hand, can reproduce several times —usually annually— or only once. In the first case they are called iteroparous —most grass species, for example— and, in the second case, semelparous —as is the case of the different species bamboos—

Stem

They have cylindrical to elliptical in cross section, jointed stems, commonly called culms, usually with solid nodes and hollow internodes (but can be totally solid as in the case of maize and some bamboos). The nodes are somewhat thicker than the internodes and the leaves and buds are born from them. The internodes are sometimes somewhat flattened in the area where the branches develop. A little above the node there is a ring-shaped intercalary meristem that determines the elongation of the stem. In some genera there are two to six knots very close to each other (which are called compound knots), each of which has its corresponding leaf. In Cynodon dactylon, for example, the nodes are in groups of two so the leaves appear opposite. In general, the basal internodes are shorter than the upper ones; when there are several very close basal nodes, the leaves appear rosetted, that is, they are arranged in such a way that they simulate forming a basal rosette of leaves. The main types of stems in grasses are the following:

• Air stalks:
  • erect or ascending stems: usually with short interns at the base and gradually longer towards the apex. They can be simple or branched.
  • Tracking stems: they grow lying on the ground, rooting in the knots; they often present compound knots, as in Stenotaphrum and Cynodon. When tracing stems originate new plants in each knot are called stungs.
  • Floating stems: which float in the water thanks to the hollow interns or the presence of aerénquima.
• Underground stems:
  • Rizomas: There are two types of rhizomes in the gramíneas. A first type of short and coveted rhizomes, of defined growth, that generate new plants next to the original kill contributing to increase their diameter, as for example in Arundo donax and Spartina densiflora. The other type of rhizome is long, of indefinite growth that propagates the plant at a certain distance from the original kill. Examples of this second type are Sorghum halepense and Panicum racemosum.
  • Pseudobulbs: they are thickenings of the basal interns wrapped by their foliar pods. They are rare in grammar, some examples are Amphibromus scabrivalvis and Phalaris tuberosa.

Leaves

They have alternately arranged leaves, distichus, typically composed of a sheath, ligule and blade. The sheath tightly surrounds the stem, their margins overlapping but not fused together (they can only occasionally be found forming a tube). The ligule is a small membranous appendage, or rarely a group of hairs (trichomes), located in the area where the blade joins the sheath, in the adaxial part. The limb (or lamina) is simple, usually linear, with parallel venation. It can be flattened or sometimes rolled into a tube, it can be continuous with the sheath or have a petiole. In addition to this description, it is necessary to address the variability that can be found for each of these organs:

Membranous lead in oats (Avena sativa).

Wheat particles (Triticum aestivum).

• Profilo: is the first blade of every innovation, it is usually reduced to a membranous pod, with two conspicuous lambs, without lip or foil, which protects innovation.
• Vaina: the pod is born in a knot and envelops the rod, being shorter or longer than the nude. Often the pod is ignited to the base, although its edges are overlapping around the rod, but in many species it is partial or totally welded by its edges (e.g., on Bromus) and even the limp can form a continuous ring around the reed, as it happens in Melica and some species Poa. The nerves of the pod are numerous and uniform, although in species with compressed reeds the pods can present a conspicuous keel.
• Ligula: usually membranous, but in some tribes, for example in the Eragrostídeas, Arundinóideas and Panicóideas, is formed by a band of hairs or cilia, or does not exist. In some genres of Trityceas and Feasts on the sides of the lip there are two atriums that embrace the rod.
• Pseudopecíolo: in many Bambuseas there is a contraction between the foil and the pod that resembles a petiole; frequently this Pseudospect is articulated with the foil and is persistent. There are also pseudo-species in gender Pharus and Setaria palmifolia.
• Lamina: usually the foil is linear or lanceolate, whole in the margin and with parallel nerves. In tropical genres, oblong foils, and even broadly elliptic, appear as in Pharus and Olyra. The foil of Neurolepis (Bambusoideae) can be up to 4 meters long. When the sheets are wide and flat, as in Zea and Sorghum, There's a conspicuous central nerve. Instead in linear leaves, narrow, nerves are more or less equal to each other. In these cases there are usually sheets that are folded longitudinally (they are said to be Appliedor that they roar (volutes). This feature is perfectly appreciated in the first prefoliation. It also allows to identify certain species. For example, conduplicated leaves are typical of Stenotaphrum secundatum, Bromus brevis, Dactylis glomerata, Axonopus compressus, Poa lanuginosa, among others. Convolut leaves can be seen in Lolium multiflorum, Bromus unioloids, Paspalum dilatatum, among several other examples. In some cases the foil is modified by becoming thick and subulate, as in Sporobolus governs.
• Epidermis: The epidermal characters of the leaf and gramlin bracts are of great importance in systematic and often serve to differentiate certain subfamilies or tribes. In general, the arrangement of epidermal cells is different in the upper and lower sides of the foil. Also different is the layout on vascular beams (called coastal areas) and between such beams (intercostal areas). The cells of the epidermis of the gramines can be grouped into 5 distinctive categories:
  • Silicone cells, do not have coloration, are recognized for their particular restraining or shine as they refracte the light in a way different from the rest of the epidermal cells because they possess the lumen (the inside of the cell) occupied by silica. They can be round, lengthened longitudinally, in the form of a double-edged axe, in the form of tibia or assumed in its central part.
  • Subtle cells are short cells, dead to maturity, with the subdued cell wall.
  • Exodermal cells that understand the elements that stand out from the surface of the epidermis, which include: unicellular, bicellular or multicellular hairs; captive hairs, rigid hairs and punctures (aggrants) more or less silicified and papills (cells with conical prominences) very short.
  • Stomas, formed by two types of cells: the occlusive cells or closure, in the form of tibia, elongated and thickened at its ends, which surround the hole where the gas exchange is made or by the stomatic, and the annexed cells, semilunares, rectangular or trapezoidal.
  • Long cells with smooth or undulating walls and colorless buliform cells that form bands in the bottom of the grooves on the top side of the foil. Certain epidermal elements are common to all Gramines, such as long cells, subdus, unicellular hairs and eagles. Others are characteristic and particular of certain groups.
• Foliar anatomy: the anatomy of the gramnea sheet is of great importance in systematic. There are two types of extreme anatomy:
  • Festucoide type, with the outer pod of vascular beams (called Parisian pod) undifferentiated, with chloroplasts, and the inner pod (called Mestomatic pod) well developed and heavily thickened, without chlorophyll; besides chlorénquima is formed by cells without invaginations or lobes, it is not ordered in any particular way. The colourless parenchyma is not present. This type of anatomy is typical of Festúceas and certain tropical tribes such as Oríceas.
  • Panicoid type. Mestomatic pod is barely developed or missing completely, while the parenchematic pod is very developed. In this type of anatomy chlorénquima tends to be ordered radially around vascular beams, with lobed cells. The colourless parenchyma is present. This anatomical type, characteristic of the Paníceas, Eragrósteas, Clorídeas and other tropical tribes, is called Kranz anatomy and is characteristic of the C gramíneas₄. In contrast, the anatomical festucoid type is characteristic of the C grams₃. In addition, there are intermediate types such as in the Bambuseas, where both the parenchematic pod and the mestomatics are very developed. The differences in the anatomy of the leaves are associated with different photosynthetic pathways. The C3 track is more efficient in cold temperate climate regions, while the C4 track is advantageous in high temperatures and low soil humidity. physiology C₃ was directly documented in 366 genres while physiology C₄ was directly documented in 335 genres. Intermediate physiology between C₃ and C₄ was observed in Neurachne minor, Steinchisma decipiens (chuckles) Panicum decipiens), S. hians (chuckles) S. milioides) and in S. spathellosum (chuckles) S. schenckii).

Tillers

Tillers or tillers are the structural unit of most grass species. They are formed from the axillary or secondary buds of the basal meristem of the main axis. Each of these secondary shoots or tillers begin to appear when the plants present between two and three leaves. Each of them, after producing its first leaves, generates its own root system. The sum or addition of tillers is what makes up the structure and shape of a grass plant. When grasses are in a vegetative state, they continually produce new tillers and leaves. Each tiller, in turn, will begin to produce new tillers in due course.

Inflorescence

Scheme of a spike: 1. Glumas; 2: Lemma; 3: Arista; 4: Pálea; 5: Lodicles; 6: Androceus; 7: Giineceus.

Detail of a graminea sprig, the glumes, the lemma and the palea are observed.

The elementary inflorescence of grasses is called spikelet and consists of a small spike formed by one or more sitting or sessile flowers on an articulated rachis, often very short, called the rachilla and protected by sterile bracts called glumes, which are inserted on the rachilla, one lower than the other.

The flowers can be unisexual or hermaphrodite and have a rudimentary perianth of two or three pieces called lodicules or glumélulas, which are the organs that determine the opening of the anthecium or flower box by becoming turgid during flowering, allowing feathery stigmas and stamens to be exposed. The anthecia are made up of two glumellae:

• The lower glumela, called lemma or slogan, attached to the locker. It can be multiple (no points) or glazed (one arist it is born in the extremity of the lemma or in its back), has a form of quilla and embraces with its edges to the palea.
• The upper glumela, called Pale, inserted on the floral axis that is born on the beech in the axill of the lemma and supports the floral organs themselves. The pale is lanceolate, binervada and is like a lid that closes the forefather of the flower.

All these elements are highly variable, so it is convenient to analyze them separately.

• Hit it. It's the stalk that holds the sprig that can be longer or less long or completely absent, in that case the spikes are sesile.
• Nozzle, or axis of inflorescence. It tends to be unsweetened with an ancestry at every angle. The lining may be articulated with the pedicel below the glumes (which then fall with the spike) or above the glumes (which, then, are persistent). The razzle can be tenacious or fragmented in arteges to the maturity of the fruits. Sometimes the hairpin is persistent, detaching the fruits with the glumelas (as in the case of Eragrostis bahiensis). It can also be extended in the form of glabra or hairy edge beyond the top ancestry, as in Deyeuxia.
• Glumas. Typically they are two and because of their consistency they can be herbaceous, membranous or papyraceous. Its shape is variable, being ovated or lanceolate, contracted laterally and more or less lacking, or rounded and almost flat, according to the species considered. They can carry one or more nervations and be mucronated, or glazed in the apex. In Orices glumes are rudimentary or absent. On the other hand, in many Paniceas there are apparently three glumes, the superior being actually a sterile lemma; sometimes there is only one gluma as in the genres. Monerma and Lolium.
• Ancestors. The ancestors (flower box, in Greek) can be from one to several. They are formed by two bracts, the glumelas, which lock the flower. In some genres the ancestors are detached with a fragment of the locker, frequently covered with hairs forming the Antopodium or Callus (Aristida, Stipa). In other genres with glyma caducas with the forehead, there is a callus formed by the pedicel apex (e.g., in Heteropogon). Lower glumela is generally more developed and called lemma; it is the bract in which the flower develops; the lemma is ovated or lanceolate; lateral or dorsally compressed, with one or more nerves, acute or obtusa, mútica or with one or more apical or dorsal edges. Upper glumela or Pale It is the prophylaus between the flower and the rake; it is usually less than the lemma and is more or less covered by the edges of it; it is usually of membranous consistency, often hialin and usually has two prominent nervatures forming two lambs. Pale may be atrophied or even missing.
• Flower. The flower is bare (peace of chalice and corolla), but is usually accompanied by two (rarely three) small translucent squamous pieces, called lodicleswhich constitute a rest of peranto. The flower can be hermaphrodite or unisexual. In many genres the ancestors carry hermaphrodite flowers, except the superiors that are male. On the other hand, in many Paníceas, there is a lower male forehead and a superior hermaphrodite. Other times there are male spikes and female spikes on the same floor (the case of Zea mays, species dilino monoica) or in different plants (e.g., in the subgender Dioicopoa of Poa, made up of divinic species. Lodicles appear to be traces of a trimer perianto and there are still 3 lodicles in some Bambuseas genera and in some species of Stipa. The turgence of the lodicles determines the opening of the antece allowing the outward exit of the stamens and stigmas (moment that is called flowering or before). The morphology of the lodicles is of systematic importance. Also, the flowers usually possess one of these two types of compression, important taxonomically: or they are laterally compressed (so that the lemma and the pale are observed on either side of the compressed face), or are ventrally compressed (so that each compressed face possesses the lemma or the palea).
  • Androceo. The androceum in the grams is cyclic and generally trimer (i.e., it is composed of three stamens, or a multiple of three).
    

    Flowers in Beforeis Holcus mollisObserve the plumy stigmas and stamens.
    The most frequent number of stamens is three, but in many species of Oríceas and Bambuseas there are 6 to 9 (sometimes more). Instead, Imperata brasiliensis There is only one stamen and two in the gender Anthoxanthum. The antennas are basifixes, biloculars and are inserted over thin and longer filaments. Pollen grains are relatively small, with very thin, light walls, have a single germinative pore (they are said to be monoporous) and are adapted to be carried by the wind from the stamens to the stigmas of other plants (politicization is anemophile).
  • Gineceo. The gin consists of a globose ovary, piriforme or fusiforme, usually bicarpelar, unilocular, with two short styles and feathery stigmas. In some Bambuseas the ovary is tricarpelar and has three styles. When there are three carpals, the adaxial part is fertile (Kircher 1986). In Euchlaena and Zea There is only one acrescent style. Within the ovary, supero, there is only one anthropo or semi-anatropo, subapical or almost basal, of parietal placentation. Some pastures have pendulum eggs, spoils. The megasporangio wall can be thin or thick. In many species of gramineas there are cleistogamous flowers in reduced inflorescences located in the axills of the lower leaves.

Secondary inflorescence

The spikelets, in turn, are gathered or grouped in racemose compound inflorescences. The most frequent are:

• Lax panojas, are clusters of spices with very long pedicels. Example: Avena.
• Dense pants are clusters of sprigs with very short pedicels. They can be continuous (called espiciformes) or interrupted. Example of the first is Phalarisand the second, Dactylis.
• Panolate panojas, in which each terminal cluster or each small panoja has a bract that separates it from the others. Example, Schizachyrium.
• We grew spinyforms, with very briefly pedicelled sprigs arranged on both sides or on one side of the raquis. Example, Paspalum.
• Swords of spikes, with sessile spikes arranged on one side of the unilateral rake or spikes; in two series altering on the opposite sides of the rake, or distic spikes (Lolium), or in several series on rake, or cylindrical ears (Zea).

Fruit

Scheme of a cariopside of graminea, showing the endosperma, embryo and various embryo structures: the spit or spit, the plumula, the coleorriza, and the cotiledonar knot. Surrounding the whole endosperma is appreciated the layer of aleurone and the other end of the embryo, the brush.

Rice cariopsides (Oryza sativa) in longitudinal cut, the embryos are dyed in blue, the endosperma in white.

The fruit or grain of the grasses is a caryopsis, an indehiscent dry fruit, with a seed whose testa is welded to the pericarp, forming a very thin casing. This envelope encloses the embryo and the albumen or endosperm. This fruit is basically a variant of the achene, although a certain variety of fruits can be found in the family (see for example Werker 1997). In some genera such as Zizianopsis or Eleusine, the pericarp is not welded to the seed, so that the fruit is an achene (or a utricle according to other authors). In some Bamboos the fruit is a nut or a berry, while in the genus Sporobolus the pericarp is mucilaginous and lets out the seed when soaked in water. Many genera, such as Aristida, Stipa, Piptochaetium, Oryza and almost all the Paníceas have caryopses that detach from the plant surrounded by the lemma and by the palea. In Andropogonea, it is the glumes that persist, enclosing the caryopsis. In Pennisetum and Cenchrus the entire spikelet is detached, surrounded by an involucre of bristles or spines. The shape of the caryopsis varies greatly according to the genera, being almost circular as in Briza, oblong as in Hordeum, lanceolate as in Poa up to almost linear, as in Vulpia. In the lower part of the caryopsis, in dorsal view, the more or less elliptical embryo covered by the transparent pericarp can be appreciated. On the other side, corresponding to the carpel groove or suture, the hilar macula or thread (or zone where the seed joins the carpel) is also distinguished by transparency., which can be punctate, as in Poa and in the Paníceas, ovate, as in Briza subaristata, or linear, as in Hordeum, Vulpia or Festuca.

The grass embryo is structurally very complex and consists of the seedling attached to its highly modified lamellar cotyledon, called the shield. The cotyledon is thin, parenchymatous, carrying on its outer part a layer of epithelial cells that, during germination, secrete enzymes that hydrolyze the reserve substances located in the endosperm. The plant consists of a cotyledonary node, where the cotyledon is inserted, a bud covered with a cap or coleoptile and a radicle covered by another cap or coleorriza. In many genera, on the outside of the cotyledon node there is a tiny scale, the epiblast, which some authors consider to be the remains of a second cotyledon, while others consider it to be an appendage of the coleorhiza..

Caryology

The size and number of chromosomes are of great importance in the systematics of grasses. There are two extreme chromosome types: the festucoid type, characterized by large chromosomes and a predominantly x=7 basic number, and the panicoid type, with small chromosomes and predominantly x=9 and x basic numbers. =10. The fescuoid type is found in almost all tribes of the pooidea subfamily, with a few exceptions. For example, the tribe Stipeae of this subfamily has small chromosomes and basic numbers x=9, 10, 11, 12, 14, 16, and 17. The remaining grass subfamilies have the panicoid chromosome type, with small chromosomes and a predominance of the number basic x=9 and 10. In bambusoids, erartoids and arundinoids the chromosomes are small and the basic number is x=12. In the subfamily Danthonioideae there are chromosomes of intermediate size and basic number x=6 and 7. The subfamily Chloridoideae has small chromosomes and several basic numbers, x=7, 8, 9, 10, 11, 12 and 14. Panicoideae always have small chromosomes, with basic numbers x=9 or x=10, although there are species with other basic numbers, which vary from x=4 to x=19.

The grass genome

Grasses are morphologically different from any other plant family, and are also highly diverse in terms of morphology and growth habit. The different species of grasses —as described in the previous section— differ in their sizes and chromosome numbers. They also differ in the size (or DNA content) of their genomes.

The rice genome, for example, is more than 11 times smaller than the barley genome, despite the fact that both species are diploid and appear to have the same morphological and physiological complexity.

The gene content of different grass species, however, does not vary as widely as the total DNA content. Rice and barley, again, differ by no more than two-fold in the average number of restriction fragments that hybridize to the same probes.

Most of the differences in genome size between grass species are due to differences in repetitive DNA. Larger genomes, such as barley or wheat, are made up of 75% repetitive DNA, while smaller genomes, such as rice, only contain less than 50% highly repetitive DNA. Furthermore, it has been determined that much of this repetitive DNA is made up of retrotransposons inserted between genes.

Genomic mapping studies in many grass species using the same DNA probes have shown that not only gene content is highly conserved, but also gene order within chromosomes.

The extensive conservation in gene content and gene order between maize and sorghum is not unexpected since both species "only" They have 15 to 20 million years of independent evolution. However, similar observations for rice and maize, which diverged 60 to 80 million years ago, indicate that all species in the family come from the same common ancestor and that they all conserve the same repertoire of genes in the same gene pool. approximate order.

The large genomic rearrangements that differentiate all grasses from one another are the result of chromosome inversions, translocations, or duplications involving most of the chromosome arms.

Most, if not all, grasses are polyploid. Based on the assumption that all the genera and families that present a basic chromosome number x=12 are derived from ancestors that underwent chromosome duplications during their evolution and that the most primitive subfamilies of grasses (Anomochlooideae, Pharoideae, and Puelioideae) have a basic chromosome number x=12, it can be deduced that the ancestor of the grasses was already a polyploid. It further follows that all grasses classified as diploid are actually paleopolyploid (i.e. ancient polyploids that display disomic inheritance and whose parents cannot be identified by cytogenetic tools or markers). molecular). The family contains more than 60% of species, distributed in all clades, which are classified as neopolyploids, that is, they have undergone an additional cycle of genomic duplication. In these species, the duplicated genomes have not diverged much from the genome of their ancestors, and their chromosome number and cytological behavior during meiosis are indicative of the chromosome duplication that caused them. Most of these neopolyploids (more than 65%) have been derived from interspecific or intergeneric crosses, which is why they are classified as allopolyploids.

Phytochemistry

The hemicelluloses and pectin polysaccharides of the primary cell wall of grasses are very different from those of other spermatophytes, both in structure and in particular compositional characteristics of the xyloglucans. The polysaccharides are less branched than in all other plant families, although this statement is based on a still scanty sampling of species. Poaceae may or may not be cyanogenetic. When cyanogenic, cyanogenic compounds are derivatives of tyrosine. They may present alkaloids (sometimes isoquinoline, pyrrolizidine and indole). Proanthocyanidins and cyanidins can rarely be present, in trace amounts, and only in representatives of the Panicoideae and Chloridoideae subfamilies. Flavonoids have been found only in a few genera, Bouteloua Glyceria and Melica, when present they are quercetin, or kaempferol together with quercetin. Ellagic acid and arbutin have not been found in any member of the family. Saponins and sapogenins are rarely found, as well as free oxalates (eg in Setaria).

Reproductive Systems

A generalization about the mode of reproduction of grasses is that the members of this family are hermaphroditic plants, that present cross-fertilization (they are allogamous) and are pollinated by the wind. Obviously, a family with about 10,000 species has many exceptions to this rule, which are described below.

Dioecy

This type of reproductive system, in which there are female plants and male plants, is not very common in grasses. Only 18 genera are dioecious or present dioecious species, Poa being the best known of them. In fact, the dioecious species of Poa are included in a separate subgenus, Dioicopoa.

Gynodyoecia

This reproductive system describes the fact that in the natural populations of a species, female individuals and hermaphroditic individuals coexist. This condition is quite rare in grasses. Bouteloua chondrosioides and some species of the subgenus Andinae of Poa are gynodioecious, although Cortaderia is the most conspicuous example.

Monoecy

In this system, the sexes are separated spatially but in the same individual, that is, each plant presents female and male inflorescences. Zea, Humbertochloa, Luziola, Ekmanochloa and Mniochloa are examples of genera with monoecious species. Much more common among the grasses are the andromonoecious species, a very common condition in the Andropogonea and Paníceas. In the former, the two sexes occur in different spikelets of heterogamous pairs. A heterogamous pair of spikelets usually consists of a sessile spikelet, with a neutral and a hermaphroditic flower, and a pedicelate spikelet, with a neutral and a male flower. In the bifloral spikelets of Paniceas, on the other hand, the lower flower is usually male or neutral, and the upper one is hermaphroditic. Some of the genera that exemplify this type of system are Alloteropsis, Brachiaria, Cenchrus, Echinochloa, Melinis, Oplismenus, Panicum, Setaria, Whiteochloa, and Xyochlaena. Some species within these genera may have only hermaphroditic flowers as the lower flower is always neutral, rarely both flowers are hermaphrodite. Apart from Paníceas and Andropogoneas, Arundinelleae is another tribe with andromonoecious species. In the rest of the family, andromonoecious species are found very sporadically, as for example in Arrhenatherum, Hierochloe and Holcus.

Self-incompatibility

The vast majority of grass species are hermaphrodites, however, they are frequently unable to produce seeds when a plant's pollen pollinates its own stigmas. This is due to the fact that a large part of the species of the family present self-incompatibility, of a gametophytic type and due to the action of two independent genes (called S and Z) with several alleles each. This self-incompatibility system has been observed in several genera of the family (Festuca, Secale, Lolium, Hordeum, Dactylis, among many others) and is not perfectly efficient. In fact, from most self-incompatible species a proportion, albeit a small one, of seeds can be obtained by selfing a plant.

Self-Compatibility

Self-pollination and self-fertilization are widely distributed among grasses. In general, it is a more common mechanism among annual species than among perennials, and decidedly much more common among colonizing species. This mechanism has been determined in approximately 45 genera of grasses, among which are economically very important genera such as Triticum, Oryza, Secale, Oats, Agropyron and Lolium. A condition of extreme autogamy is cleistogamy, in which pollination and fertilization within the anthecium without anthesis occurring. This last system is distributed in more than 70 genera belonging to 20 grass tribes.

Apomixis

Apomixis is defined as asexual reproduction through seeds. In this reproductive system, embryos develop by mitosis from an unreduced oosphere without fertilization taking place. In other words, each embryo produced is genetically identical to the mother plant. In grasses, apomixis was first described in 1933 in a species of Poa. Since then this mechanism has been identified in hundreds of Poaceae species, particularly in the Panaceae and Andropogonea. Some of the genera that have apomictic species are Apluda, Capillipedium, Heteropogon, Themeda, Sorghum, Bothriochloa, Dichanthium, Cenchrus, Setaria and Paspalum.

Ecology

Distribution

View of the Tanzanian savannah.

Grams have developed various adaptations to extreme environments or conditions. The image shows an ice-covered gram line, Salzburg, Austria.

Grasses are a cosmopolitan family that inhabits everything from deserts to freshwater and marine habitats, and all but the highest elevations on the planet. In the world, extensive native biomes dominated by grasses have developed where there are periodic droughts, flat or sloping topography, frequent fires, and on some occasions where there is grazing and under certain particular soil conditions. Communities dominated by grasses account for 24% of the planet's vegetation, examples are the North American prairies, the South American pampas, the "veldt" or the savannah in Africa, and the Eurasian steppes. Outside of grasslands, woody bamboos play a central role in the ecology of tropical and temperate Asian forests.

Grasses have been ecologically successful and have diversified extensively due to many key adaptations. The spikelet protects the flowers but, at the same time, allows pollination when the lodicules open the antechia. Likewise, the spikelets (lemmas with hairs or hooks) have several adaptations for the dispersal of the fruit. The versatility in mating systems, including selfing and apomixis, allowed various grass species to be successful colonizers of new environments. The leaf anatomy, which can be either C₃ or C₄, allows these species to explore and adapt to a wide range of habitats. The meristems are located at the base of the internodes and at the base of the pods, protected by the entire plant, resulting in an adaptation to grazing and fire that is unmatched among all plants. Grassland development during the Miocene (about 25 to 5 million years ago) may have fostered the evolution of large herbivores, as well as representing an important food source and stimulus for the evolution of Homo sapiens.

Grasses have also developed certain physiological characteristics that have allowed them to conquer habitats where suboptimal conditions for plant growth prevail. One such characteristic is the ability to accumulate glycine betaines and other compounds that are associated with plant adaptation to growth in saline conditions. Pooideas, on the other hand, store carbohydrates as fructans, which are in much lesser amounts. concentration in the remaining species of the family. This characteristic is associated with the adaptation of such species to conditions of water stress (drought) and low temperatures (frost). Finally, another unique physiological mechanism of grasses is that they are apparently the only family of angiosperms that acquires ions by chelation. of ferric ions with siderophores that are absorbed by the roots.

Partnerships with Other Agencies

Mycorrhizas and mycophils are two types of mutualisms in which higher plants and fungi are involved. Mycorrhizae are mutualisms between fungi and plant roots. Mycophiles, meanwhile, are mutualisms between endophytic fungi (those that grow inside plants) and the aerial part of plants. On the other hand, grasses can also associate with different genera of free-living bacteria that fix atmospheric nitrogen.

Mycophiles

Cells of a high festuca leaf (Festuca arundinacea) colonized by the fungus Neothypodium coenophialumwhose mycelium appears like blue cords.

The presence of endophytic fungi can modify the survival of plants in several ways since three different types of associations can be produced between fungal symbionts and plants, which vary according to the taxonomic group of the host, with the fungal and vegetal structures involved and with the intrinsic characteristics of the symbiosis. Thus, type I has members of the Juncaceae family and various subfamilies of Gramineae as hosts. These fungi colonize the entire host and produce their sexual structures in stroma that replace the fruits that the plants should produce, so that they lose the ability to reproduce sexually. The interaction is extremely aggressive for the host and determines its gradual decay. Type II has as hosts the grasses of the poóideas subfamily. Not all individuals of the colonized population have stroma instead of caryopses. In specimens without external symptoms, the endophytic mycelium colonizes the caryopses without impairing the sexual reproduction of the plant. In this way, the pathogenic nature of the interaction is less than in the previous case. Finally, in type III, which also occurs in the pooidea subfamily, stroma never appear in the hosts; the colonization of the fungus is systemic, reaching the caryopses through which this association is propagated. This type of interaction is considered a mutualistic symbiosis because the endophytes benefit the hosts by increasing their growth, biomass, photosynthetic rate, tolerance to frost and drought, resistance to nematodes and insects, thus increasing the competitiveness of their hosts. Also, because they produce alkaloids, they protect plants from attack by a wide spectrum of herbivorous animals, thus constituting part of their defense system. The presence of endophytes affects the palatability of pastures for herbivores and also the palatability of seeds for granivorous birds, animals that eat infected material present various symptoms of intoxication. The level of infestation by aphids and that of their parasites and parasitoids, and even the pattern and rate of decomposition of dead grass, are also affected by mutualism. i>) live in the stroma of fungal endophytes of the genus Epichloë, and the adults transmit the fungal spermatium in a manner analogous to insect pollination of flowers. In turn, the endophytes they benefit by receiving the direct contribution of carbohydrates produced by their hosts. Of the 232 known mycophiles in the world, 209 have members of all grass subfamilies as hosts and represent all three types of interaction. The distribution areas of these interactions cover both cold and temperate and tropical zones. In the Northern Hemisphere, type I, II and III interactions are very frequent, while in the Southern Hemisphere, type III interactions prevail. Endophytic fungi of the Clavicipitaceae family are widely distributed among grasses. One genus of this family of fungi, Epichloë, is an endophyte restricted to the subfamily Pooideae, Neotyphodium is the asexual or imperfect stage of Epichloë . More than 30% of pooid species are involved in such associations, and there is transmission of these fungi (the subfamily Balansiae of the Clavicipitaceae) both vertically and horizontally The mycophils are an association that seems to date from about 40 million years ago and one of its consequences is the production of alkaloids such as lolina. The alkaloids produced, such as the aforementioned lolina, are active mainly in the defense of plants against insects. Many other endophytic species, apparently asymptomatic, can grow together in grasses, but very little is known about their relationships. Márquez et al. (2007), for example, reported that the grass Dicanthelium lanuginosum can only grow in soils heated by volcanic action when the endophytic fungus Curvularia, with which it is associated, is infected with a virus. This indicates that the relationships between grasses and endophytic fungi may be extremely complex and their effects unsuspected. There are lists of endophytic fungal species associated with countless species of grasses. In fact, there are at least 1,933 described fungal species in bamboo alone.

Mycorrhizae

The oldest fossil records indicate that this association is about 400 million years old, indicating the complex coevolution between plants and their associated fungi, manifested in the wide distribution of the phenomenon (it has been estimated that 90% of terrestrial plants are mycorrhized) and in the diversity of morphological, physiological and ecological mechanisms involved. During symbiosis, the host plant receives mineral nutrients from the soil taken up by the fungus (mainly phosphorus), while the fungus obtains carbon compounds derived from photosynthesis. The fungi that form arbuscular mycorrhizae constitute mycorrhizae that colonize the internal tissue of the roots of the host plant, where they develop characteristic structures of symbiosis (arbuscules and vesicles), as well as extraradical mycelium, which interacts with the rhizosphere ecosystem and is the one in charge of extracting nutrients from the soil. In this sense, grasses are no exception. Many of the species in this family form mycorrhizae, which favors and optimizes their adaptation to different types of environments.

Association with nitrogen-fixing bacteria

Molecular nitrogen (N₂) is the only accessible reserve of nitrogen in the biosphere. Virtually unlimited, this reserve is not directly used by plants and animals. Nitrogen is an essential constituent of fundamental molecules of all living beings: amino acids, proteins, nucleic acids, vitamins, among the most important. In order for atmospheric nitrogen to be assimilated, it needs to be reduced. Grasses are capable of associating with diazotrophic bacteria belonging to the genera Azospirillum, Azotobacter, Azoarcus and Herbaspirillum which They carry out the biological fixation of atmospheric nitrogen (N₂). These bacteria are free-living organisms capable of fixing nitrogen from the rhizosphere, that is, from the area surrounding the root system of the plant and including it in compounds (such as ammonium) easily available and absorbable by plants. In addition to fixing atmospheric nitrogen, diazotrophic bacteria favor the development of the root system of the plant with which they live, apparently through the production of growth regulators or hormones. In this way, they favor a greater absorption of nutrients by the plant. Increases of the order of 5% to 30% in the yields of grasses such as sugarcane, corn, rice, wheat and forage grasses have been reported as a result of this association. These associations do not develop differentiated structures in which microorganisms are housed, as occurs in the case of legumes and bacteria of the genus Rhizobium. In 1998 another type of association of diazotrophs has been described in which the bacteria (called endophytic bacteria) are located inside the root, stem and leaves of the plant. This association was discovered in isolates of diazotrophs from forage plants from Pakistan, where a new nitrogen-fixing bacterium called Azoarcus was identified. This microorganism is located in the outer layers of the cortex; once inside the plant it spreads to the aerial tissues probably through the xylem vessels.

Other ecological aspects

Natural grammar grass. Oglala National Grassland, United States.

Grasses are anemophilous, meaning that pollen is transported from one plant to another by means of the wind to carry out pollination. None of the poaceae have nectaries, although some tropical forest grasses—especially small bamboos—are pollinated by insects. Seed dispersal is mainly by animals, and some species even have specialized structures to attract them, such as elaiosomes., most of the species have hooks or needles through which the fruits or diaspores adhere to passing animals. Many species disperse with the wind, for which they have long hairs on the awns. Finally, Spinifex and some other genera are tumbleweeds, which are uprooted at maturity and carried whole by the wind, dispersing their seeds as they roll. Ridges can aid in both wind dispersal and animal dispersal; the microstructure of the surface of the awns may result in the caryopse being directly "planted" on the ground.

Woody species of bamboo are known to flower synchronously. Many of them, moreover, are monocarpic perennials, that is to say that they vegetate for many years, flower only once and die after giving the seeds. This feature is also found in some herbaceous bamboos. In plants that show this type of reproduction, all the members of the same clone flower simultaneously, wherever they have been transported around the globe, and all the plants, after a reproductive period that can be said to be delayed, die.

In the hollow stems of the bamboos, water often accumulates, and a distinctive fauna lives in it. Poaceae provide food for both adults (pollen) and larvae (roots) of different species of beetles in the Galerucinae subfamily of chrysomelids. Caterpillars of butterflies of the family Nymphalidae, particularly the brown Satyrinae and related Morphinae, are common in members of this family (found in about 10% of censuses). Insects of the taxon Hemiptera-Lygaeidae-Blissinae are more commonly observed in species in the clade called PACCMAD than in the BEP clade.

Parasitic fungi of the order Uredinales and those of the class Ustilaginomycetes are common in poaceae. Those that attack the subfamilies Bambusoideae and Pooideae (including Stipa and close relatives) are particularly distinctive. Two-thirds of the Ustilaginales (about 600 species) are found in the Poaceae.

Evolution, phylogeny and taxonomy

Evolution

The base group of the grams would have originated and diversified in Gondwana before the isolation of the Indian subcontinent.

Grasses and their extinct relatives date to about 89 million years ago, with the main group diverging about 83 million years ago. Grasses are known from the Paleocene-Eocene boundary, about 55 million years ago, and this figure is roughly in line with an estimate of the age of a grass genome doubling of about 70- 50 million years ago. However, the fossil of a monocot (Programinis burmitis) belonging to the early Cretaceous (about 100-110 million years ago) is similar to a bambusoid grass. Although this fossil has a number of vegetative characters that are common among Poaceae, its identity still needs confirmation. Silicified plant tissues (phytoliths) preserved in fossilized feces (coprolites) of Late Cretaceous dinosaurs found in India indicate that at least five extinct grass taxa were present on the Indian subcontinent during that geological period (about 71-65 million years ago). of years). This diversity suggests that the basal group of grasses would have diversified and distributed in Gondwana before India became geographically isolated.

C4 plants present a special foliar anatomy called Kranz anatomy. The image shows a vascular corn beam surrounded by a parenchimatic cell pod, which is characteristic of plants with this type of photosynthesis.

C₄ photosynthesis appears to have been present in early to middle Miocene grasses, both on the North American Great Plains and in Africa, about 25-12.5 million years ago. Perhaps this type of photosynthesis was initially associated with adaptive changes in response to a decrease in the concentration of CO₂ in the atmosphere, although the great expansion of this physiological mechanism occurred only about 9-4 million years ago. of years. It is not yet clear if this event was also favored by increases in temperature, decreased rainfall, increased winds, and the concomitant increase in fires, which would have removed trees from some habitats in that period. The details of the mechanisms of C₄ photosynthesis and the morphologies associated with it are highly diverse and show considerable variation, particularly in the case of the subfamily Panicoids. In fact, C₄ photosynthesis apparently originated and evolved independently up to eight times in this subfamily. This mechanism also originated independently in other subfamilies, such as Micrairoideae, Aristidoideae, and Chloridoideae. Because of their higher photosynthetic efficiency, C₄ grasses have lower nitrogen content, more sclerenchyma fibers, and may be less palatable than C₃ grasses. Of these characteristics, there was a radiation of herbivorous mammals in the Miocene, which could be associated with the expansion of grassland-dominated grasslands and savannahs. However, when prairie grass species spread into Nebraska in the early Miocene—about 23 million years ago—hypsodont ungulates existed by then.

The inflorescence of grasses is a new structure in the reproductive repertoire of flowering plants. It is intricate from the point of view of developmental biology, it is of central importance in agronomy and, finally, it is a true evolutionary puzzle. Its architecture controls the type of pollination and seed production, making it a very important target both for natural selection and for genetic improvement and biotechnology. It is noteworthy that the diversity of structures that the spikelets and spikes of grasses present are controlled by genes that affect development and that are not present in any other family of plants. These genes have originated after extensive genome duplications and their subsequent functional diversification.

The palea (and apparently also the lemma) may be derived from the calyx, and the lodicules may be derived from the corolla (Ambrose et al. 2000). On the other hand, a study of comparative morphology suggests that the lemma is a bract and that the palea represents two connate tepals of the outermost whorl (Whipple and Schmidt 2006). Taking into account the close related relationships between Ecdeicoleaceae and Joinvilleaceae recently found by Marchant and Briggs (2007) and the probability that the flowers of Anomochloa are sui generis, the Floral morphology of Streptochaeta may be plesiomorphic (ancestral) in the family. Interestingly, the flowers of Ecdeicolea are also remarkably monosymmetric, with the two adaxial tepals of the outer whorl longer and keeled, and while this is not directly relevant, a comparable differentiation in the outer whorl of the perianth occurs in Xyridaceae, these are all probably parallels. A more common interpretation of the palea is that it is profilar/bracteolar in nature, monocots commonly have bicarinated prophyla, however it seems that bracteoles had to reappear in Poaceae, as the more related clades but outside the family (&# 34;outgroups" in cladistic analysis) do not possess them. Lodicules appear to be involved in the opening of staminate and perfect flowers, while they may be absent in pistillate flowers (Sajo et al. 2007).

Phylogeny

Phylogenetic analyzes using specific DNA sequences and the general structure of genomes suggest that grasses differ much more from other monocots than they differ from dicots. Such conclusions about the relationships of Poaceae with other families of monocots and with the dicots are not surprising. In fact, grasses are easily recognizable and identifiable from any other family, and their monophyly is supported both by morphology and by molecular DNA analysis. The phenotypic characters that support the monophyly of the family are the inflorescence with bracts, the reduced perianth, the type of fruit, and the characters of the embryo and of the pollen grain wall. The similarities with sedges (Cyperaceae) in growth habit and spikelet type represent convergent evolution, not synapomorphy. In fact, the sedges are more closely related to the rushes (Juncaceae) than to the grasses, which belong to the nucleus of the Poales.

The economic and ecological importance of the family has motivated a significant number of systematic studies. In the early 19th century, differences between the spikelets of pooids and panicoids led Robert Brown to split the family in these two basic groups. At the beginning of the XX century, the characters of the epidermis of the leaves and the number of chromosomes led to the separation of the chlorids of the pooids. By the middle of the XX century, the internal anatomy of the blade (in particular, the presence or absence of Kranz anatomy and the characters of the embryo (presence or absence of epiblast and certain characteristics of the cotyledon node), led to the recognition of five to eight subfamilies Since the end of the century XX, phylogenetic studies based on various gene sequences have been shown to be consistent with many of the phylogenetic relationships previously inferred through structural and physiological characters.The most advanced molecular studies support the recognition of 13 subfamilies. The first three lineages to diverge are the Anomochlooideae (native to Brazil), the Pharoideae (native to the Old and New World tropics), and the Puelioideae (native to West Africa).The members of these three groups are only about 25 of the cas i 10,000 species of the family. The rest of the species are distributed in two large groups. The first, called the BEP clade, groups together the Bambusoideae, the Ehrharttoideae, and the Pooideae. The second, called the PACCMAD clade, groups the Panicoideae, Arundinoideae, Chloridoideae, Centothecoideae, Micrairoideae, Aristidoideae, and Danthonioideae. The PACCMAD clade is supported by a character of the embryo: a long internode in the mesocotyl. The C₃ anatomy is the plesiomorphic or ancestral state in the family. All C₄ species are found in the PACCMAD clade.

All the subfamilies mentioned are monophyletic, although only a few have morphological synapomorphies that characterize all their members. Rather, their monophyly is supported by groups of morphological characters that must be observed together. The relationships within the large PACCMAD and BEP clades are still largely unclear, indeed the position of the pooids is unclear in some analyses. The cladogram showing the relationships between the 13 mentioned clades is as follows:

Inflorescences of sugar cane (Saccharum officinarum a panicoid).

Arundo donax, An arundynoid.

Cortaderia sealana, a dantonoid.

Taxonomy

The family was recognized by all plant classification systems, beginning with that of Carlos Linnaeus in his work Systema naturae, going through those of Adolf Engler, Arthur Cronquist, Armen Takhtajan, among many others. Modern classification systems (APG III classification system, and the two previous phylogenetic systems, APGI and APGII) also recognize this family.

Among the diagnostic characters of the family are several of the organs already described: the leaves that have long and open sheaths and the ligules at the junction between the sheath and the blade, the round stems and usually hollow in the internodes, and the inflorescences whose basic unit is the spikelet. Individual flowers are small, with an inconspicuous perianth and a gynoecium that usually has two feathery stigmas and a single ovule. In the dry fruit, of the achene (caryopse) type, the embryo —relatively large— occupies a lateral position and is located next to the seed coat.

Spikelet characters such as size, compression plane, presence or absence of glumes, number of flowers per spikelet, presence of sterile or incomplete flowers, number of flowers per spikelet, are particularly useful for genera identification. number of veins in the glumes and glumes, the presence or absence of awns, and the shape of the secondary inflorescences.

Subfamilies

The following is a description of the grass subfamilies and the most important or representative genera within each one.

Anomochlooideae

The anomocloids are herbaceous plants and present inflorescences with a characteristic morphology that do not resemble the typical or usual spikelets of the other grasses. They appear to be the sister clade to all the rest of the family, Soreng and Davis 1998, which suggests that the characteristic spikelet of grasses probably originated after the anomocloids diverged from the rest of the grasses. The members of this subfamily present a pseudopetiole with an apical pulvinulus, the ligule of the leaves transformed into a tuft of hairs, and the branches of the cymose inflorescence. In the inflorescence they present two bracts along each branch and two more under each flower, or else the flowers are arranged in a spiral shape along racemose axes, with several bracts under each flower. The anthers are centrifix (in Anomochloa) or nearly basifix (Streptochaeta). Characteristically, the first leaf of the seedling lacks a leaf blade. The basic chromosome numbers are x=11 and 18. The species belonging to this subfamily were originally included within the bambusoideas, but it is currently recognized that they only have a distant relationship with them. The subfamily includes two genera —Anomochloa and Streptochaeta— with four species that inhabit forests and are distributed from Central America to southeastern Brazil.

Pharoideae

Faroids are characterized by their resupinate leaves, that is, with the leaf blades facing upwards abaxially. The spikelets are uniflorous and have six stamens with centrifix anthers. The coleoptile has a blade. This subfamily includes four genera and about twelve pantropical species, which inhabit the forests. This subfamily, along with Anomochlooideae and Puelioideae, was traditionally included within the Bambusoideae, but phylogenetic studies on molecular data have shown that all three are the basal clades of the entire grass family. Pharoids differ from puelioids by their uniflorous spikelets and trifid stigmas.

Puelioideae

The puelioids together with the other subfamilies of grasses, but with the exception of the puelioids and the anomocloids, have gynoeciums with two stigmas and spikelets that disarticulate above the glumes at maturity. They are also characterized by their androecium with six stamens. It includes two genera —Guaduella and Puelia— and approximately eleven species distributed throughout tropical Africa.

Bambusoideae

Bamboo forest, Japan.

Distribution of bambooids in the world.

Bambusoids, in a restricted sense, include both herbaceous and woody species and are almost exclusively tropical in distribution. The leaves are pseudopetiolate. The flowers have three lodicules and an androecium with six stamens, rarely from two to 14. The ovary bears two or three stigmas, rarely just one. The first leaf of the seedlings does not present a lamina. The basic chromosome numbers are x=7 and x=9 to 12. It includes 112 genera with approximately 1,647 tropical to temperate species.

Diversification within bamboos occurred 30 to 40 million years ago. The woody bamboos form a monophyletic sister group to the clade containing the herbaceous species. Woody bamboos, with their stems up to 40 meters tall, certainly do not resemble grass. Flowering in many of these species is also unusual, occurring in cycles of up to 120 years. Even though individual stems live for only a decade or a few, some form of "clock" it causes the stems to flower all at the same time throughout the distribution range of the species, sometimes causing abrupt ecological changes, such as those associated with ratates. Some genera of woody bamboos are Bambusa (120 species), Chusquea (100 species), Arundinaria (50 species), Sasa (50 species), and Phyllostachys (45 species).

Ehrhartoideae

Rice cultivation, one of the most important cereals in the world, which is included within the estroids.

The members of this subfamily have spikelets with very reduced glumes and the androecium with six stamens, rarely with only one. The basic chromosome numbers of this subfamily are x=10 and x=15. It comprises 21 genera and 111 species, including members of the Southern Hemisphere Ehrharteae tribes, as well as the cosmopolitan Oryzeae. The latter is aquatic or wetland. The best known representative of the Oryzeae tribe is the Asian rice Oryza sativa, one of the most important crops in the world. Another of the 22 Oryza species is also cultivated in North Africa: O. glaberrima. In the United States, another species of the tribe, Zizania aquatica, the North American wild rice, becomes important.

Pooideae

This subfamily is the largest of the grasses. It consists of 194 genera that make up some 4,200 species. They are distributed in temperate climate regions around the globe. Notable genera include important cereals such as wheat, barley and oats, as well as rye (Secale cereale), grasses used for turf (such as Poa, with 500 species), for hay (Festuca, 450 species), for pastures (such as Phleum, Dactylis), and some weeds (such as Agrostis, with 220 species, and Poa). Other important genera of this subfamily are Stipa (300 species), Calamagrostis (270 species), Bromus (150 species), and Elymus (150 species). Pooideas are distinguished by the fact that the main branches of the inflorescence are couplets, the lemma usually consisting of five veins. In addition, they present oligosaccharides derived from fructose in the stem. They have usually long chromosomes and the basic chromosome number is x=7, more rarely x=2, 4, 5 or 6.

Chloridoideae

Inflorescences of Eragrostis tefA chloride.

Members of Chloridoids have disarticulated spikelets above the glumes and distinctive bicellular hairs on the epidermis of the leaves. However, this last character may be a synapomorphy of only one subgroup of the clade. All but two species of the clade show photosynthesis via the C₄ pathway. The prevailing basic chromosome numbers in the subfamily are x=9 and x=19, although there are genera with x=7 and 8. The subfamily is distributed mainly in arid and semi-arid tropical regions, where it is assumed that C_{4 is advantageous. Distribution centers located in Africa and Australia suggest a Northern Hemisphere origin. Some important genera are Eragrostis (350 species), Muhlenbergia (160 species), Sporobolus (160 species), Chloris (55 species), Spartina (15 species) and Eustachys (10 species).}

Panicoideae

Different colors of grains in corn spikes (Zea mays, a panicoid).

Panicoids have been taxonomically recognized for a long time, due to their characteristic spikelets. The culms are usually solid, the spikelets are dorsally compressed, do not have a rachilla, and are biflorous. Disarticulation of the spikelet at maturity occurs below the glumes. The prevailing type of photosynthesis physiology is C₄. Starch granules in the endosperm are simple. The most typical basic chromosome numbers are x=5, 9 and 10, although species with x=7, 12 and 14 are also found. The subfamily is mainly tropical and contains two large tribes, the Andropogoneae and the Paniceae, along with a number of smaller groups. Andropogonous are relatively easy to recognize due to their paired spikelets. Panaceae are not as homogeneous as members of the Andropogoneae. The subfamily comprises 203 genera and 3,600 species. Major genera include Panicum (470 species, polyphyletic), Paspalum (330 species), Andropogon (100 species), Setaria (100 species), Sorghum (20 species), and Zea (4 species). Sorghum and maize are two crops of great economic importance and both are included in this subfamily.

Centothecoideae

The centotecoids are a poorly studied subfamily. It is made up of about 30 species distributed in 11 genera that inhabit warm temperate to tropical forests. Its most distinctive characteristic is the presence of a style in the flower and an epiblast in the embryo. The most frequent basic chromosome number is x=12, although there are also genera with x=11.

Arundinoideae

With 14 genera and between 20 to 38 species, the arundinoideas are a subfamily whose exact delimitation is still not clear. They are hydrophytic to xerophytic grasses that inhabit temperate to tropical regions. Its basic chromosome numbers are x=6, 9 and 12. Arundo (with 3 species, Arundo donax is the Castilian cane) and Phragmites (2 species) are the best known genera of this subfamily.

Micrairoideae

This monophyletic subfamily belonging to the PACCMAD clade was reinstated and circumscribed again in 2007. Its members present stomata with dome-shaped subsidiary cells, ligules with tufts of hairs, small embryos, C₄, single starch grains in endosperm. It comprises 8 genera and about 170 species, mostly tropical. Some of the genera (such as Eriachne) were not assigned to any family until recently, and others were included in other subfamilies (such as Isachne in the Panicoideas). Includes the genera Isachne (100 species), Eriachne (35 species), and Micraira (8 species).

Aristidoideae

Aristoideans include 3 genera and 300 to 385 species from warm temperate regions, with awns with a basal column and type C₄ photosynthesis. The basic chromosome numbers are x=11 and x=12. It comprises the large genus Aristida (230 to 330 species) and Stipagrostis (50 species).

Danthonioideae

Danthonioideae is a subfamily widely distributed throughout the globe, especially in the Southern Hemisphere. They present bilobed profiles and the synergids of the haustorial type embryo sac. The basic chromosome numbers are x=6, 7 and 9. It comprises 19 genera and about 270 species. Among the genera with the largest number of species are Danthonia (100 species) and Rytidosperma (90 species). Cortaderia selloana is the popular "cortadera" an ornamental grass that is included within this subfamily.

List of genres

The list of all known genera of grasses, arranged alphabetically, is provided in the annex called genera of Poaceae. In many cases synonyms are listed. Synonym links lead to the preferred genus name.

Synonymy

The family or one of its subfamilies have the following synonyms:

Economic importance

Wheat harvest (Triticum aestivum), one of the world's main cereals.

Maize leaflets (Zea mays)), one of the countless cereal products.

Cows grazing on a grassy grass.

Chrysopogon zizanioides, a perfumed species. They are seen tied to vetiveria roots ready for sale, Island of the Meeting.

Malta used for making whisky.

Many grams are excellent dunes fixers. In the image one of such species is observed, Ammophila arenariaIn Grenen, Denmark.

The grass family is probably the most important to the human economy. In fact, about 70% of the world's arable land is planted with grasses and 50% of the calories consumed by humanity come from of the numerous species of grasses that are used directly for food, or indirectly as fodder for domestic animals. In terms of global production, the 4 most important crops are grasses: sugarcane (Saccharum officinarum), wheat, rice and maize. Barley and sorghum are in the top 12. On the other hand, various grass species are used in the industry.

• Food plants. Cariopse or grain is usually used directly as food or ground in the form of flour. The species used in this way are the so-called cereals. Man has grown cereals for at least 10,000 years. From the beginning of its domestication, wheat (Triticum aestivum), barley (Hordeum vulgare) and oatmeal (Avena sativa) in the Fertile growth of the Near East, the sorghumSorghum bicolorand the millet (Pennisetum americanum) in Africa, rice (Oryza sativa) in Southeast Asia, and corn (Zea mays) in Meso-America have made possible the settlement of human communities and the development of civilizations. Wheat (especially Triticum aestivum, the so-called bread wheat, a Pooideae), provides a fifth of the calories consumed by humans, and began to be domesticated about 10,000 years ago. Most of the domestic forms are polyploids, and the plasticity of the genome in connection with polyploidy is involved in the success of the cultivation of this cereal (Dubcovsky and Dvorak 2007). Hard wheat (Triticum durum) is used to make noodles or pastas. The corn (Zea mays, a member of Panicoideae) is a cereal with multiple applications, from its direct consumption as "choclo", the use of its flour in the elaboration of many regional dishes and even alcoholic beverages, the industrial use of its grains for the production of oil, fructose syrup, and many other applications, to which the manufacture of biodiesel has recently been added. The rice (Oryza sativa, Ehrhartoideae) is, most likely, the species of greater global importance as food, given the enormous amount of people who consume it daily. The oatmeal (Avena sativa, Pooideae), barley (Hordeum vulgare) and the rye (Secale cereale) are three other cereals that are commonly used as food. In addition to cereals, some grams like Phyllostachys edulis and Sinocalamus beecheyanus are used as vegetables in Asia.
• Forage plants. Many species of gramineas are excellent pasture producers for livestock, both in natural grasslands and in cultivated pastures. Thus, many perennial pasture species are cultivated for this purpose, both in temperate climates and in tropical or subtropical climates. Tempered fodder species produce pasture during autumn, winter and spring and the most popular are perennial raigrás (Lolium perenne), cebadilla criolla or bromo of the meadows (Bromus unioloides), high festuca (Festuca arundinacea), elongated agrimy (Thinopyrum ponticum ), the fling (Phleum pratense) and the bulbous falaris Phalaris tuberosa). Grams cultivated as perennial forages from tropical or subtropical climates are of summer production and among them are honey pasture (Paspalum dilatatum), the elephant grass (Panicum elephantypes), the round chart (Gay Chloris), Mijo Perla (Pennisetum americanum), the horquette pasture (Paspalum notatum) and the weeping pasture (Eragrostis curvula). Several other fodder species are annual, so they are used to produce large amounts of pasture (called green) during a certain period of production: winter or summer. Among the species for winter greens are the oats (Avena fatua, Avena sativa), the rye and the barleyHordeum vulgare). For the summer greens the corn and the forage sorghum are used (Sorghum Sudanese). There are also cereals used to feed animals, such as millet (Panicum miliaceum) and the apiste (Phalaris arundinacea) to feed birds, or corn and sorghum to feed cows, pigs and poultry.

• Industrial. The industrial uses of grammares are as varied as the family itself is. There are perfumed species, whose extracts are used in the preparation of a large number of perfumes, such as lemon grass (Cymbopogon citratus, from which an essence called citronella is extracted and vetiver (Vetiveria zizanioides). Other gramineas, such as barley (Hordeum vulgare), are used for the production of malt, a product indispensable for the manufacture of beer, whisky, gin, gin and other alcoholic beverages. Other grains of cereals are used to produce alcoholic beverages for fermentation, such as sake (or nihonshu) from rice in Japan. The oil industry also uses grams grains (such as corn) to produce edible oils. Some genres have great interest in lanyard, basketwork and manufacture of traditional footwear, such as Spartagues. Such is the case of the albardin (Lygeum spartumand, above all, of the atocha (Stipa tenacissima), both very employed in Spain and North Africa to produce spartum, raw material for the preparation of all these elements. It is also used to Sorghum technicum to make brooms, to Epicamps microura and Aristida pallens to manufacture brushes and Stipa tenacissima to elaborate ruins.
• Grams for grass: numerous species of the genus Poa, Lolium, Festuca, Axonopus, Stenotaphrum and Paspalum are used to form lawns in parks and gardens. Agrostis palustris, In particular, it is used to form the "green" of the golf courses.

• Ornamental grams. Until many years ago the only grams cultivated in the gardens were those species that make up the grass. Currently many perennial large porte grams have increased their popularity to be used as central elements in the design of parks and gardens. This is not only due to its characteristics of longevity, adaptation, rusticity and low maintenance, but also to its ornamental values or particular aesthetic qualities related to movement, transparency, lighting and the spectacular seasonal changes that take place. Some of the species used as ornamental are Alopecurus pratensis, Cortaderia stampana, Festuca glauca, Imperata cylindrica, Leymus condensatus, Miscanthus sinensis, Pennisetum setaceum and Phyllostachys aurea, among many others.

• Weeds: there are numerous species of grams that are weeds of different crops, several of them very difficult to eradicate or control, which cause great losses of yield every year due to competition with the cultivated species, such as the sorghum of Aleppo (Sorghum halepense), the salmon (Cynodon dactylon)The cape (Echinochloa crus-galli) the Lent pasture (Digita sanguinalis) and the (Extended). In one of the parables of Jesus is used the example of tares (Lolium temulentum), which is also a common place in colloquial speech.

• Other uses: grams are also used for erosion control and as dunes fixers. Examples of species used for this purpose are Sporobolus arundinaceus, Panicum urvilleanum, Spartina ciliata, Poa lanuginosa, Ammophila arenaria and Elymus arenariusamong many others. Certain musical instruments are manufactured with grammar rods, such is the case of the quena through the use of Arthrostylidium harmonicum. Other species have been used for centuries as medicinal plants, in particular as diuretics, such as the boticar chart Elymus repens and Cynodon dactylon. The bambooes, finally, are economically important in many tropical areas for their young edible stems, for their fiber used to make paper, for their pulp for the rayon, and their thick stems for construction.