Tracheophyta
Tracheophyta or Tracheobionta is a taxon that encompasses vascular plants or tracheophytes. They are organisms made up of plant cells, which have a life cycle in which gametophytic and sporophytic generations alternate, the latter being the dominant phase (on whom the most pressure of natural selection acts); whose sporophytic phase is photosynthetic and independent, and has tissues and organ systems; it is organized in a "corm" (a system that has an aerial stem, an underground root and a vascular conduction system that links them) which is what people commonly refer to when they say "plant"; whose gametophytic phase is reduced and can be from a "thallus" (body not organized into tissues or organs) in ferns and the like, to a few cells protected and nourished by the sporophyte, in gymnosperms and angiosperms. Natural selection strongly directed the evolution of tracheophytes towards less dependence on environmental conditions on land for reproduction and dispersal, a characteristic that becomes evident when comparing the oldest tracheophytes (Lycophyta) with the most modern (flowering plants).).
Evidence from molecular DNA analyzes today has shown that the tracheophytes are a monophyletic (comprising all descendants of a common ancestor) group within the embryophytes. This means that tracheophytes are probably descendants of plants very similar to bryophytes, with the gametophyte being the dominant phase, and the unbranched sporophyte nutritionally dependent on the gametophyte.
Within the tracheophytes there are two main lineages, Lycophyta and Euphyllophyta, differentiated mainly by the construction of their leaves (in lycophytes they are lycophylls and in euphyllophytes they are euphylls, the euphylls roughly correspond to the megaphylls, although in some groups may be reduced secondary to their acquisition). Euphyllophytes in turn comprise two large lineages, Monilophyta (ferns, Equisetaceae and Psilotaceae) and Spermatophyta, which differ from each other because the first has free-living gametophytes and the second has them enclosed in the seed and pollen grain. In turn, spermatophytes are made up of two large living monophyletic groups, Gymnospermae and Angiospermae or Magnoliophyta, which differ from each other because the first lineage has its seeds visible on the fertile leaf, while the second has its seeds enclosed within the walls of the fertile leaf or carpel.
The Lycophyta and Monilophyta are still being studied as grouped in the paraphyletic group of "ferns and related" or Pteridophyta.
Tracheophytes are an important group both because they dominate most terrestrial ecosystems and because they are widely used by humans.
Synonymy
The name Tracheophyta has been used by Sinnott (1935), Barkley (1949), Whittaker (1969), Kubitzki et al. (1990), Cavalier-Smith (1998), Ruggiero et al. (2015) and is considered a clade and a taxon with the level of phylum or superphylum. It has also been considered as subkingdom Tracheata (Margulis & Chapman 2009) or Tracheobionta and other terms such as Cormophyta (Haeckel 1866) have been used. or Cormobionta, although the latter can also mean Embryophyta. Tracheophyta comes from tracheo (referring to tracheids, specialized for transporting liquids within the plant) 'phyta, a root of Greek origin meaning "plant". It is Spanishized as tracheophytes or tracheophytes.
The sporophyte of vascular plants
The vegetative body
The vegetative body of the sporophyte is a «corm» (stem + root + vascular conduction system, made thanks to the thickening of the cell walls of elongated cells), which, in addition to the vascular tissues, has protective tissues and supporting tissues, and that grows thanks to the action of its meristems anatomy/11.html (broken link available at Internet Archive; see history, first and last version). anatomy/14.html (broken link available at Internet Archive; see history, first and last version). span> anatomy/13.html (broken link available at Internet Archive; see history, first version and the last)..
The «corm» is specialized for terrestrial life. It consists of a stem, root and a vascular conduction system that links them. The stem is the region of the corm that specializes in photosynthesis, using the water and mineral salts that come from the root. The root is the region of the corm that specializes in the absorption of water and salts, and that uses the sugars provided by the stem to do so. This specialization is possible thanks to the fact that the corm has a vascular conduction system formed thanks to the fact that the walls of the plant cell can become rich in lignin, a compound that gives them hardness.
Meristems
The corm grows thanks to the activity of its meristems (from the Greek meros: 'to divide') which are a group of cells in a permanent embryonic state, capable of dividing indefinitely, forming tissues that in their youth are undifferentiated anatomy/11.html (broken link available at Internet Archive; see history, first and last version). anatomy/14.html (broken link available at Internet Archive; see history, first and last version). anatomy/13.html (broken link available at Internet Archive; see history, first version and the last one)..
The main types of meristems are two:
- in all the cormos we will find in the apex of the stems and the roots, primary meristhes of the Primary growth the sporophyte.
- in the cormos with secondary stem growth, we will find secondary along the stem, responsible for its growth in thickness.
Tissue systems
The botanist J. Sachs in the XIX century distinguished three main tissue systems in the corm of the sporophyte of the cormophytes, classified according to their function in the plant: protective tissues, fundamental tissues, and vascular tissues.
Protective fabrics
The tissues that fulfill the function of protection form the outermost layer of the corm.
- If the cormo only has primary growth, we will find a epidermiscovered by a layer of skin (lipid complex that prevents water loss in terrestrial life but also avoids gaseous exchange with the environment), and with allies/Psilophyta/Psilotum nudum/Stem xs/Epidermis stomata.html and lenticelas (both ensure gaseous exchange with the environment).
- In the taxa with secondary stem growth, the epidermis with cutin is replaced during secondary growth, by a peridermis partially waterproofed with suberine (lipid very similar to the cutin, responsible for the formation of the sober or cork).
Fundamental tissues
The fundamental tissues form a continuous system and are made up mainly of the various types of parenchyma (from the Greek: «meat of the viscera»), to which the supporting tissues (such as collenchyma and sclerenchyma).
The support tissues, as their name indicates, fulfill the function of maintaining the structure of the plant, a function that they fulfill thanks to the lignin present in the wall cell phone. The most common support fabrics are:
- Colénquima (from Greek: rubber, tail, name given by the ease with which cell walls swell when hydrated) formed by living cells.
- Sclerénquima (from the Greek scleros: hard, name given by its thick very hard and resistant walls) formed by cells almost always dead at maturity.
Vascular tissues
They are responsible for the transport of liquids and substances throughout the body of the plant, in which the phloem (from the Greek floeos: 'yolk of the bark') can be distinguished. ', 'proper to the bark', specialized in transporting sugars) and the xylem (from the Greek xylos: 'to lignify', formed by open or closed tubular dead cells at their ends, with very lignified walls, which form a vascular bundle specialized in transporting water and salts) [1] [2] [3] [4] [5]. These tissues are complex and are often associated with others (parenchymal and supporting). The vascular tissues are located within the fundamental tissues in a different way depending on the different organs of the plant (root, stem, etc.), which in a cross section form patterns (called stele ), which are important systematic. Thus, the stem of most monocotyledons presents a pattern or stele called atactostela, while the (primary) stem of dicotyledons and conifers, on the other hand, presents a eustela, and that of ferns and related species presents a great diversity that allows differentiating the families from each other.
Reproduction in vascular plants
 Schematic diagram of the life cycle of vascular plants (Tracheobionta). References: n : haploid generation, 2n : diploid generation, m! : mitosis, M! : meiosis, F! : fertilization |
The adult sporophyte develops the so-called sporangia (multicellular structures of the sporophyte within which the spores are formed), where meiosis will occur that will give haploid spores. The spores dividing by mitosis become "gametophytes" (which will be a thallus or just a few cells) that will give the male and female gametes by meiosis, the female gamete will always be immobile and protected at least by the female gametophyte giving together the ovule, the male gamete will be mobile or will be transported by external agents to the ovule where fertilization occurs, which will give a diploid zygote. The new sporophyte will divide nourished at the beginning by at least the female gametophyte, until giving the embryo formed by plumule (where the apical meristem is) and radicle (where the radical meristem is), which when growing forms the new adult sporophyte starting the cycle again.. In taxa without leaves (Psilotum) the sporangia are located directly on the stem, in taxa with leaves, the sporangia are located on the leaves (these becoming «sporophylls »).
Meiosis occurs inside the sporangia that forms haploid spores, which will then be released or not, and when dividing by mitosis they will give haploid gametophytes. In older cormophytes (such as ferns and the like) the gametophytic stage is still free-living, but in more recent ones it has become totally dependent on the vegetative body of the sporophyte for nutrition and protection. Pteridophytes (ferns and the like) can be isosporic (all spores are the same, they will give hermaphroditic gametophytes) or heterosporic (with two spore morphs, megaspores which will give the female gametophyte and microspores which will give the male gametophyte); On the other hand, spermatophytes (gymnosperms and angiosperms) are all heterosporic. When the sporophyte is heterosporic, the gametophyte develops entirely within the spore, without being free-living. The gametophytes will produce female and male gametes (from the Greek gametes: husband). The immobile female gamete waits to be fertilized inside a structure where it is protected and will be nourished (oogamy phenomenon). This structure comes from the gametophyte (in pteridophytes) or from the gametophyte and the sporophyte (in spermatophytes). Upon receiving the male gamete, fertilization occurs, which will give the diploid zygote of a new sporophyte (from the Greek zigos: couple, yoke). The zygote will begin to divide, passing through an embryo phase before becoming an adult, the embryo nourishes itself on its protective coverings. Only in spermatophytes can the embryo remain in a state of dormancy (in the form of a "seed") until the conditions are right (and what is known as "germination" occurs), the more primitive pteridophytes develop directly until adults without latency. In the embryo, the primary meristems can already be distinguished: the apical meristem that gives rise to the stem which also gives rise to the leaves if there are any anatomy/15.html (broken link available at Internet Archive; see history, first and last version). span>, and a radical meristem that gives rise to the primary root anatomy/19.html (broken link available at Internet Archive; see history, first version, and last version). The taproot or embryonic root can either develop or die, either that the scion can also emit roots that in this case will be called adventitious roots.
Systematics and phylogeny
Original systematics
Vascular plants are divided into two large groups according to the traditional systems of Eichler, Engler or Wettstein:
- Pteridophyta: Paraphylical group considered a subdivision or phylum. The most accepted hypothesis up to a few years ago argued that pteridophytes are a monophytic group derived from some green algae ancestor. Today it is known composed of two phylogenetic lines, are the oldest of the current cormophytes, it includes the ferns and related. They are characterized by an alternation of well manifest generations. Current pteridophytes are concentrated in the tropics and humid mountain areas. It is composed of three monophyllic classes, two of which (ekeches and ferns) form the Monilophyta nail:
- class Lycopsida or Lycopodinae (licopodes)
- class Sphenopsida or Equisetinae (equities)
- Filicopsida or Filicinae class (facts)
- Spermatophyta▪ Traditional systems generally considered a division called Phanerogamae, Siphonogamae or Anthophyta. They can be defined as vascular plants where the alternation of generations is given in masked form, as the gametophyte develops within the structures of the sporophyte (even in the most primitive groups can still be seen beforehands and archgones). It includes gymnospermas and angiospermas, in the following monophytic lines:
- Gymnospermae subdivision
- subdivision Angiospermae (magnoliophytes)
Phylogeny
The latest genetic analyzes determined that the phylogeny of living tracheophytes is as follows:
| Trachaeophyta |
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In particular, it is noteworthy that a very strong molecular evidence of the basal division between lycophytes and euphyllophytes is the presence in euphyllophytes of an inversion of about 30 kilobases in the chloroplast DNA (Raubeson and Jansen 1992), what is remarkable is that investments of so many kilobases are very rare, and it is rare that they give viable products, so it becomes very strong evidence of monophyly.
If we take extinct groups into account, the relationships are approximately as followsː
| Tracheophyta |
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Evolution of tracheophytes
 Reconstruction of the sporophyte Rhynia. General aspect, longitudinal cutting of sporangio, spores tetrada and spora detail with trilet mark. Note the dichotomous branching of the stem, the lack of leaves and roots, and the terminal sporangios. |
Much of what we know about the first vascular plants and their transition to terrestrial life is found in the fossil record, since those valuable lineages are all extinct.
Ancestral tracheophytes were derived from the earliest embryophytes, the first plants to colonize the earth's surface. Everything indicates that the first embryophytes were small and very simple in their structure. In the case of the lineage that led to vascular plants, the sporophyte was basically a dichotomous branching stem, initially about the height of a matchstick, with the sporangium (where meiosis to give haploid spores occurs) produced at the tips of the vascular plants. branches. These plants had neither leaves nor roots, in some cases (such as Rhynia from the "Rhynia Chert" in Scotland) the preservation of these plants is spectacular, and it is possible to discern many anatomical details, including stomata, spores, and vascular tissue within the stem.
In analyzes based on these fossils, it was recently discovered that the earliest polysporangiophytes (plants with branching sporophytes) did not actually produce true water-conducting cells (tracheids) in the xylem, and therefore both must have been entirely dependent on turgor pressure to stay upright. Truly water-conducting cells evolved later and characterize the clade of tracheophytes or "true vascular plants" (Kenrick and Crane 1997a, 1997b).
Tracheids are elongated cells with thickened walls, dead at maturity. Where one tracheid connects to the next, characteristic secondary wall openings (pits) are observed, but the primary cell wall of the cells that formed the tracheids remains intact, giving the pit membrane. Water must therefore pass through the primary cell walls in order to move forward. In early tracheophytes (represented by Rhynia) the tracheids were of a distinctive type, with some "decay resistance" conferred by lignification of the fibers. of cellulose, which was present only as a very thin layer. The most resistant cell walls are those that characterize the eutracheophyte clade, which includes all living vascular plants (Kenrick and Crane 1997a). In these species, the strongly lignified tracheids allow for more efficient water conduction, and provide internal resistance, allowing plants to grow much taller.
For a discussion of the evolution of sporophyte dominance over gametophyte evolution, see Embryophyte evolution.
Phylogenetic relationships between lines of living vascular plants, shown in the systematic section figure, show a basal split, which occurred in the early to middle Devonian (about 400 million years ago), separating the lineage from the current lycophytas of the lineage of the current euphyllophytes. This division is marked by a considerable variety of morphological characteristics. A notable one is the presence of multiflagellate sperm in the euphyllophytes as opposed to the biflagellate sperm of the lycophytes and of the lineages detached from older groups (the sensu lato bryophytes). The only two exceptions are Isoetes and Phylloglossum, where multiflagellate sperm originated independently.
The first seed plants appeared in the late Devonian, and that led to the typical Mesozoic flora dominated by gymnosperms. The oldest fossils of angiosperms found to date date from approximately 140 million years ago (early Cretaceous), and the number of species found increases markedly with geological time, compared to other non-angiosperm plants (the so-called « Darwin's Abominable Mystery). For a discussion of radiation in angiosperms see Evolution of angiosperms.
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Economic importance
Tracheophytes are an important group for man, as well as dominating most terrestrial ecosystems: they provide us with medicines, ornamental plants, fibers for paper and clothing, and most of our food. Historically, information on these and other attributes of tracheophytes was essential to the development of human civilization. Survival depended on knowing which plants were good to eat, which were poisonous to people or potential food animals, which were good for weapons and tools, which could heal, and which could be useful in many other ways. senses.
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