Psilotaceae

See Pteridophyta for an introduction to seedless vascular plants

The Psilotaceae (scientific name Psilotaceae, "whisk ferns" in English) are a family of plants related to ferns, which are characterized by having a very simplified sporophyte (without roots, with reduced leaves, and with a stem that branches dichotomously), which is probably due to its association with fungi. Many of them are epiphytes. They are the only family of the order Psilotales.

The simplification that the body of the sporophyte suffered when it was associated with fungi for its development, led to the belief for a long time that the psilotaceae were plants that retained many primitive characters similar to those found in the first vascular plants in the fossil record such as Rhynia, although no fossils had been found to support this argument. Therefore, they would have been plants that had emerged from the phylogenetic tree of vascular plants before the appearance of ferns in the broad sense (monilophytes).

Today, modern phylogeny analyzes based on DNA studies have agreed that the Psilotaceae as defined here are positioned as a sister group to the Ophioglosaceae (which is a group of primitive ferns with whom they share important unique characters that they inherited from the common ancestor to the two families) in a basal node of ferns in the broad sense or monilophytes. There are only two living genera, Psilotum with 2 species and Tmesipteris with about 10 species.

Description

Psilotaceae are vascular plants with a haplodiplontic life cycle where the alternation of generations is very evident, therefore they have a sporophyte and gametophyte that develop to be multicellular and are independent of each other (each depending exclusively on itself). to survive), with spores as a unit of dispersal and resistance. The gametophyte is a "thallus" (body without tissue organization) and produces gametes that, when fertilized, develop a sporophyte whose structure is a "corm" (organized into root and stem and a vascular system that links them), the sporophyte in its sporangia gives spores that, under favorable conditions, develop into new gametophytes. Due to these characteristics, psilotaceae are traditionally grouped with the rest of the seedless vascular plants or "pteridophytes".

Absent roots, unique character in pteridophytes. The plant is anchored to the substrate by underground stems that can carry "gems" ("gemmae" in English), see description below.

The aerial stems are erect or pendulous, glabrous (hairless), simple or dichotomously branched. The leaves are arranged in a spiral along the stem (in some species the arrangement is distichu).

The leaves are small, scaly or awl-shaped to lanceolate, simple or with a fork. In the anatomical section it is observed that a single vein runs through them, or they have no veins.

Two or three sporangia join together by means of parenchymatic tissue to form a synangium (can also be described as a single 2-3 locular sporangium instead of a synangium with 2-3 sporangia), the synangium has as many lobes as sporangia, and appears to arise on the adaxial side of the forked leaf (according to Judd et al. 2002, it can also be sessile—located directly on the stem—). The sporangia are homosporic (only have 1 spore morph). Many (more than 1000) spores per sporangium. The sporangia are considered eusporangiate type.

Under the microscope it is observed that the spores are reniform ("bean-shaped"), with a monolete mark.

Along with the ophiglosaceae and unlike the rest of the monilophytes, the gametophyte is axial, that is, it is attached to the stem, and in Psilotum it is also underground. The gametophyte is mycorrhizal (associated with fungi that explore the soil), not photosynthetic. The propagules of the gametophyte, responsible for vegetative propagation, are called "gems".

Ecology

Having a pantropical distribution and also present in temperate climates where there are no very cold seasons (warm temperate climate in English), they do not develop in very dry areas. The largest number of species is found in Southeast Asia and the South Pacific, mainly at low altitudes.

They are herbaceous plants. They can be terrestrial but the most common thing is that they are epiphytic, when this is the case they are found especially on the trunks of tree ferns. They depend on the presence of certain fungi with which they form endophytic mycorrhizae, necessary for the production of spores.

Evolution and phylogeny

Theoretical Introduction in Philogeny

As circumscribed according to Smith et al. (2006) (see taxobox), is monophyletic (Hasebe et al. 1995, Hauk 1995, Pryer et al. 2001a and 2004).

Until 2001 Psilotum was considered a "living fossil", a true living exponent of the first vascular plants such as Rhynia. However, this hypothesis never received support from the fossil record of this family, which does not support a possible relationship with Rhynia, although it is not very extensive.

DNA analyzes showed that together with Ophioglossaceae they form a monophyletic group sister to the rest of the ferns, together with which and together with Equisetum they form the division Monilophyta.

Now that it is agreed that they are unequivocally related to the rest of the monilophytes, their morphological features have been reinterpreted, and it is now considered that the simplicity of their characters is a reduction (that is, that they underwent a simplification from a state most complex of its characters, a state corresponding to the common ancestor of all ferns along with the horsetails and this clade). Apparently the reduction was a consequence of its association with fungi or mycotrophy (Juddet al. 2002).

Now that the phylogeny of the psilotaceae is known, it is known that the sporophytes of their ancestors had euphyllous leaves (and therefore are descendants of the first euphyllophytes), and although all plants with broad leaves (leaves called for this reason fronds or megaphylls) are euphyllophytes, in psilotaceae the euphylls are considerably reduced.

For a discussion of synapomorphies shared with Ophioglosaceae, see Psilotopsida.

Taxonomy

Theoretical Introduction in Taxonomy

The most updated classification is that of Christenhusz et al. 2011 (based on Smith et al. 2006, 2008); which also provides a linear sequence of the lycophytes and monilophytes.

• • Order F. Psilotales Prantl, Lehrb. Bot., ed. 5: 183 (1884).

1 family.

• • • Family 6. Psilotaceae J.W.Griff. & Henfr., Microgr. Dict.: 540 (1855). Synonyms: Tmesipteridaceae Nakai, Chosakuronbun Mokuroku [Ord. Fam. Trib. Nov.]: 206 (1943).

2 gendersPsilotum, Tmesipteris). References: Bierhorst (1977), Brownsey & Lovis (1987), Gensel (1977).

Classification sensu Smith et al. 2006

Clades and higher taxa: Plantae (clade), Viridiplantae, Streptophyta, Streptophytina, Embryophyta, Tracheophyta, Euphyllophyta, Monilophyta, Class Psilotopsida, Order Psilotales, family Psilotaceae.

Constituency: 2 genders:

• Psilotum with about 2 species, cosmopolitan.
• Tmesipteris with about 10 species, in Australia and the South Pacific.

Synonym: Tmesipteridaceae.

Other classifications

Due to the dichotomous branching of the stem and the general simplification of the sporophyte, in traditional classifications this clade was placed in its own class, as an ancient pteridophyte. The sensu Engler classification is as follows:

• Plant Kingdom (poliphytic), Embryophyta asyphonogama (paraphytic), Pteridophyta subdivision (paraphylatic), class Psilotopsida, order Psilotales, family Psilotaceae (monophylaxis, equivalent to Psilotales / Psilotaceae sensu Smith et al.).