The pteridophytes, "Lower" vascular plants, or vascular cryptogams, comprise a group of spore-producing terrestrial vascular plants, which do not produce flowers or seeds. Ferns, horsetails and club mosses belong to this group..
Pteridophytes, unlike angiosperms and gymnosperms (the main groups of terrestrial vascular plants), do not produce flowers or seeds and their reproduction occurs through spores..
However, they are classified within the group of tracheophytes, since they do have a vascular system, which makes them considerably different from mosses, liverworts and hornworts (Bryophyta sensu lato), which are non-vascular land plants.
Although they are not phylogenetically related to each other, botanists have included in the group of pteridophytes the 2 classes of existing "lower" vascular plants: Polypodiopsida (that of ferns and horsetails) and Lycopodiopsida (that of lymphophytes).
There are around 15,000 species of pteridophytes today, making them a truly abundant group, preceded only by angiosperms. They are also an extremely diverse group, not only in size and shape, but also in habits and distribution..
As is true for other "higher" plants, it is in tropical regions that we find the greatest diversity of pteridophytes, since very few species have been described for temperate climates. However, a large number of fern species, for example, are grown as ornamental plants all over the world..
It is also important to comment that, since they are plants with relatively simple structures and life cycles, they have great value from a phylogenetic and systematic point of view for the general understanding of the structure and evolution of "higher" plants..
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Pteridophytes have very particular characteristics that distinguish them from other groups of plants such as bryophytes (non-vascular), angiosperms and gymnosperms (vascular with flowers and seeds or only seeds, respectively)..
The main characteristics of pteridophytes are:
- They are vascular plants, that is, they have an internal system for conducting water and processed matter (xylem and phloem)..
- They have leaves, often known as fronds, also called microfilms the smallest, and megaphiles the largest; roots and stems. Some species of ferns, for example, have true trunks.
- They do not produce flowers or seeds, but they do produce spores.
- Unlike the bryophytes and in a similar way to the rest of the terrestrial vascular plants, this group has the sporophytic phase as the dominant phase, that is, the plant body that we observe corresponds to the diploid sporophyte (2n) and not to the haploid gametophyte (n ).
- They are heterosporic plants, which means that they produce two different types of spores: a megaspore and a microspore. Megaspores germinate giving rise to female gametophytes (archegonia) and microspores germinate to give rise to male gametophytes (antheridia).
- Although they are extremely common plants, none of the species described so far is cultivated as a food plant, although some foliar shoots of some are consumed in some latitudes.
In all vascular plants, that is, in tracheophytes, the vascular conduction system includes two types of tissues:
- The xylem, formed by tubular cells known as tracheids and specialized in the conduction of water and mineral nutrients.
- The phloem, formed by cells known as the sieve elements and specialized in the conduction of sap or elaborated nutritional material.
The way in which these vascular tissues are configured inside plant organs is, many times, group-specific, but we can point out some generalities.
In the "interior" of the stems and roots, the vascular tissues form a cylinder known as a stele, whose simplest conformation (protostela) consists of a central xylem, surrounded by a "band" of phloem; where both tissues are surrounded by an endodermis called the pericycle.
Most of the pteridophytes have what is called siphonostela, which consists of a central "medulla" around which the vascular tissues are located forming a cylinder that surrounds it..
An important difference between pteridophytes and other vascular plants has to do with the fact that when the organs of the former mature, they do not increase in diameter, which does occur with the latter thanks to special cells that form a tissue called cambium..
The field of systematics is extremely dynamic and changing, and what was previously considered as “the traditional classification” of the edge of the pteridophytes, today is considered as “alien” to reality..
Before delving a little more into its classification, it is convenient to highlight that pteridophytes do not form a group monophyletic, that is, they do not all come from the same common ancestor, so the term "Pteridophyta" is often not considered valid as a taxon, although it is still used informally.
According to R. Walkowiak's report in 2017 for the International Botanical Research Group, pteridophytes can be classified rather in 4 divisions, 6 classes and 20 orders, namely:
- Lycopodiophyta: with the classes Lycopodiopsida (order Lycopodiales, 400 species), Sellaginellopsida (order Selaginellales, 450 species) and Isoetopsida (order Isoetales, 130 species).
- Equisetophyta: with the class Equisetopsida (order Equisetales, 15 species)
- Psilotophyta: with the class Psilotopsida (order Psilotales, 12 species)
- Polypodiophyta: with the class Polypodiopsida (orders Osmundales, Hymenophyllales, Gleicheniales, Schizaeales, Plagiogyriales, Dicksoniales, Cyatheales, Marsiliales, Salviniales, Pteridales, Davalliales and Polypodiales; more than 10,000 species)
In the division Lycopodiophyta the “garrote mosses” are included, although they are not a species of bryophytes and the “ground pines” (order Lycopodiales). There are also the “spike mosses” (order Selaginellales) and the isoetes or quillworts in English (Isoetales order).
Divisions Equisetophyta Y Psilotophyta They comprise two more or less known classes: Equisetopsida (where “horsetail” plants are classified) and Psilotopsida (where “brush ferns” are classified).
Finally, the division Polypodiophyta, which includes the class Polypodiopsida and its numerous orders, comprises what we colloquially call the “true ferns”. It is the most important division within the group of pteridophytes, not only due to diversity but also due to abundance.
Pteridophytes have a life cycle that presents a "true" alternation of generations, since both the haploid and diploid phases are free-living.
As we have already mentioned, the dominant phase is that of the sporophyte, which is responsible for the production of meiotic spores (by meiosis) during asexual reproduction. Remember that they are heterosporic plants, which produce mega and microspores.
The gametophytes arise from the germination of these spores and are those that are responsible for originating, by mitosis, the gametes (female and male).
Both sporophytes and gametophytes are free-living, photosynthetic, multicellular "individuals." Gametophytes, often known as "Protest them”, Grow in places much more humid and colder than sporophytes, as they are highly dependent on water.
This dependence is due to the fact that the antheridia (the male gametophytes) release the male gametes (anterozoids) so that they reach (through the water) the archegonia (the female gametophytes) where the ovocells are found, so that the gametic fusion takes place and the formation of the zygote, that is, during sexual reproduction.
The zygote product of this fusion divides to give rise, later, to the diploid sporophyte (2n) that, when producing spores by meiosis, forms new gametophytes that return to fulfill the cycle.
The gametophytes of the pteridophytes can be classified as dioecious, if there is an individual male and a female organism, or as monoecious, when the same gametophyte produces antheridia and archegonia..
When it comes to monoecious gametophytes, these can, in turn, be defined as protandric and as protogynic, depending on whether the antheridia mature before the archegonia or vice versa, respectively..
Like most terrestrial vascular plants, pteridophytes are autotrophic plants, that is, they are organisms that “produce their own food” through photosynthesis (photosynthesis)..
With the above we understand that this group of plants fundamentally requires: light energy (mainly from the sun's rays), carbon dioxide (CO2) and water, in order to carry out the metabolic processes necessary for the formation of their tissues..
They also need mineral nutrients, which they can obtain directly from the soil through their roots (they are absorbed with water) or from the substrates where they grow, which can be other plants (for epiphytic species) or decomposed plant matter, for example..
Since the most important division within the pteridophytes corresponds to the "true ferns", we will mention some relevant species:
Platycerium bifurcatum, Better commonly known as "elkhorn" fern or "deer horn" fern, it is a fern of Australian origin that grows on other plants (generally trees) or on rocks, in an epiphytic way..
This well-known fern belongs to the Polypodiaceae family, which is classified within the Polypodiales order of the Polypodiopsida class, of the Polypodiophyta division..
It is well known for its popularity as an ornamental plant, so it represents some economic interest in horticulture.
Not all ferns and other pteridophytes are small plants, the Australian tree fern, Cyathea cooperi, It has a large wingspan that characterizes it, as it can reach up to 15 meters in height and trunks up to 30 cm in diameter in its natural environment.
Despite what its name suggests, this fern is also widely cultivated around the world for garden decoration, as it is easy to grow and resists low temperatures and weather seasonality quite well..
Of Asian origin, the red fern, also known as the sunset fern, is a well-known fern of the species Dryopteris lepidopoda. Its popularity also has to do with its wide use as an ornamental plant, since the tonality of its leaves is usually very attractive in the world of landscaping..
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