BSC 314 Lecture Notes - Lecture 56: Polypodium, Prothallium, Antheridium
27 Jun 2018
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Phylum Psilotophyta: Whisk Ferns
Two living genera, Psilotum and Tmesipteris, with only two species of the former and
less than 30 of the latter, constitute the entire phylum. Both genera are weeds in the
tropics and subtropics. Tmesipterisis confined to the islands of the South Pacific,
including Australia and New Zealand whereas Psilotum is more widespread, reaching
parts of the southern United States.
The sporophyte of Psilotum looks like a survivor from the Devonian age; it has no leaves nor
roots, a protoxylem, a dichotomously branching green stem with small scales that bears
bright yellow synangia (formed from three fused sporangia) on short lateral branches. It is
homosporous and the spores develop into bisexual gametophytes that resemble pieces of
sporophyte rhizome.
While many of the morphological and anatomical features fit, data from current
RNA sequencing and other chemical analyses cause many botanists to reject
the long held belief that Psilotum is a living ancestor of the seed plants.
Instead, Psilotum appears to many botanists to be a descendent of the ferns by
loss and simplification of structures. There are no known fossils of the
Psilotophyta. Phylum Pterophyta: Ferns
The largest group of living seedless vascular plants—and probably the most familiar—
are the ferns with about 12,000 species, over two thirds of which are tropical. Ferns are ‐
an ancient group. Spores and leaf impressions of plants that lived 400 million years ago
in the Middle Devonian have been found, but almost all of these early types (grouped
simply as “preferns”) were extinct by the Permian. The group to which most modern
ferns belong, the Filicales, first appeared in the Lower Carboniferous, 300 million years
ago.
Characteristics
The ferns are an extremely diverse group, and there is no single characteristic that
defines them. The following features are present in most:
Leaves, called fronds, are megaphylls. Most are compound with a rachis and
numerous pinnae (or compound once again with pinnules). Almost all
have circinate vernation—they are coiled (circinate) tightly in “shepherd's crook”
or crozier fashion over the growing tips. These unroll as they mature (growth
from the base to the tip like this is termed acropetal). The croziers are
called fiddleheads and are eaten by some people, although many species are
toxic.
Stems, for the most part, are rhizomes that grow at, or just under, the ground
surface. They have only primary tissues. “Tree” ferns have erect, thick trunks, the
bulk coming from roots clustered around the small true stem. The more primitive
species have a protostele, most have siphonosteles, and some have complex
dictyosteles.
Roots are simple, uncomplicated and arise adventitiously along the rhizomes
near the base of the fronds.
Sporangia are located, for the most part, on the undersides of ordinary leaves in
clusters called sori (singular, sorus). In early ferns, and some living ones, sori
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Document Summary
Two living genera, psilotum and tmesipteris, with only two species of the former and less than 30 of the latter, constitute the entire phylum. Both genera are weeds in the tropics and subtropics. Tmesipterisis confined to the islands of the south pacific, including australia and new zealand whereas psilotum is more widespread, reaching parts of the southern united states. It is homosporous and the spores develop into bisexual gametophytes that resemble pieces of sporophyte rhizome. While many of the morphological and anatomical features fit, data from current. Rna sequencing and other chemical analyses cause many botanists to reject the long held belief that psilotum is a living ancestor of the seed plants. Instead, psilotum appears to many botanists to be a descendent of the ferns by loss and simplification of structures. The largest group of living seedless vascular plants and probably the most familiar are the ferns with about 12,000 species, over two thirds of which are tropical.
