BSC 314 Lecture Notes - Lecture 54: Archegonium, Corm, Gametophyte
27 Jun 2018
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Phylum Lycophyta: Club Mosses and More
The lycophytes are the oldest of the seedless vascular plants that have living
representatives. They constitute one of the two major lines (clades) of vascular plants,
which split probably in the Silurian Age, but at least by the Devonian. For the last 400
million years, therefore, they have developed independently from the rest of the
vascular plants. During this time, they evolved from small, semiaquatic herbaceous
plants to huge trees that dominated the Coal Age forests for 40 million years and then,
as continental masses shifted and the climate dried, they declined in importance until
most became extinct by late Carboniferous early Permian time. Their structural features‐
show convergence with taxa on the line leading to the flowering plants. Leaves, wood,
trees, and reproductive structures that resemble seeds evolved in both lineages.
There are about 1,200 species today in three lycophyte families:
Lycopodiaceae, Selaginellaceae, and Isoetaceae. Both of the latter two
families have only one genus each— Selaginella with about 700 species
and Isoetes with about 100. None of the lycophytes is over a meter or so tall,
even in the tropics where they flourish and are the most abundant. Many are
epiphytes growing high in the tree crowns. The temperate zone plants are
small, trailing, evergreen plants that once were collected in quantity to place
as crudely woven evergreen “blankets” on graves in cemeteries.
SomeSelaginella species are known as “resurrection plants” because they
grow in arid sites and shut down metabolically during dry periods, rolling
their aerial stems into tight balls and appearing lifeless. When moisture is
available, they uncurl and flash green leaves into the sun, making and
storing sufficient photosynthates to weather the next dry period. A number
of lycopods are present in the arctic flora, and many form a groundcover on
the forest floor in the northern and montane conifer forests.
The distinguishing features of the lycophytes are the arrangement of their
vascular tissues and their leaves—microphylls with only a single vascular
strand. The sporangia on the modern plants are kidney-shaped, like those of
the ancestral forms, and borne on sporophylls clustered in strobili. A
distinguishing ligule (scale-like outgrowth) is present in the Selaginella-
Isoetes group.
The life cycles of members of the three groups vary. The lycopods are
homosporous and the spores give rise to bisexual gametophytes, which in
some species develop underground and live with the assistance of a
mycorrhizal fungus; others develop on the ground surface. The
gametophytic phase of the cycle may last several years (15 in some) before
gametes are released and zygotes produced. The embryo develops slowly
into the sporophyte, and the latter may remain attached and drawing
sustenance from the gametophyte for a long period.
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Document Summary
The lycophytes are the oldest of the seedless vascular plants that have living representatives. They constitute one of the two major lines (clades) of vascular plants, which split probably in the silurian age, but at least by the devonian. For the last 400 million years, therefore, they have developed independently from the rest of the vascular plants. Their structural features show convergence with taxa on the line leading to the flowering plants. Leaves, wood, trees, and reproductive structures that resemble seeds evolved in both lineages. There are about 1,200 species today in three lycophyte families: Both of the latter two families have only one genus each selaginella with about 700 species and isoetes with about 100. None of the lycophytes is over a meter or so tall, even in the tropics where they flourish and are the most abundant. Many are epiphytes growing high in the tree crowns.
