As for the ferns, the sporophyte is dominant while the gametophyte is short-lived. If it is generally true that ferns have big leaves, fern allies have small leaves or none at all. You might want to remind yourself of some ferns and fern allies by taking a virtual tour of the pteridophyte greenhouse at the University of Georgia.
We are going to be quite selective in which groups we study and in what depth.
Club mosses or lycopods
There are only two surviving genera today of which Lycopodium is
the more important.
Lycopodium cernuum - a Caribbean species
|Lycopodium spp. are essentially creeping plants with tiny leaves,
termed microphylls, spirally arranged around the stem. Note how the leaves at the shoot
tips look different. These are spore-bearing leaves called sporophylls.
These sporophylls are tightly grouped into
cones or strobili.
(Some ferns do have spore-bearing and sterile leaves but grouping the sporophylls into a cone is quite different and distinct.)
|At left is an L.S. of a Lycopodium cone.
Note the sporangium in the axil of each sporophyll.
Each sporangium is full of numerous tiny spores, similar in size to those of ferns.
Incidentally, these spores were once used in fireworks (their high oil content makes them burn readily) and more recently to "dust" condoms.
|The spores of Lycopodium germinate to produce a tiny,
short-lived, peg-shaped gametophyte. This usually forms an association with a fungus and
some species of Lycopodium have lost their photosynthetic capacity and rely on the
fungus to provide carbohydrate! The gametophyte bears antheridia and archegonia and motile
sperm cells swim to the eggs to fertilize these. The young sporophyte, as for ferns,
develops initially as a parasite on the gametophyte.
A = antheridia
Check out the Lycopodium images at the University
Lepidodendron was a giant tree-like club moss. It towered to 45 m in height and flourished in the swamp forests of the Carboniferous.
Click here to learn more about fossil
There is only one surviving genus, Selaginella
Selaginella serpens - a Caribbean native
|There are about 700 Selaginella species world wide and several in the Caribbean. The sporophyte has a creeping habit and is dorsiventrally flattened. As in Lycopodium, there are separate sterile and fertile leaves and the spore-bearing leaves are grouped to form cones or strobili at the shoot tips.|
|In Selaginella the roots are typically borne on a rhizophore which extends from the stem.|
|Biodisc photomicrograph||In the LS of the cone of Selaginella (left), you should be able to see that
there are two kinds of spores: -
microspores (small, many)
This is termed heterospory and evolved several times in several groups of land plants.
(Homospory is where there is only one kind of spore. Except for a few water ferns which are heterosporous the ferns are homosporous.)
The consequence of heterospory is separate male and female gametophytes;-
| Each megasporophyll bears a megasporangium which by meoisis produces 4
megaspores which in turn form female gametophytes and female gametes.
|Each microsporophyll bears a microsporangium which by meoisis produces many
microspores which in turn form male gametophytes and male gametes.
In Selaginella, gametophytes develop within the actual
spores, a phenomenon termed
Microspore split exposing sperm
| Each haploid microspore divides internally by mitosis to form ultimately an
antheridium with 128 or 256 biflagellate sperm cells. These are released from the
microspore wall and swim in surface water to megaspores .
Cross section of megaspore showing archegonia at exposed top surface
|Each haploid megaspore divides internally by free
nuclear division to form a female gametophyte with archegonia opening to the spore
surface. Sperm cells swim down the necks of these to fertilize eggs and produce the new
diploid sporophyte generation.
Three features distinguish Selaginella from Lycopodium;-
|Heterospory: Selaginella produces microspores and
megaspores; Lycopodium is homosporous, producing only microspores.
|Endospory: Selaginella microspores and megaspores develop
their gametophytes within the actual walls of the spores; in Lycopodium the
gametophyte grows out of the spore.
|Ligule: Each leaf in Selaginella (fertile and sterile) bears in
its axil a tiny tongue-like structure called a ligule; these are not present in Lycopodium.
You can check out more Selaginella images at the University of Wisconsin
There are 3 other fern allies you should be aware of, although no
detailed knowledge of
these is required in this course.
|The horsetails are recognizable by their erect, jointed stems ringed by
microphylls and topped by cones. Only a single genus Equisetum survives today with
a few temperate species but these plants were well represented in the swamp forests of the
Equisetum - entire sporophyte (left) and strobilus (right)
|Calamites is an extinct horsetail that flourished in the Carbononiferous.
It superficially resembles a Christmas tree.
|The Casuarina tree (Casuarina equisetifolia) is a flowering plant commonly
cultivated in the Caribbean. It gets it specific name from the resemblance of its
branches to Equisetum. (equisetifolia = leaves like Equisetum)
can check out more Equisetum images at the University
Psilotum nudum in a gully in Barbados.
|We met Psilotum when we looked at Rhynia, to
which it bears a superficial resemblance.
Rather than being living relatives of Rhynia, the whisk ferns are viewed by many experts as being highly evolved relatives of the true ferns.
Check out further Psilotum images at the University
|Only two genera are known, Isoetes and Stylites. They resemble clumps of
Stylites was only discovered in 1957 in the Andes. It is an a swamp-dwelling plant with no stomata, taking in CO2 through its roots!
Isoetes is more widespread.
Find out more about Isoetes at the University
of Wisconsin or the University of
Palermo's Isoetes page.
like to look at some sample questions on this part of the course?
If so, click the button.
now completed our look at the Fern Allies.
Click the button to move on to the Evolution of Seed Plants.