Fern Allies

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.)

Biodisc photomicrograph

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
B = archegonia
C = antherozoids

Check out the Lycopodium images at the University of Wisconsin

Extinct lycopods

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 lycopods.

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Spike mosses

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)
megaspores (large, few)

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
endospory;-

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

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Other fern allies

There are 3 other fern allies you should be aware of, although no detailed knowledge of
these is required in this course.

Horsetails

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 Carboniferous.

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)

You can check out more Equisetum images at the University of Wisconsin

Equisetum sp.

Whisk ferns

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 of Wisconsin

Quillworts

Only two genera are known, Isoetes and Stylites. They resemble clumps of grass-like leaves.

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.

Isoetes

Find out more about Isoetes at the University of Wisconsin or the University of Palermo's Isoetes page.

Would you like to look at the lab for this part of the course?
If so, click the button.

Would you like to look at some sample questions on this part of the course?
If so, click the button.

We have now completed our look at the Fern Allies.
Click the button to move on to the Evolution of Seed Plants.

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C.M. Sean Carrington, 13 April 1998