Club mosses and horsetails

Club mosses and horsetails, together with quillworts, ferns, and whisk-ferns, are known as pteridophytes (fern plants). These primitive plants were much more abundant at least 300 million years ago in the Pennsylvanian period than they are today. Their ancestors reached an enormous height—more than 100 feet (30 meters)—and were dominant over large areas of land. Superficially, many of them resemble mosses, and they are sporophytic— that is, they reproduce from spores instead of seeds—but they differ from mosses in that they have a vascular system.

Stag’s horn club moss (Lycopodium clavatum) is named after its long white stalks, which usually have two fertile cones (strobili). Its trailing stems, which are often 10 feet (3 meters) long, give rise to fertile and sterile branches. The stalks grow from the fertile branches. Their spore leaves (sporophylls) are smaller than those of the sterile and fertile branches and grow more closely against the stem. This club moss is homosporous—its spores are all identical.

Club mosses and quillworts

The members of the class Lycopsida are grouped into three living orders: Lycopodiales (with about 200 species) and Selaginellales (more than 700 species) both contain the club mosses, and Isoetales (about 70 species) contains the quillworts. Most lycopsids are found in the tropics and subtropics but some occur in temperate, desert, arctic, and alpine regions. They usually grow on the ground, although a few species live on other plants com-mensally (when they are known as epiphytes). The quillworts are found worldwide, and most live largely below the ground level, with only the tips of their sporophylls (fertile leaves) showing.

Most club mosses creep or trail although some have erect stems. The roots generally grow directly from the stem, and some species produce rhizophores, which grow down from the stem to the soil with true roots issuing from their tips. The leaves, which photo-synthesize, are small with an unbranched midrib—those of Lycopodium are needlelike.

In most species the leaves are spirally arranged, but in some species of Selaginella they are attached in four rows—two on the upper side and two laterally. The leaves may all be the same size or there may be a regular pattern of large and small leaves. In Selaginella, a membranous scale (ligule) grows at the base of each leaf, the function of which is not known. Ligules are also found in quillworts during the development of the sporophylls.

In addition to the ordinary green leaves, club mosses have fertile leaves (sporophylls).

In contrast to the club mosses all the leaves of the quillworts are sporophylls. These leaves, which project above the soil level, are the only photosynthetic part of the plant; the rest survives as a corm (a condensed stem) underground.

Alternation of generations in some club mosses involves the underground germination of a spore. The resulting prothallus (the ga-metophyte) has female sex organs (archegonia) and male ones (antheridia) on its upper surface. When the sex cells (gametes) mature, the necks of the archegonia open and the antheridia release the sperm cells whose walls rupture. The sperm “swim” to the archegonia where they fertilize the egg. The developing embryo begins the sporophyte phase.

In most species of club mosses the sporophylls are grouped at the top of the stem to form a cone (strobilus). Each sporophyll has a single large spore receptacle, or sporangium, on its upper surface near the base of the leaf. The sporangium may be globular or kidneyshaped. Lycopodium bears sporangia that are identical and produce one type of spore only.

It is, therefore, described as homosporous. Selaginella and the quillworts, however, are heterosporous, having both small microsporangia and larger megasporangia.

In club mosses the spores are released when they are ripe—the sporangium splits open and the spores are dispersed by the wind. In quillworts the spores have to wait until the sporangia decay before they can be released. In suitable conditions, each spore germinates in the ground to form a tiny pro-thallus. In some species, the prothallus lives underground in a symbiotic association with mycorrhizal fungi from which it gets its nutrients. In other species the prothalli are surfacedwelling and photosynthetic. In Selaginella and the quillworts, the prothalli are endo-sporic—that is, they develop within the spores.

The life cycle of club mosses consists of two alternating phases—the gametophyte generation and the sporophyte generation. The gametophyte phase begins with spores that germinate into prothalli and ends with the fertilization of the female gametes by the male sperm.

The sporophyte generation begins with the fertilized egg (zygote), which develops into the spore-producing plant. In Lycopodium the male and female gametes are produced in the same prothalli. Selaginella and quillworts, however, have minute microprothalli, which have a single male organ (antheridium) and much larger megaprothalli, which are produced by the megaspores and which have several female organs (archegonia). Sperm and eggs are thus produced on separate prothalli. To achieve fertilization the sperm must swim through a film of water from the microprothal-lus to the archegonia on the megaprothallus.

Reproduction among the club mosses is, however, not always sexual—in some species of Lycopodium, leafy stem structures called bulbils detach themselves from the plant and develop into new plants.

A cross section through the stems of club mosses and horsetails reveals their primitive vascular systems. Lycopodium has a central cylinder which contains lobes of xylem interspersed with phloem. Equisetum has a hollow stem ringed by small xylem and phloem bundles.

Horsetails

The horsetails (class Sphenopsida) are today represented by a single genus—Equisetum— with about 25 species. They are found worldwide, except in Australia and New Zealand, and in a varied range of habitats. These plants have long underground rhizomes that give rise to aerial stems that are usually 4 to 24 inches (10 to 61 centimeters) in height. The stems normally contain chlorophyll, but some species alternate the growth of special fertile stems without chlorophyll with green sterile ones.

The stems are grooved and have thick silici-fied walls. They are simple (unbranched) and carry whorls of slender branches, which are simple or have further whorls of branches. The small leaves, too, are produced in whorls, each cluster being fused into a tubular sheath, except for the tips, which form a serrated edge around the margin of the sheath. Most of the leaves have no chlorophyll, and photosynthesis is carried out by the green stems.

The reproductive organs of horsetails are grouped together to form a terminal cone, or strobilus. Each strobilus consists of a central axis bearing whorls of mushroom-shaped (peltate) sporangiophores, each with several sporangia on the lower surface. The sporangia split to release the spores. Each spore has four long strips called elaters, which coil and uncoil in response to humidity changes and assist its dispersal in water and wind. Some of the spores germinate to form male prothalli, which bear antheridia, whereas others produce female prothalli with archegonia. In others still, antheridia and archegonia may be borne successively on the same prothallus. Fertilization is achieved in the same way as in the club mosses; the horsetails also develop by an alternation of generations.

The great horsetail (Equi-setum telmateia) produces pale photosynthetic fertile stems with strobili at their tips in early spring. Later in the year sterile stems with green leaves develop, which are seen here. This plant grows in damp woods and on hedge banks.