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| Lycophytes |
The lycophytes, which compose the phylum Lycophyta, are one of four phyla of seedless plants having vascular, or conducting, tissue. The living lycophytes are all small and herbaceous, whereas the extinct lycophytes included large trees, which were important in the formation of coal.
There are at least twelve genera and twelve hundred species of living lycophytes. These include plants known as club mosses and spike mosses (though none are true mosses) and quillworts. The lycophytes consist of three families, each belonging to a separate order. The family Selaginellaceae has a single genus, Selaginella.
Similarly, the family Isoetaceae has a single genus, Isoetes. The remaining genera belong to the family Lycopodiaceae. The living lycophytes are widely distributed but reach their greatest species diversity in the tropics. The lycophytes are similar to the higher vascular plants—the gymnosperms and angiosperms—in having vascular tissue and true leaves, stems, and roots.
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Reproduction
The alternation of generations in lycophytes resembles, in an important way, this life cycle in the higher vascular plants: The sporophyte (the spore bearing generation), rather than the gametophyte (the gamete-bearing generation), is the larger, more obvious generation.
In contrast, in the bryophytes (mosses and their relatives, in the phylum Bryophyta), which are an earlier, nonvascular evolutionary line, the gametophyte is the larger, dominant generation.
Unlike the higher plants, however, the lycophytes do not produce seeds. Like all seedless plants, the lycophytes require water for the sperm to swim to the egg. In addition, the stem tip in most lycophytes forks repeatedly, resulting in branches that are of about equal length, whereas in higher plants, there is a single main axis, from which lateral branches arise.
Evolutionary Origins
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| Selaginella erythropus frond |
Some of the features that lycophytes and the higher vascular plants have in common differ in their evolutionary origins. Such characteristics are considered convergent, or analogous, having arisen to meet similar environmental demands. The different origins are attributable to the early divergence of the lycophytes from the main lineage of vascular plants.
The lycophytes first appear in the fossil record in the early Devonian period of the Paleozoic era, about 400 million years ago; they probably arose from early members of the Zosterophyllophyta, a phylum of seedless vascular plants that became extinct during the Devonian.
Leaves are one of the features that developed independently in lycophytes and higher plants. The leaves of lycophytes are called microphylls and are characteristic of all members of the group, extinct and living. In two of the three living lycophyte families, the microphylls are short.
In all lycophytes, the microphylls are narrow and have a single, unbranched vein. In contrast, the leaves of the higher vascular plants typically have a complex system of branching veins and are called megaphylls.
Carboniferous Period
Some extinct lycophytes had, in addition to microphyllous leaves, other features analogous to, but derived independently of, those of higher vascular plants. These included tree forms and structures analogous to seeds. These do not occur in living lycophytes. Especially note worthy among the extinct lycophytes were the scale trees.
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For most of the Age of Coal, in the Paleozoic’s late Carboniferous period (322 million to 290 million years ago), these trees were among the dominant plants of swamp forests in what is now North America and Europe. They grew 10 to 35 meters tall and included the genera Lepidodendron and Sigillaria. The remains of these large trees contributed greatly to formation of the world’s coal beds.
The trunks of most tree lycophytes branched only near the top. Stabilizing them at their bases were shallow, forking, rootlike structures, which produced spirally arranged rootlets. The stems were supported mostly by a massive bark surrounding a relatively small amount of secondary xylem, or wood.
The reproductive structures were borne in cones. Like the living genera Selaginella and Isoetes, lycophyte trees were heterosporous, forming spores of two different sizes rather than a single size. Some species produced structures analogous to seeds.
The tree lycophytes vanished in the late Carboniferous period, about 296 million years ago, as the climate changed and swamps began to dry. The nearest living relative of the tree lycophytes is the herbaceous plant Isoetes in the family Isoetaceae.
Herbaceous members of the two other living lycophyte families, Lycopodiaceae and Selaginellaceae, also existed in the Carboniferous, and representatives of some of them survive today.
Lycopodiaceae
The family Lycopodiaceae includes 10 to 15 genera, with a total of 350 to 400 species, most of them tropical. The family is also represented in the temperate zone and even in the Arctic but not in arid regions.
In the United States and Canada, there are seven genera, with twenty-seven species which occur in the East and Northwest but not in the dry Southwest. The most familiar of these are probably the club mosses, which belong to the family’s largest genus, Lycopodium.
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| club mosses |
Lycopodiaceae consists mainly of trailing plants. A horizontal, branching, underground stem, or rhizome, produces roots and upright, aerial branches. The small, microphyllous leaves are generally spirally arranged.
In some genera, the sporophylls—the leaves that bear the spores—look like ordinary microphylls and are interspersed with them along the aerial stems. In other genera, the sporophylls do not resemble ordinary microphylls and are grouped in cones at the ends of the aerial stems.
The spores in Lycopodiaceae all the same size. Each spore gives rise to a free-living gametophyte that bears both male and female reproductive structures.
In some species, the gametophyte grows above ground and produces its own food photosynthetically; in others, it grows below ground, is not photosynthetic, and is associated with a fungus. Young sporophytes may remain attached to the gametophyte for some time before becoming independent.
Selaginellaceae
Selaginella, or spikemoss, is the sole genus in the family Selaginellaceae. With more than seven hundred species, the Selaginellaceae is the largest of the living lycophyte families. As with the Lycopodiaceae, most species of Selaginella are tropical, but some occur in temperate zones. In the United States and Canada, there are thirty-eight species.
Many species of Selaginella grow in damp places, but a few occur in deserts, where they become dormant during the driest season. The resurrection plant, Selaginella lepidophylla, which grows in Texas, New Mexico, and Mexico, can recover from several months of complete drying.
Selaginella has a branched, prostrate stem,which produces roots and upright branches that grow a few inches tall. In some species, the horizontal and upright stems are sheathed with small leaves in four longitudinal rows. Both microphylls and sporophylls bear a small, scale like outgrowth, called a ligule, at the bases of their upper surfaces.
Selaginella forms cones at the ends of the branches. The spores are of two sizes: large ones, called megaspores, and smaller ones, called microspores. Megaspores germinate to form megagametophytes, which bear egg-producing archegonia. Microspores germinate to form microgametophytes, which bear sperm-producing antheridia.
In contrast to the fully free-living gametophytes of the Lycopodiaceae, the gametophytes of Selaginella develop mostly within the spore walls. When a microgametophyte is mature, the spore wall ruptures, releasing the sperm.
The megaspore wall also ruptures, allowing the megagametophyte to protrude from the wall; archegonia develop in the exposed part. Neither the microgametophyte nor the megagametophyte has chlorophyll; thus, the gametophytes, and the developing embryos in the archegonia, must obtain their nutrition from food stored within the spore.
Isoetaceae
Known as the quillwort family, the Isoetaceae is the smallest of the lycophyte families. Its single genus, Isoetes, includes about 150 species. Unlike the Lycopodiaceae and Selaginellaceae, which are predominantly tropical, Isoetes occurs mostly in cooler climates. In the United States and Canada, there are twenty-four species. Most Isoetes species grow partially or totally submerged in fresh water.
Unlike the Lycopodiaceae and Selaginellaceae, Isoetes is not a trailing plant. Instead, it has a short, thick, fleshy, bulblike underground stem, or corm. Roots grow on the lower surface of the corm, and microphyllous leaves grow on the upper surface. The corm undergoes secondary growth, which thickens it.
The microphylls are not small, as they are in the Lycopodiaceae and Selaginellaceae, but instead are elongated, slender, and stiff, reaching 15 to 50 centimeters in height. They resemble quills, giving Isoetes the look of a young onion or a tuft of grass or rush. They are spoon-shaped at the base, where they surround the corm. As in Selaginella, the leaves have ligules.
Isoetes species do not produce cones. Eachmicrophyll is a potential sporophyll. As in Selaginella, there are two kinds of spores: microspores and megaspores. The sporophylls that bear microspores are located nearer the center of the plant than are the sporophylls that bear megaspores. Gametophyte development takes place largely within the sporewalls, as in Selaginella.
Some species of Isoetes from highlands in the tropics obtain their carbon for photosynthesis from the soil rather than in the usual way, from the atmosphere. These plants have an unusual kind of photosynthesis, called CAM (crassulacean acid metabolism) photosynthesis.
