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BRYOPHYTA. VASCULAR PLANTS. PSYLOPHYTA. LYCOPHYTA

The bryophytes represent about 25,000 species of mosses, liverworts, and hornworts. On the basis of any one characteristic it is difficult to distinguish them from the thallophytes, or from what are sometimes referred to as the true algae. In contrast to algae, which tend to be composed of either single cells, or filaments (sheets of cells), which can intertwine to create more complex body structures, bryophytes are rarely filamentous, except during one stage in the life history of the mosses. Instead, bryophytes are composed of cells that form tissues called parenchyma; they are characterized by loosely fitting cells that have thin walls of cellulose. In between the cells are intercellular spaces incorporated within the cellular network.

In bryophytes, the principal photosynthetic pigments are chlorophyll a and b. These are biochemically similar to the pigments of the chlorophytes, from which bryophytes probably arose. Their energy reserves are stored in the form of starch, and their cell walls contain cellulose.

Bryophytes are usually terrestrial. But they remain somewhat dependent on their ancestral aquatic environment. This has kept their distribution limited to moist environments or to environments that are moist during a critical period each year. These small plants need water for their flagellated sperm cells to swim from the antheridia to the egg cells in the archegonia. Without any vascular tissue, their ability to move fluids internally across long distances is limited. And since xylem, the vascular tissue in higher plants, is necessary for support, the upper size limit of these plants without such supportive tissue is kept at a minimum.

All the bryophytes have an alternation of generations, with a sporophyte generation (diploid) and a gametophyte generation (haploid). Among the larger, more complex algae, most of which have an alternation of generations, there is some tendency toward a reduction of the gametophyte, multicellular stage, and an emphasis on the sporophyte, multicellular stage. In both the brown and red algae, the sporophyte generation is prominent, as is the case in the vascular plants. This tendency is not apparent in the bryophytes, in which the haploid gametophyte stage is clearly dominant. It is the leafy green gametophytic stage of the bryophytes that produces the gametes. These swim through moisture, present as a film of either rain or dew, from the antheridia to the archegonia. Here the sperm fertilize the eggs, producing zygotes that make diploid sporophytes by mitotic division. The sporophyte plant is attached to the gametophyte and grows directly from it. The sporophyte produces the sporangia, organs that contain and release the asexual spores. Spores are asexual because unlike gametes, they never meet in a sexual union.

The moss gametophyte generation possesses what appear to be, but aren’t, true roots, leaves, and stems. Bryophytes have no vascular tissue, a critical component in such structures. Rootlike organs in plants without vascular tissues are called rhizoids; they function like roots, by anchoring the plant and absorbing water and nutrients. The stem is “stem-like” and sometimes is referred to as the stalk. The “leafy” parts are sometimes referred to as“ leaves ” because of a lack of better terminology.

Some of the earliest known vascular plants had roots that functioned as hold-fasts and absorbed water. They also had vascular tissue for water and nutrient movement. This tissue also provided strength and helped hold the plant up in the air. In addition, these early vascular plants had a waxy cuticular layer covering the leaves for water retention. The fossil record indicates there was a trend toward the reduction of the gametophyte generation in favor of the more dominant sporophyte generation, which contained the sporangia. The earliest vascular plants probably produced only one kind of spore from one kind of sporangium, a process called homospory. After germination, these spores developed into gametophytes with antheridia and archegonia. They produced the sperm and eggs, respectively. However, the trend toward heterospory (the production of two different kinds of spores) is also evident in the overall evolution of the vascular plants.

Following the evolution of the aquatic vertebrates, which took place about 500 million years ago, the first vascular plants to colonize the land appear in the fossil record; at about the same time, some arthropods also began colonizing terrestrial habitats.

A significant innovation unique to the vascular plants is the seed, which consists of an embryo and some stored food enclosed within a protective coat. The earliest known fossilized seeds date back 350 million years.

Five major groups of tracheophytes, or vascular plants, are discussed below. To date, more than 260,000 species of vascular plants have been described.

The most primitive of the vascular plants are the psilophytes. These resemble some of the branching filamentous green algae (Chlorophyta) from which they probably arose. The psilophytes have true stems, with simple vascular tissue, branching from slender rhizomes, which are elongated, underground, horizontal stems; they are not true roots. The rhizomes have unicellular rhizoids, thin, rootlike structures that are similar to root hairs. No true leaves are present, although the aerial stems are green and perform photosynthesis.

Sporangia develop at the ends of the stems and produce haploid spores. These fall to the ground and give rise to the subterranean gametophytes. The gametophytes bear both archegonia and antheridia. Each gametophyte produces both eggs and sperm. The sperm travel to the eggs, where fertilization occurs. Then the diploid zygote begins developing into a sporophyte.

The psilophytes evolved during the Silurian period and thrived more than 300 million years ago. However, with the exception of three surviving species, the entire group is extinct. Many botanists believe some psilopods evolved into the ferns.

Lycophyta, or the club mosses, also appeared during the Silurian, about 400 million years ago. They were among the largest and most dominant plants during the Devonian and Carboniferous periods. Toward the end of the Permian, they were superseded by more advanced vascular plants. About 1000 species are still found throughout much of the world, although all are quite smallandamount to little more than ground cover.

Club mosses have true leaves, stems, and roots. They may have evolved from algae that penetrated the ground, occasionally sending branches above the surface. Some of their leaves are specialized. Called sporophylls, these leaves have sporangia which bear spores. Many species have club-shaped structures called strobili at the ends of their stems. The strobili, formed from clusters of sporophylls, are the source of the group’s common name. It should be pointed out, however, that the club mosses are not related to the true mosses, or bryophytes.

Some club mosses are heterosporous, having two types of sporangia. The larger spores, known as megaspores, develop into the archegonia-bearing female gametophytes. The other sporangia produce smaller spores, the microspores, which develop into antheridia-bearing male gametophytes.

Дата добавления: 2015-11-16