BRYOPHYTES, CLASSIFICATION AND LIFE CYCLE
Bryophytes are unique among land plants in that they possess an alternation of generations, which involves a dominant, free-living, haploid gametophyte alternating with a reduced, generally dependent, diploid sporophyte. Bryophytes are small, herbaceous plants that grow closely packed together in mats or cushions on rocks, soil, or as epiphytes on the trunks and leaves of forest trees. Bryophytes are remarkably diverse for their small size and are well-adapted to moist habitats and flourish particularly well in moist, humid forests like the fog forests of the Pacific northwest or the montane rain forests of the southern hemisphere.INTRODUCTION
The term Bryophyta is used by botanists to describe the mosses and liverworts and is derived from a Greek word meaning moss. Bryophytes are small, herbaceous plants that grow closely packed together in mats or cushions on rocks, soil, or as epiphytes on the trunks and leaves of forest trees. Bryophytes are distinguished from tracheophytes by two important characters. First, in all bryophytes the ecologically persistent, photosynthetic phase of the life cycle is the haploid, gametophyte generation rather than the diploid sporophyte; bryophyte sporophytes are very short-lived, are attached to and nutritionally dependent on their gametophytes and consist of only an unbranched stalk, or seta, and a single, terminal sporangium. Second, bryophytes never form xylem tissue, the special lignin- containing, water-conducting tissue that is found in the sporophytes of all vascular plant. Bryophytes are classified into 3 groups:
Mosses (Bryopsida) (1)
Mosses and Liverworts have been called up-side-down or role-reversal plants. The green and often leafy part underneath that we would think of as the moss or liverwort itself is equivalent to tiny parts within a flower, or to a small, rarely seen part of the fern. The part that is equivalent to all of the flowering plant or fern that normally see is the fruit of the moss or liverwort. One of the most attractive and flower-like structures of mosses is found on the fruit. The fruit consists of a fruit stalk whose foot remains embedded in the moss plant, and a spore-producing capsule. When the spores are ripe, the lid falls off, but the release of spores is usually controlled by a fringe of flexible teeth round the capsule mouth. (2)
BRYOPSIDA (Mosses): The mosses are plants that possess both stems and leaves. The leaves, which frequently have a nerve or midrib, are usually arranged spirally round the stems of the plant Mosses do not have roots like those of higher plants but instead have root-like threads or rhizoids that attach the plant to the substratum.There are about 14,000 species of moss worldwide.(3). It may be small, but they may also be as complex as flowering plants. They have stems with leaves, and there is just about as much variation in the form and size of these plants as there is in the flowering plants. The 20,000 species range from being microscopic to over a metre; they may be upright, or creeping and much branched. They may grow in streams or deserts, on mountain tops or in sea spray, from the antarctic through tropical rain forests to the arctic, and in fact just about anywhere except in the sea itself (2). Mosses can be distinguished from the other two groups because of the multi-cellular rhizoids and by differences in the leaves and stem structures of the plants. Mosses lack a cuticle (waxy covering of the plant) they usually require moisture because they can lose water through the surface of the plant. They typically live in wooded areas or along streams but are found in places all over the world. Mosses can’t survive in dry, hot environments that are windy or cause the moss to be in direct sunlight (4)
HEPATICOPSIDA (Liverworts): The gametophytes are dorsiventrally differentiated. They may be thalloid (thallose) or differentiated into leaves and stem (foliose). In foliose types the leaves are arranged in two or three rows on the axis and are always without mid-rib.The sex organs develop from superficial cells on the dorsal side of the thallus, except when they are terminal in position.The sporophyte may be simple, or differentiated into foot and capsule, or into a foot, seta and capsule. The sporogenous cells develop from the endothecium of sporogonium. The sporophyte is completely dependent on gametophytes for its nutritive supply. The wall of sporogonium is one to several layered thick. The stomata are not present on the wall of sporogonium. The dehiscence of sporogonium is irregular.The class Hepaticopsida is further divided into several orders.
(a) Sphaerocarpales; (b) Marchantiales; (c) Metzgeriales; (d) Jungermanniales; (e) Calobryales (f) Takakiales. (5)
ANTHOCEROTROPSIDA (Hornworts): Gametophytic plant body is simple, thalloid; thallus dorsiventra without air cambers shows no internal differentiation of tissues.. Scales are absent in the thallus. Each cell of the thallus possesses a single large chloroplast with a pyrenoid. Sporophyte is cylindrical only partly dependent upon gametophyte for its nourishment. It is differentiated into bulbous foot and cylindrical capsule. Seta is meristematic. Endothecium forms the sterile central column (i.e., columella) in the capsule (i.e. columella is present). 6. It has only one order-Anthocerotales. (6)
In bryophytes (liverworts, hornworts, and mosses), the gametophyte stage is dominant. The leafy green structures are haploid, and perform most of the photosynthesis. These gametophytes produce organs for sexual reproduction: male antheridia and female archegonia. In dioecious (“two house”) species, individual plants are either male or female, producing just one type of sex organ. In monoecious (“one house”) species, both antheridia and archegonia form on the same plant. Occasionally, the environment determines whether a species is monoecious or dioecious. Each archegonium produces a single egg cell by mitosis. Within each antheridium, mitosis produces many biflagellate (two flagella) sperm. These microscopic structures and the Lilliputian “umbrellas” that raise them above the leafy surface in the liverwort, Marchantia. Sperm must swim through a layer of moisture from antheridia to archegonia to fertilize the eggs. The resulting zygote divides by mitosis – still within the archegonium – to produce a diploid sporophyte.(7)
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