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Bryophytes
Bryophytes, commonly known as mosses, hornworts, and liverworts, exhibit a pronounced alternation of generations during which the conspicuous "leafy" plant is a part of the gametophyte generation and the sporophyte is frequently smaller and dependent upon it. Bryophytes are usually found in moist habitats, but they do occur in drier areas as well. These habitats vary widely, ranging from animal dung and insects (in the tropics), to bare rock. Bryophytes are frequently small and always lack xylem (specialized water-conducting tissue) and phloem (specialized food-transporting tissue). The absence of these vascular tissues sets bryophytes apart from the more advanced ferns, gymnosperms, and angiosperms.
An individual moss plant, for example, consists of an upright or prostrate, slender, leafy stalk that may have absorptive root-like structures called rhizoids. Mosses usually form large colonies. These colonies are found in a variety of habitats such as moist soil, rocks, and wood. Mosses are usually small, but some species in the Southern Hemisphere can grow a foot high.
Bryophyte Reproduction [Illustration]
The starting point for the sporophyte, as noted previously, is the zygote, which is the product of syngamy, or the fusion of gametes. In mosses, syngamy occurs in a structure called an archegonium, where haploid eggs are produced. The zygote grows from, and remains permanently attached to, the archegonium, from which it gets its nutrition. The cells of the developing sporophyte soon become photosynthetic, making it able to or at least partially synthesize its own nutrients. As the needle-like sporophyte grows, it becomes visible above the tip of the leafy gametophyte plant.
Once the sporophyte has reached its mature length, an enlarged, spore-bearing region called the capsule or sporangium forms at its tip. The capsule is situated atop a stalk, or seta. At the base of the seta is a foot that is buried in the top of the leafy gametophyte. The seta gets progressively longer as the sporophyte develops, raising the capsule high above the rest of the plant. As the capsule matures, certain cells in the sporangium separate, forming sporocytes. Each of these spore mother cells is capable of undergoing meiosis-having its chromosome number reduced from the diploid condition to the haploid condition-and also a process called cytokinesis, in which the remaining contents of the cell are divided. The result is the formation of four spores.
Spores, the products of meiosis, are widely distributed in the atmosphere, and those that come to rest in a favorable environment can germinate. The spores of many moss species require light before they can germinate. In many mosses, the spore develops into a prostrate, many-branched, filamentous structure called the protonema. The protonema is capable of photosynthesis, as its cells contain chloroplasts. After a period of growth, the protonema forms buds, which become leaf shoots. While the protonema serves as a primitive root system for its shoots, additional root-like structures called rhizoids develop from the bases of the "leaves." Archegonia and the male sex organs, called antheridia, are produced at the tips of some branches of mature "leafy" gametophyte plants. Some mosses are monoecious, having both archegonia and antheridia on the same plant (for example, Funaria), while others are dioecious, with male and female sex organs on separate, unisexual plants (for example, Polytrichum). When there is enough moisture present, mature antheridia release their sperm, some of which may swim to the archegonia. Raindrops sometimes help the process along by splashing sperm to the archegonia, especially in monoecious species. Once in the vicinity of the mature archegonium, the sperm swim down the neck canal to the egg. The resulting fusion of the gametes produces a zygote.