Bryophyte And Pteropyte Life Cycles :: essays research papers

Although Bryophytes and Pterophytes are both plant divisions with a common ancestor, they have little in common. While Bryophytes (mosses) are generally nonvascular and very short in height, Pterophytes (ferns) are vascular plants that usually grow much taller than mosses. While the dominant generation in mosses is the gametophyte, ferns exhibit the sporophyte generation. The details of their alternations of generation vary as well, although both have diploid and haploid stages. Alteration of generations is defined as a life cycle in which there is both a multicellular diploid form, the sporophyte, and a multicellular haploid form, the gametophyte. Meiosis in mosses produces haploid spores. This process occurs in a sporangium, a capsule in fungi and plants in which meiosis takes place and haploid spores develop. After fertilization, the sporophyte zygote grows out of the parent gametophyte. At the end of this stalk is the sporangium. Meiosis occurs and the haploid spores disperse. In ferns, the mature sporophyte (2N) has small spots on the undersides of its leaves. These are clusters of the Pterophyte sporangia, called sori. As in mosses, meiosis occurs in the sporangia, which then release spores, continuing the cycle. Mitosis in Bryophytes begins germination, or growth. It directly follows meiosis. As mitotic division continues, protonemata (1N) are formed. They grow until they are mature gametophytes. In Pterophytes, mitosis occurs directly after fertilization. The zygote divides and grows until it peeks out of the parent gametophyte, then it matures until it can release its own spores. Mosses exhibit the embryophyte condition, which means their gametes develop with protective sheaths of cells. This was an adaptation that allowed plants to become hardier. Mosses have two sexes of gametangia; the male gametangium is called an antheridium, and the female is called an archegonium. Eggs are produced in the archegonium and sperm in the antheridium. In order for fertilization to take place, the sperm must swim through moisture to the archegonium. This is why mosses must live in damp environments to survive- they cannot reproduce without moisture. The zygote develops within the archegonial cavity. Ferns are usually homosporous as well (there is a single type of spore; the gametophyte is bisexual). In ferns, the archegonia and antheridia mature at different times, which means there is cross fertilization between gametophytes and thus more genetic variation. Sperm from the antheridia swim to the archegonia where the zygote develops. Like mosses, ferns also need a moist envir onment for fertilization to occur.