Bryophyte And Pteropyte Life Cycles – A Compare/Contrast Essay, Research Paper

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 environment for fertilization to occur.

The chromosome number varies throughout the cycle of alternation of generations. Humans are dominantly diploid (2N). Our 2N number is 46; therefore our 1N is 23. When 1N and/or 2N are mentioned, it refers to a single or double set of chromosomes. A diploid organism has two sets of chromosomes, one from each parent, while a haploid only has one set, cut down from two during meiosis. The gametophyte generation of plants is haploid, and the sporophyte is diploid. In ferns, the gametophyte generation releases spores (1N) which combine to form a zygote (2N). The zygote grows to an embryo and then into a young sporophyte. Therefore, the stalks growing out of the parent are 2N, while the parent gametophyte is 1N. Meiosis in the sporangium cuts the spores? chromosome number in half, making them 1N, so that they can then grow into gametophytes themselves. In ferns, a spore (1N) grows into a young gametophyte. Fertilization yields a 2N zygote. The sporophyte (2N) grows out of the gametophyte (1N). The mature sporophyte eventually makes spores of its own.

The gametes of both Bryophytes and Pterophytes are 1N. Eggs are produced in the archegonium and sperm in the antheridium.

Egg and sperm unite when sperm swims from the antheridium to the archegonium. The result is a zygote, a diploid (2N) organism that grows into a sporophyte.

Spores are haploid (1N). In both mosses and ferns, the spore germinates into a gametophyte. If a fern is heterosporous, the spore can germinate into a male or a female gametophyte. However, gametophytes are usually homosporous.

The dominant generation of mosses is the gametophyte, which is haploid. The sporophyte which grows from it is dependant on it and is much smaller. The dominant generation of ferns is the sporophyte, which is diploid. In most plants, the dominant generation is the sporophyte.

Both examples of alternation of generations include meiosis and mitosis. Both ferns and mosses have antheridia and archegonia, and zygote growth takes place in the archegonium. Both need water to reproduce, and both have sporangia that disperse spores and house meiosis. Both are seedless plants. They also evolved from the same ancestor- charophytes.

The major difference between these two cycles is the dominant generation- gametophyte in mosses and sporophyte in ferns. Ferns can also be heterosporous, while mosses are always homosporous. Ferns themselves are vascular, while mosses are not.

Bryophytes and Pterophytes follow the same general cycle through their lives- the alternation of generations. However, it is obvious that many differences exist within the details of these cycles and the plants themselves.