![]() The prefix "mega-" comes from the Greek word for "large" ( megas). In heterosporous (from the Greek heteros, different) plants, two size classes of spores are produced: larger megaspores and smaller microspores. ![]() Sometimes, the gametophytes of these plants may develop as unisexual, or female (archegonia/egg-producing) or male (antheridium/sperm-producing), due to the influence of environmental factors or the production of chemical signals by other nearby gametophytes. Such plants have a life cycle like the generalized fern life cycle described above. The gametophytes of homosporous plants are often bisexual, meaning that they produce eggs in archegonia, as well as sperm in antheridia. Homospory is considered the ancestral condition in land plants. Homosporous (from the Greek homos, the same) plants produce one size class of spore, sometimes called isospores (from the Greek isos, equal). One variation is whether plants are homosporous or heterosporous. In addition to differences in the appearance, persistence, and independence of the sporophyte and gametophyte generations in the life cycles of land plants, there are several other variations that occur in the land plant life cycle. They can thus be described as heteromorphic (Greek, heteros + morphē = different form). Regardless of which generation may be dominant, the sporophyte and gametophyte generations in land plants are typically very different in structure and appearance. The ovule is released from the pine cone sometime after fertilization, when it has matured into a seed containing a sporophyte embryo and stored food. The pollen is released and is carried by wind to the ovule, where it can deliver sperm so that fertilization can occur. The male (sperm-producing) gametophyte is the pollen grain, whereas the female (egg-producing) gametophyte is found in the immature seed (called an ovule). ![]() In a pine, the sporophyte is a tree, whereas the gametophytes are very tiny and strictly unisexual (male or female). The simple sporophyte grows on and is dependent on the gametophyte the sporophyte is typically unbranched and makes only one sporangium during its lifetime. For example, in a moss, the gametophyte is green and leafy. In some plants the gametophyte is the dominant generation, whereas in others the sporophyte is dominant. ![]()
0 Comments
Leave a Reply. |
AuthorWrite something about yourself. No need to be fancy, just an overview. ArchivesCategories |