megaspore

…sporangia; the larger spore (megaspore) gives rise to the female gametophyte, the smaller spore (microspore) to the male. This condition is referred to as heterospory. The gametophytes, or prothalli, of other club mosses and most horsetails and ferns are sexually undifferentiated and arise from one kind of spore, a…

…microspores, or male spores, and megaspores (macrospores), or female spores. In pteridophytes, megaspores are typically larger than microspores, but the opposite is true in most seed plants.

…sizes, the larger designated as megaspores and the smaller as microspores. The megaspores develop into female gametophytes and the microspores into male gametophytes. Accordingly, strobili bear megasporophylls that contain megasporangia, which will produce megaspores, and microsporophylls that contain microsporangia, which will yield microspores. Although the evolutionary origin of two kinds…

…a single linear tetrad of megaspores. Three of the four megaspores degenerate, and the surviving one enlarges. The resulting megagametophyte produces the female gametes (eggs). This development (called megagametogenesis) involves free-nuclear mitotic divisions. The cell wall remains intact while the nucleus divides until the megagametophyte, or embryo sac, is formed.…

…variations in the female (megasporangiate) reproductive structures among the conifers are the main basis for their classification. Most living conifers have a seed cone that is interpreted as a compound strobilus; each cone scale, inserted in the axil of a bract, is equivalent to an entire simple pollen cone.…

…free nuclear divisions in the megaspore. At the end of those divisions, there may be up to 2,000 nuclei in a thin layer of cytoplasm pressed against the megaspore wall by a giant central vacuole. Cell walls then form between adjacent nuclei and gradually extend into the central vacuole until…

…magnoliids three of the four megaspores formed from the megakaryocyte (megaspore mother cell) degenerate. A female gametophyte of eight nuclei, including the ovum (egg), develops from the surviving megaspore (see angiosperm: Reproduction). About 70 percent of angiosperms have this type of female gametophyte development. With few exceptions in the subclass,…

…one producing a few large megaspores (holding food reserves for the early development of the embryo) and the other producing many small microspores. The microspore divides to form a reduced gametophyte, merely a jacket of cells and a few sperm cells; the megaspore divides to form a mass of tissue…

…sizes, the larger designated as megaspores and the smaller as microspores. The megaspores develop into female gametophytes and the microspores into male gametophytes. Both types of spores are thick-walled, and both have prominent three-part (triradiate) ridges.