Spore

Spore, a specialised reproductive cell, in itself asexual, capable by itself of giving rise to a new organism. The spore may originate either asexually - i.e. from a single mass of protoplasm - or sexually from the fusion of two masses. It is generally a single cell or nucleated mass of protoplasm. It may have no cell-wall and may then be motile, when it is termed a zoospore (q.v.). Zoospores may be ciliated, as in many Algae and in a few Fungi, or amaeboid, as in the Myxomycetes (q.v.) and in a few A1gae. The non-motile naked spores of the Florideae are either tetragonidia, produced asexually, or carpospores, produced sexually. When a spore has a cell wall it is commonly thick, and may consist of two layers - the outer cuticularised extine, exine, or exospore, and the more delicate inner intine or endospore. The asexually-produced spores of the sporophyte are either all alike as in most ferns, horsetails, and Lycopodium, when the plant is termed homosporous or isosporous or they are of two kinds, differing in size and in the sex of the organs to which they give rise. The plant is then termed heterosporous, as in the case of the Selaginella, the Hydropterideae, and the Spermaphyta, the smaller spores, which give rise to male organs, being termed microspores or pollen-grains, and the larger megaspores, macrospores, or embryo-sacs. Asexually-produced spores originate in a sporangium, or by abstriction, a process of budding on a branch hypha. In thallophytes the sporangium is unicellular; in higher plants it is multicellular; and in heterosporous forms two kinds of sporangium occur, the microsporangium or pollen-sac, and the megasporangium or ovule (nucellus). Sporangia are generally borne on special leaves or sporophylls, such as the staminal and carpellary leaves of Spermaphytes, but are in some cases axial. In ordinary ferns and Hydropterideae the sporangium originates from one cell (leptosporangiate); in other vascular plants, from a group of superficial cells (ensporangiate).