HET'EROS'PORY (from Gk. grepos, hetcros, other ar6pos, sporos, seed). The production, by plants, of two kinds I if sexless spores. Ikter ospory is one of the most important phenomena in conneetion with the evolution of the plant king dom, since through it the seed has appeared, and since a clear conception of flowering plants is im possible without some knowledge of the hegin wings of heterospttry. The phenomenon appears first, and in its simplest form, among certain of the fern-plants ( Pteridophytes), and is universal among the seed-plants (spermatophytes). To un derstand the significance of heterospory it will he necessary to rend the article on A LTERNATION OF GENERATIONS, since this deals with the two kinds of spores produced by the sexless genera tion (sporophyte).
In ordinary ferns the spores are alike. and in germination each one produces a sexual plant (gainetophyte I, which hears hoth the male and the female sex organs. Such a condition is called 'homospory' (q.v.) or `isospory,' which means in each ease 'spores similar.' Very gradually the spores begin to differ in size. until presently some become very much larger than others. The large spores are called 'megaspores' and the small ones `inierospores.' Alore important than the fact of ditierenee in size, however, is the other fact that t he micros1.ore: produce sexual which bear only male organs, and the megaspores sexual plants which bear only female There IS a very definite relation between the amount of nutrition and the appearance of the female organs, so that the larger and hence bet ter-nourished spore is expected to produce a fe male plant. In consequence of heterospory. therefore, the sexual plants become distinetly male and female. The unfortunate thing is that the sexual diameter has often been applied to the mierospores and the megaspores, which are simply the sexless spores of a sporophyte, which necessarily produce gametophytes.
When spores become thus differentiated in size and in product, the spore-eases (sporangia) are also differentiated, so that certain sporangia produce only microspores, arid others produce only megaspores. The former were naturally
called 'microsporangia' and the latter 'mega sporangia.' These sporangia are ordinarily pro duced upon leaf-like structures, and later the leaves bearing sporangia become differentiated in the same way, so that certain leaves hear only mierosporangia and other leaves only mega sporangia. Naturally the former were called `inierosporophylls' and the latter `megasporo phylls."These names are used in connection with those fern-plants which are beterosporous, as the water-ferns, the little club-mosses (Sela-• ginella), and the quillworts (Isoetes). Among the seed-plants (flowering plants), however, the corresponding structures already possessed names long in use, and not to he discarded. It is impor tant, however, to know the structures among the seed-plant.s that correspond to those among the heterosporous fern-plants. It is found that the stamen (q.v.) of a flowering plant is a micro sporophyll, that its pollen-sacs are mierospo rangia, and that the pollen-grain is a miserospore. The interesting conclusion is reached. therefore, that the pollen-grain is a sexless spore, a eonelu sion quite contrary to the general impression that it is a male cell, and that the stamen is male organ. it is found that the earpel (q.v.) of a flowering plant, which organizes the ovary, style, and stigma, is a megasporophyll; that the ovules are megasporangia; and that the so-called embryo-sae is a single large unshed megaspore. It follows, therefore, that the ovule is by no means the egg or ovule structure which its name would imply, but the spore-ease (sporangium) of a sexless plant.
An important further fact in connection with heterospory is that upon its development the sex ual plants (gametophytes) become very much reduced in size, for the most part not escaping from within the spores which produce them. As a consequence, the male plant or generation in a flowering plant is contained entirely within the pollen-grain; and the female plant or generation is completely inclosed within the ovule. For fur ther development of the subject in connection with the evolution of the seed, see SEED.