SPERMATOPHYTES ( from Gk. Tripno., sperma, seed + phyton, plant), SEED PLANTS. The highest of the four primary divi sions of the plant kingdom, distinguished from the other groups by the production of seeds. The much used name phanerogams, meaning evident sexual reproduction, is unfortunate because in this group sexual reproduction is least evident. The once used name anthophytes, meaning flow ering plants, which last is probably the most com monly used popular name, is also inappropriate, since the production of flowers is not coextensive with the group. The most recently proposed name siphonogams, meaning sexual reproduction by means of a tube, referring to the passage of the male cells to the eggs through pollen-tubes, has not been extensively adopted. Since the seed-production seems to distinguish the group more than any other character, the name sperma tophytes, which is in common use, seems likely to prevail.
This group. which includes practically all the conspicuous vegetation (herbs, shrubs, and trees), is by far the most useful group to man, so useful, indeed, that until the closing years of the nine teenth century elementary botanical training dealt with no other group and botanists were thought of chiefly as students of flowers. More than 100, 000 species of seed-plants have been described, and grouped in two distinct but very unequal divisions—gymnosperms (q.v.) and angiosperm-3 (q.v.). They are distinguished by the position of their seeds, which are naked or freely exposed in the former, but inclosed in a seed-ease in the latter.
In all seed-plants the alternation of generations (q.v.) is very much obscured by the great reduc tion of the sexual plants, which are not popularly recognized, and are undiscoverable except by laboratory manipulation, the whole visible body of these plants, contrary to the popular notion, being the sexless phase or sporophyte. All the members of the group are also heterosporous. (See HETEROSPORY.) The pollen-grain is a sex less microspore that in germination gives rise to a small plant consisting of only a few cells, among them two male cells, whieh are formed within the pollen-grain and are to function as sperms. The pollen-grain is transferred to the
immediate neighborhood of the female plant, usually by the wind or by insects. See POLLI NATION ; FERTILIZATION.
The megaspore, a large sexless spore, that pro duces the female plant, is developed within the ovule, which is, therefore, a sporangium. The fact that in this sporangium there is but a single mega spore, and that is not discharged, but retained, is what makes a seed possible; for the retained megaspore germinates within its sporangium • ovule ) and produces the female plant there. l'his megaspore within the ovule was once thought to be merely a sac-like cavity, within w 'Itch the embryo appeared, and hence was called Fig. 1: s, Fig. 2 ). In germina tion the megaspore produces a female plant, with several or many cells, which has long been called `endosperm,' and recognized as a prominent nutri tive tissue within the seed. The female plant, therefore, is entirely inclosed within the ovule, and produces an egg (0, Fig. 1) which is reached by a pollen-tube fig. 1) and fertilized. This act of fertilization is followed by two conspicuous results, namely (1) the development of the em bryo (c, Fig. 2). and (2) the development of a hard superficial tissue (testa) in outer part• of the ovule, which hermetically seals the female plant and embryo within, the whole com plex structure constituting the seed.
In the seed condition the plant passes into a resting period of greater or less duration, and then, under favorable conditions, the seed is said to germinate. This simply means the renewed growth of the young plantlet, whose embryo, which had begun to develop, was checked by the hard investment of the seed. A seed, therefore, is a complex of three generations: (1) the old sexless generation (sporophyte), represented at least by the seed coats; (2) the following female sexual generation (gametophyte), reprAented by the endosperm; and (3) the new sexless genera tion (sporophyte). represented by the embryo.