ANGIOSPERMS, iin'ji-tespi;rmz (Gk. (1) yeiov, anyeion, vessel ± arippa, spernaa, seed). A name applied to the greatest group of seed plants, Spermatophytes, as distinguished from the other group, Gymnosperms, in which the "seeds are naked." The two great divisions of Angiosperms are the Monocotyledons and Di cotyledons, once called the "Endogens" and "Ex ogens" respectively. The Monocotyledons are characterized by the single terminal seed leaf (cotyledon) of the embryo, the scattered woody bundles of the stem, the closed venation (often called "parallel veined"), and the three-parted flowers. To the group belong such forms as the common pondweeds, grasses, palms. aroids, lilies, and orchids. The Dicotyledons are char acterized by the lateral cotyledons, the organiza tion of the woody bundles of the stem into a hol low cylinder, the open venation (often called "net-veined"), and the five- or four-parted flow ers. To this group belong such forms as the common trees (poplars, oaks, elms, etc.), but tercups, roses, peas, umbellifers, heaths, mints, composites, etc. The Angiosperms are estimated to comprise over 100,000 species, and they form the most conspicuous part of the vegetation of the earth. Since the Gymnosperms comprise only about 400 living species, it is evident that the Angiosperms are the chief modern represent atives of seed-plants. It is among Angiosperms also that the true flowers are developed. with elaborate relations with insects for securing pol lination. The group is often called "true flower ing plants," because it is characterized by the ordinary conspicuous flower.
The members of the group are of every possible variety of habit, from minute floating forms to gigantic trees. The roots, stems, and leaves are more elaborately and variously organized for work than those of any other plant groups, and the whole structure of the body is the most com plex found in the plant kingdom.
It is among the Angiosperms that "stamens" and "carpels" become definite and distinctly de veloped. The stamen of the Angiosperm corre sponds to a spore-bearing leaf of the fern-plants, but shows no resemblance to an ordinary leaf in form. The region devoted to producing the spores is called the "anther." In observing the development of an anther it is found that four sporangia usually appear. and that as these ap
proach maturity they fuse in pairs, resulting in the appearance of two pollcu-saes. each of which has been derived from two sporangia. Occasion ally iu Angiosperms the four original sporangia of the stamen remain distinct.
The carpels of Angiosperms give name to the group, for these structures inelose the ovules that become seeds, the name angiosperms mean ing, as has been said, "seeds in a ease." In this regard they differ decidedly from any carpels which exist among the Gymnosperms, in which group they are fiat and open, exposing the ovules, and giving rise to the name, which means "seeds naked." In order to understand the following state ment of the technical characters of angiosperms, it will to read the articles ALTERNA TION OF GENERATIONS, and HETEROSPORY.
In its germination the pollen grain (micro spore) produces within itself usually three cells, which represent a very much reduced male plant. One of these cells later develops the pollen tube, which penetrates to the egg, while dated with it are two other cells called syner !fir's (helpers). This group of three cells is called the egg apparatus. At the opposite end of the sac is a group of sterile cells, usually three in number, and called the antipodal cells. More centrally placed in the sac is the seventh cell, that has been formed by the fusing of two cells, and, after the fertilization of the egg. is to de velop the endosperm (nutritive tissue of the seed). This cell is called the "definitive nucleus" or "primary endosperm nucleus." Before fer tilization, the pollen grain containing the male plant is transferred by the wind or by insects to the stigma (receptive region of the pistil), the process of transfer being called pollination (q.v.). After pollination the pollen tube is de veloped, which penetrates the pistil and finally reaches the ovule, carrying in its tip the two male cells or sperms. The tip of the tube then enters the micropyle, crowds its way to the egg, and discharges its contents. One sperm passes to the egg and fuses with it, this act being called fertilization. The other spdrm has recently been observed to pass on in the sac and fuse with the endosperm nucleus; but it remains to be seen how general this phenomenon, called double fer tilization, may be in the group.