small male spore of the higher cryptogams takes the name of pollen-grains, and the larger female spore is known as the embryo sac. The latter does not sever its connection with the mother-plant until after an embryo plant has formed. On this account the prothallium—which we have seen as an independent structure in ferns and diminishing gradually as we ascended in the scale of flowerless plants—is here but feebly developed. The flower-bearing plant, whether herb, shrub, or tree, is the asexual generation producing two kinds of structures, which, by their development and union of parts, produce a plant like the one from which the sexual generation sprang. The pollen-grain is usually a small spherical or oval body that, when mature, separates from the case (anther) in which it was formed. Figs. 16 shows the form of some simple pollen-grains. Grain A shows the rudimentary Fig. 16. prothallium as a small cell, y; B is a pollen-grain forming the tube. Much the same is seen at C and D, excepting that the prothallium is made up of three small cells. In structure and function these pollen-grains are almost identical with the male spores of higher cryptogams. The embryo sac is more or less surrounded by the substance of the parent-plant, and develops within itself a prothallus of small size which is known as the endosperm, and is a store-house of nourishing matter for the young embryo. One or more cells form the homologue of the archegonia in higher cryptogams with its female germ. The male cell or pollen-grain no longer develops a number of mobile, fertilizing antherozoids or spermatozoids; but, instead, the whole pollen-grain passes to a receptive surface (stigma) situated somewhere near the female organ, from which it sends out a tube that penetrates the tissue provided for its passage (style) until it reaches its destination and mingles its contents with those of the female cell. Circumstances obtain in the flowering plants which render mobile bodies like spermatozoids worthless as a means of fertilization. In many cases the male element needs to pass from one tree to another, and even from one country to another. The first observed result of fertilization is the formation of the suspensors, mentioned under Selaginella. At the lower end of the suspensor the young plantlet is formed with its one or more small seed-leaves and a short root and stem. In this growth the food-material in the endosperm is frequently entirely exhausted. The ovule, as the female cell with its immediate surrounding tissue is
Page:Popular Science Monthly Volume 25.djvu/178
THE POPULAR SCIENCE MONTHLY.