. Lectures on the evolution of plants. Botany; Plants. ANGIOSPERM^ 179. spore or embryo-sac (Fig. 44, A, ma, B), which in ita origin corresponds closely to that in the Gymnosperms and in such Pteri- dophytes as Isoetes. As in the Gymno- sperms, the macro- spore remains per- manently within the ovule. The gametophyte is usu- ally exti'emely re- duced, showing in the typical forms a very constant structure. The single nu- cleus of the ma- crospore divides, and one of the two resulting nuclei moves to each end of the spore-cavity or embryo-sac. Here each nucleus divides twtee, so that there resul
. Lectures on the evolution of plants. Botany; Plants. ANGIOSPERM^ 179. spore or embryo-sac (Fig. 44, A, ma, B), which in ita origin corresponds closely to that in the Gymnosperms and in such Pteri- dophytes as Isoetes. As in the Gymno- sperms, the macro- spore remains per- manently within the ovule. The gametophyte is usu- ally exti'emely re- duced, showing in the typical forms a very constant structure. The single nu- cleus of the ma- crospore divides, and one of the two resulting nuclei moves to each end of the spore-cavity or embryo-sac. Here each nucleus divides twtee, so that there result four nuclei at each end of the sac. Three of them re- main at the ends, while the fourth one from each end moves toward the centre of the embryo-sac where these two " polar nuclei" Fig. 44.—A, diagram showing the arrange- ment of parts in a typical angiospermous flower; ca, the calyx made up of individual leaves, sepals; co, the corolla, composed of petals; an, the andrcecium, composed of the stamens; gy, the gynoecium, made up of the carpels (here but a single carpel). The upper part, anther, an, of each stamen has usually four microsporangia, or pollen- sacs; the macrosporangia, or ovules, are contained in the ovary, formed by the base of the carpel, or united carpels ; p, a pol- len-spore germinating upon the stigma, and sending its tube, pt, down through the cen- tral part of the gynoecium or pistil; B, dia- gram showing the structures within the embryo-sac at the time of fertilization. The three cells at the upper end form the "egg-apparatus," consisting of the two synergids, sy, and the egg-cell, o. At the lower end are the three " antipodal-cells," ant, and In the ceritre are the two " polar nuclei," pn, which afterward unite into a single one, the "endosperm-nucleus"; C, young pollen-spore of Naias, showing the antheridial cell, x; D, a germinating pollen- spore of the sweet-pea; pt, pollen-tube; a;,
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Keywords: ., bookcentury1900, bookdecade, booksubjectbotany, booksubjectplants
