REPRODUCTION IN PLANTS 



271 



microspores and megaspores of plants give rise asexually to a haploid 

 gametophyte generation which produces the gametes. 



The megaspore " germinates" within the ovule and undergoes three 

 successive nuclear divisions unaccompanied by cytoplasmic division. At 

 the conclusion of these divisions it is a large ovoid cell containing eight 

 nuclei — four toward each end. One nucleus from each group of four 

 migrates to the center of the cell; these two will later fuse to form the 

 primary endosperm nucleus. One of the three nuclei at the end of the cell 

 nearest the opening of the ovule grows larger than the other two and, 

 with a portion of the surrounding cytoplasm, becomes enclosed in a cell 



Fig. 17.16. The mechanism of pollination in the bellfiower (Campanula persicifolia, family 

 Campanulaceae). Normally cross-fertilized by insects, this plant can fertilize itself if 

 necessary. The opening flower (left), with anthers closely applied to the sticky but non- 

 receptive outer surfaces of the three stigma lobes. Later (center) the stigma lobes separate 

 and the anthers pull away, leaving pollen adherent to the outer faces of the stigma lobes. 

 The latter open wide (right), exposing their pollen-receptive inner faces to insect visitors. 

 If pollination fails to occur, the lobes curl until their inner faces touch the pollen adhering 

 to the pistil, so that self-fertilization takes place. (Photos by Prof. E. B. Mains.) 



membrane; this is the ovum, or egg. The two other nuclei at this end (the 

 synergids) and the three nuclei at the other end of the cell (the antipodal 

 nuclei) may also form cell membranes, but all five are destined to break 

 down and disappear. 



The product of this entire process is a structure containing seven 

 nuclei, corresponding to seven cells, and is the mature female gametophyte. 

 The ovum is a germ cell; the other six may be regarded as somatic cells. 

 The entire gametophyte is still contained within the ovule, or spore case, 

 and all of its nuclei except the primary endosperm nucleus are haploid. 



Pollination. Development of the egg is dependent upon its fertiliza- 

 tion. To accomplish this, the first necessary step is transfer of a pollen 

 grain with its contained sperm nuclei from the anther of the same or 

 another flower to the adhesive tip, or stigma, of the pistil in which the 

 egg lies. 



