VEG E TA TIVE REP ROD UCTION. 



255 



planes; or it may even be entirely unsymmetrical. This 

 unlikeness in the size and shape of the accessory leaves not 

 infrequently extends to the sporophylls (figs. 277, 278). 



The irregular form and color of the perianth (when other 

 than green), including the variegation of the ground color 

 by lines and spots, seem to be dependent upon the relation of 

 the flower to insects. (See further ^[ 484.) 



FIG. 277. 



FIG. 278. 



FIG. 277. An unopened flower of the sweet pea, halved ; showing bilateral symmetry 



(irregularity). Slightly enlarged. After Bessey. 

 FIG. 278. Diagram showing the arrangement of the parts of the flower of sweet pea. 



Outer circle, calyx <5-lobed) ; second, 5 petals, the two lower united ; third, 10 stamens, 



9 united by filaments, i separate ; center, one carpel. Only one plane will divide this 



flower into halves. After Bessey. 



358. Pollination. Since the megaspore is enclosed per- 

 manently by the ovule, and in angiosperms the ovules are 

 again enclosed by the pistil, it is necessary that the male plant 

 growing from the pollen spores be developed in the neighbor- 

 hood of the ovule whose megaspore produces a female plant. 

 (See TT 341, 386.) To insure this a portion of the pistil 

 forms a receptive surface, the stigma, upon which the pollen 

 spores may be readily lodged. It is advantageous, also, to 

 have the pollen spores of one flower lodged upon the stigma 

 in another flower of the same sort rather than upon the 

 stigma of the same flower. The process of lodgment of 

 pollen on a stigma is called pollination. If the pollen from 

 one flower is carried to the pistil of another, it is called 

 cross-pollination.* To secure pollination, and especially 



* Since fertilization of the egg is the ultimate object of pollination and 



