REPRODUCTION AND DISPERSAL 



831 



weeds and the orchids). 

 Pollen grains commonly 

 have a thick outer layer, 

 the exine, and a delicate 

 inner layer, the inline 

 (figs. 1145-1148); in 

 cases where there is a 

 single layer, it may be 

 thick and cutinized (as 

 in Senecio) or thin and 

 permeable (as in sub- 

 mersed aquatics). Pol- 

 len grains differ con- 

 siderably in shape, the 

 common forms being spher- 

 ical or ellipsoidal (figs. 

 1149-1157), and also in 

 size, those of some mallows 

 being a hundred times as 

 large as the grains of many 

 other plants (fig. 1157); they differ also in surface sculpturing, most 

 grains being smooth, but some being spiny, as in 

 the composites and the mallows (figs. 1156, 1157). 

 Many pollen grains have thin spots which upon 

 germination determine the position of the develop- 

 ing pollen tubes; in some cases the tube forces off 

 a part of the spore coat as a lid. 



The stigma. The essential elements of the 

 pistil are the ovary and the stigma, the style often 

 being short or wanting, though its presence may 

 be advantageous through its elevation of the stigma 

 into a region of optimum exposure to pollen. 

 When mature, the stigma secretes mucilaginous 

 substances, which, together with its papillate or 

 spinescent surface, facilitate the adherence of pollen 

 (fig. 1158). Stigmas also secrete substances which 

 facilitate the germination of pollen grains, and in 

 some cases they secrete very specialized substances 

 which stimulate the germination of pollen from 



1154 1155 1156 



FIGS. 1149-1157. Pollen grains: 1149, grains of 

 Euphorbia splendens, both dry (a) and moistened (b) ; 

 1150, angular grain of the nightshade (Solanum 

 nigrum) ; 1151, grains of a croton (Codiaeum varie- 

 gatum), both dry (a) and moistened (b); 1152, a 

 germinating pollen grain of Oxalis ; 1153, ellipsoid 

 grain of Impatiens Sidtani; 1154, grain of Cuphea 

 ignea with processes at the angles; 1155, grain of a 

 nasturtium (Tropaeolum) with prominent angles; 

 1156, spiny pollen grain of Bidens ; 1157, grain of 

 Hibiscus with prominent spiny processes; note the 

 relatively gigantic size; all equally magnified. 



FIG. 1158. Stig- 

 matic region of Hibis- 

 cus ; t, the upper part 

 of a style branch with 

 scattered hairs ; g, the 

 stigma with its hairy 

 surface to which pol- 

 len grains (/>) are ad- 

 hering; highly mag- 

 nified. 



