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 elhpsoidal (figs. 



1154 1155 1156 

 Figs. 1149-1157. — Pollen grains: 1 149, grains of 

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

 1 1 50, angular grain of the nightshade {Solanum 

 fjigrum) ; 1151, grains of a croton {Codiaeum varie- 

 gatum)^ both dry {a) and moistened (6); 1152, a 

 germinating pollen grain of Oxalis ; 1153, ellipsoid 

 gra.m of Impatiens SuUani; 1154, grain of Cupkea 



1140-1157)," and also "in '^"'^ ^''^ processes at the angles; 1155, grain of a 



nasturtium (Tropacolum) with prominent angles; 

 size, those of some mallows njg^ spiny pollen grain of Bidens; 1157, grain o£ 

 being a hundred times as Hibiscus with prominent spiny processes; note the 

 large as the grains of many relatively gigantic size ; all equally magnified. 



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 

 cus; t, the upper part substances, which, together with its papillate or 

 of a style branch with g jj^gg^g^t surface, facilitate the adherence of pollen 



scattered hairs ; ^, the r i • i 



stigma with its hairy (fig. 1158). Stigmas also Secrete substances which 

 surface to which pol- facilitate the germination of pollen grains, and in 

 len grams (^) are ad- ^^^^ ^^^^^ ^, secrete very Specialized substances 



henng; highly mag- . , , ■ ^- - r n i- 



nified which stimulate the germination of pollen from 



Fig. 1158. — Stig- 

 matic region of Hibis- 



