Arrangements for Securing Union of Sexes 305 



rt.^3 ^^ I 



plants of the land, which we shall consider a few pages later; but 

 a great man}^ others of simpler sort, including especially the 

 lowlier Waterweeds, cast their suitably-protected pollen out into 

 the water, to be drifted about by the currents until it reaches the 

 stigmas. In some kinds, as in 

 most of the Eel-grasses, where 

 the pollen is thread-like in 

 shape, the pollination occurs 

 under water; but in others, for 

 example the Freshwater Eel- 

 grass, VaUisneria spiralis (figure 

 107), it takes place on the sur- 

 face, to which the staminate 

 flowers rise from their place of 

 formation, and on which floats 

 the ripe ovary with widely- 

 spread stigmas. Then the 

 movements of the surface cur- 

 rents, with aid of the wind, 

 bring the pollen sooner or later 

 to the stignia. 



But far more striking and 

 important are the adaptations 

 to cross pollination found in 

 plants that live out on the land, 

 including the kinds with wliich 

 we are most familiar. These, 

 having no power at all of loco- 

 motion, have had to secure the 

 transport of their pollen in some different way and that way con- 

 sists in the utilization, by aid of suitable adaptive mechanisms and 

 methods, of such motive agencies as happen to exist in the world 

 around. Now of all such agencies, the most ubiquitous and the 

 easiest to utilize is the wind. Accordingly- wind pollination prevails 



Fig. 107. — Cross pollination in the Water- 

 weed, VaUisneria spiralis, which is shown 

 about one-third the natural size. The 

 staminate flowers may be seen rising to 

 the surface, where they open and are 

 drifted about until their stamens come 

 into contact with the long-stalked float- 

 ing pistillate flowers. (Copied, somewhat 

 simplified, from Kerner's Pflanzenleben.) 



