Ch. VI, 5] METHODS OF CROSS-POLLINATION 



287 



that the transfer is usually not between anther and stigma 

 within the same flower, but between anthers and stigmas of 

 different flowers, — and oftener than not between different 

 plants, of course of the same species. This cross-pollina- 

 tion, as it is called in contrast with close-pollination 

 within the same flower, is a matter of much botanical conse- 

 quence. 



In the simpler kinds of 

 plants, e.g. the low Algae 

 (page 12), the physiological 

 equivalent of cross-pollina- 

 tion results incidentally from 

 the fact that the free-swim- 

 ming sexual cells (or gam- 

 etes), 'usually cast out into 

 the water by many plants of 

 one kind together, inter- 

 mingle and unite promis- 

 cuously. Of course in such 

 cases a union may result be- 

 tween cells from the same 

 plant, as doubtless often 

 occurs, though in higher forms 

 there is reason to believe that 

 chemical attractions and re- 

 pulsions between the gametes 

 compel crosses between differ- 

 ent plants. In the Ferns, and some other plants having 

 free-swimming male cells, a cross is insured through the 

 fact that the sperm and egg cells produced by the same 

 plant are not usually mature at the same time. 



Some flowering plants are pollinated on the water, no- 

 tably our submerged pond weeds, eel-grasses, etc. A typical 

 case is the fresh-water eel-grass, Vallisneria spiralis (Fig. 196). 

 Growing in masses together, these plants produce pistil- 

 late flowers which float by long stalks at the surface, 



Fig. 199. — Flower of Iris, cut ver- 

 tically. 



The stamen, somewhat to the 

 right of the center, is under the style ; 

 the stigma is the upper side of the 

 small projection from the latter. 

 (From Le Maout and Decaisne.) 



