POLLENATION 



307 



3. Physiological relations. In some species of plants it is 

 found that when the pollen is placed on the stigma of the 

 same flower, it will either not germinate at all or it will produce, 

 on the whole, poorer seeds than those produced by means of pol- 

 len taken from another flower. This physiological difference 

 in favor of outside 

 pollen is called pre- 

 potency, and was 

 demonstrated by 

 Darwin in several 

 species of plants. 



Prepotency is com- 

 monly associated with 

 the presence of two or 

 three lengths of styles 

 and of filaments. 



In the flax, cow- 

 slip, Chinese prim- 

 rose, bluet, and other 

 species there are two 

 forms (Fig. 135). 



In the purple loose- 

 strife (Ly thrum sali- 

 carid) and in certain 

 species of Oxalis (re- 

 lated to our sorrel) 



there are three lengths of stamens and three lengths of pistils (in 

 different flowers) corresponding to them (see Fig. 136). 



In buckwheat, in most orchids, in certain species of day lily, and 

 in certain species of the bean family the pollen will not germinate at 

 all on the stigma of the same flower. 



There are, then, many species of plants in which close 

 pollenation cannot take place, or in which it is not very effec- 

 tive if it does take place. How, then, do these plants produce 

 seeds, or, rather, how do they secure pollenation ? In other 

 words, how is pollen carried from flower to flower? 



FIG. 139. Pollenation by birds 



Saber-billed humming bird pollenating flower with trumpet- 

 shaped corolla. (From exhibit in American Museum of 

 Natural History, New York) 



