286 



A TEXTBOOK OF BOTANY 



[Ch. VI, 5 



throughout the body of each individual, though whether 

 this is the functional aim, or only an incidental result of 

 fertilization, is uncertain. 



What now is the significance of fertilization, or, in other 

 words, of sexual reproduction ? Fertilization is by no means 

 essential to reproduction, since many kinds of low organisms 

 lack it, while even the higher plants possess many methods 

 of purely vegetative, or asexual, reproduction, as a later 



section will make clear. 

 The asexual method is 

 thus not only possible, 

 but it is actually much 

 simpler and easier of ac- 

 complishment than the 

 sexual. Why then the 

 overwhelming predomi- 

 nance of the sexual 

 method ? Unfortunately 

 we have not yet any cer- 

 tain knowledge upon this 

 point. It has commonly 

 been believed that indi- 

 viduals produced by fer- 

 tilization are more vari- 

 able than those produced 

 by asexual methods, and 

 that such variability gives advantage in competitive evo- 

 lution. Yet some investigators hold other views, and the 

 matter is one on which we must await further evidence. 



198. — Various forms of 

 grains ; magnified. 



The 3 three-lobed grains in the row on 

 the left are Pine, the two lateral lobes 

 being air-filled bladders. The roughened 

 forms are mostly carried by insects, in the 

 hairy bodies of which the various projec- 

 tions become caught. (Reduced from 

 Kerner.) 



5. The Methods and Meaning of Cross-Pollination 

 It was said in a preceding section that the transfer of pollen 

 from the anthers to the stigmas of flowers is effected, as a 

 rule, not by action of the plant itself, but by some external 

 agency, notably wind and insects. The matter, however, 

 goes farther than this, for the floral arrangements are such 



