i yo 



GROWTH AND WORK OF PLANTS 



place, there may be little or no seed or fruit developed, but when 



cross-pollination takes 

 place an abundance 

 of seed is formed. 

 Even when pollen from 

 the same and from a 

 different flower is de- 

 posited on the stigma, 

 the pollen from the dif- 

 ferent flower prevails. 

 Cross-pollination is im- 

 portant for the plant, 

 since it insures greater 

 vigor and greater plas- 

 ticity in the offspring, 

 survive in 



Fig. 128. 

 The pendent flower of Y.ucca. showing position of sta- 



mens and the ribbed ovary. After Riley and Trelease. 



which make it better fitted to 

 the struggle for existence. Still there are 

 flowers in which the usual method of pol- 

 lination is a close pollination. 



284:. Cleistogamous flowers. Close pol- 

 lination always takes place in cleistogamous 

 flowers, of course if left to themselves. Cleis- c 

 togamous flowers are those which remain 

 closed during the process of pollination and 

 fertilization. The violet is an excellent ex- 

 ample. The showy flow r ers which are so 

 conspicuous rarely develop seed. The greater 

 quantity of seed is formed in flowers which 

 are not showy and which are covered by 

 the soil or leaf mold. They remain closed, 

 and the pollen from the stamens is shed 

 on the stigma of the same flower. 



285. Pollination of the yucca by the 

 moth Pronuba. This is a remarkable case 



Fig. 129. 



and Trelease - 



of close pollination brought about by an 



insect. The pollen is somewhat sticky and without some aid 



