METHODS OF POLLINA TION 



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 

 deposited on the stigma, the pollen from the different flower 

 prevails. Cross-pollination is important for the plant, since it 



Fig. 137- 



The pendent flower of Yucca, showing position of stamens and the ribbed ovary. After 

 Rilcy and Trelease. 



insures greater vigor and greater plasticity in the offspring, 

 which make it better fitted to survive in the struggle for ex- 

 istence. Still there are flowers in which the usual method of 

 pollination is a close pollination. 



298. Cleistogamous flowers. Self-pollination always takes 

 place in cleistogamous flowers, of course if left to themselves. 

 Cleistogamous flowers are those which remain closed during the 

 process of pollination and fertilization. The violet is an excel- 

 lent example. The showy flowers 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. 



299. Pollination of the yucca by the moth Pronuba. 

 This is a remarkable case of close pollination brought about by an 



