PRACTICAL POLLINATION 249 



And even if the insects themselves are abun- 

 dant, the weather conditions, in a given season, 

 may be such as to make it almost impossible for 

 them to carry out their bargain by transferring 

 pollen from flower to flower. Every orchardist 

 knows that a protracted rainfall just at the time 

 when his apple, pear or plum trees are in bloom 

 may prevent the bees from visiting the flowers; 

 and in such case, as is only too well known, there 

 will be a partial, or no crop that season. 



With trees and other perennial plants it is 

 not a matter of absolutely vital importance that 

 there should be a crop of seeds produced each 

 season. 



Failing progeny this year, next year or the 

 year after will answer in the case of a plant 

 which grows on a permanent stalk or from roots 

 which outlast the winter. 



But the case of the annual plant is altogether 

 different. 



Should such a plant fail for a single season 

 to produce seed, its entire race might vanish 

 instantly from the earth. 



That thought is rather startling when pre- 

 sented thus tangibly. 



Yet its truth is almost axiomatic. Quite often 

 the entire seed crop of an annual plant in a 

 state of nature germinates or decays the ensuing 



