312 ELEMENTARY BIOLOGY 



two to three yards, — they will nevertheless visit only one kind of 

 flower in the course of a day, or even for many days running. Thus, 

 if a bee starts out in the morning by visiting a red clover, it will visit 

 only red clovers for the rest of the day, or as long as any red clovers 

 are to be had. 



367. The interdependence between flowers and insects. In 



some cases the relation between a seed plant and some insect 

 is so close that it affects the practice of plant raisers. 



When fig trees were first introduced into California, they 

 produced large, juicy fruit, even without pollenation. But. the 

 fruits thus produced are not as satisfactory for commercial 

 purposes as the others : they do not dry properly, and so 

 cannot be prepared for shipping or for preservation. To get 

 the normal fruit it was necessary to find the insect that 

 regularly brings about pollenation. This little wasp, the Blas- 

 tophaga, has a curious life history which is closely adapted 

 to the flowering habits of the fig tree. On the other hand, 

 thousands of fig pistils supply breeding places for wasps 

 without ever producing seeds (see Fig. 141). Thus the insect 

 and the tig tree are of great value to one another, although 

 it is difficult to see what advantage either species has in its 

 dependence upon the other. It is quite impossible for us, at 

 present, to imagine how this relationship came to be established 

 in the course of time. 



There are many cases of plants that have been transferred 

 from one region of the earth to another, and then failed to 

 bear seeds because of the absence of the suitable insect. 



When vanilla was transplanted from Mexico and South 

 America to various islands in the Indian Ocean and else- 

 where, the plants grew luxuriantly, but produced no fruit, 

 although flowers were produced in abundance. Since the 

 plant was raised for the "beans" or pods, there was no profit 

 in the business so long as the fruit failed to develop. It was 

 found that the failure was due to the absence of pollenation, 

 which is brought about in the native regions by certain insects, 



