INSECTS AND DISEASES 97 



plants where the pistils are borne on one individual 

 and the stamens on another. In order to produce 

 seed it is necessary for the pollen to be conveyed in 

 some manner from one plant to the other. There 

 are other cases where the stamens are in separate 

 flowers, but borne on different parts of the same plant. 

 Even, as is usually the case when both occur in the 

 same flower, there are many contrivances to prevent 

 the pistils from being pollinated from the same 

 flower. All of these cases require what is known as 

 cross pollination; that is, the carrjdng of pollen by 

 some means from one flower to another. There are 

 some plants, however, where the flowers are normally 

 self-pollinated. Even in these cases cross pollina- 

 tion undoubtedly often accidentally occurs. Some 

 plants produce pollen in immense quantity and de- 

 pend on the wind for carrying it from one plant to 

 the other. This is a wasteful method and unduly 

 taxes the energy of the plant in the production of 

 useless pollen. The great majority of plants depend 

 on insects of one kind or another to carry pollen 

 from flower to flower. The manifold devices that 

 have been evolved for securing insect pollina- 

 tion constitute one of the most marvelous wonders 

 of the vegetable world, and this interrelation be- 

 tween plants and insects furnishes a most interesting 

 and important field for study. 



Insects and Diseases 



Cultivated plants of all kinds are liable to be at- 

 tacked by insects, and they are, like animals, subject 

 to various diseases. The study of the diseases of 



