Rutherford Piatt 



Single Flower of Elm, Magnified 



THE DISTRIBUTION OF POLLEN BY WIND 



Staminate Catkin of Birch 



The rather dry pollen of our common trees is shed from the stamens in vast quantities 

 and scattered widely by the wind 



and float to the surface in large numbers. Here they open, and as they 

 come in touch with the exposed stigmas, the pollen is transferred directly. 



Next to the wind, the most common moving agents that pollenate flowers 

 are flying animals, like species of birds and insects that regularly visit flowers. 

 Certain tropical flowers are said to be pollenated by bats that come to them 

 for nectar. 



In thousands of species of plants the flowers are pollenated by insects, 

 chiefly varieties of bees and wasps and certain moths and butterflies. All these 

 insects have sucking mouths, and they all visit flowers that contain nectar. 

 Some of these insects also use pollen as food. In gathering the pollen or in 

 sucking nectar the insects rub off pollen on various parts of their bodies; and 

 when they visit other flowers of the same kind, they then transfer the pollen 

 to the stigmas (see illustration, p. 410). Many species of flowering plants, 

 especially among the orchids, depend so completely upon particular insects 

 that they produce barely enough seeds to maintain themselves. 



Flowers as Secondary Sexual Structures We saw that among many 

 species of animals males and females difl"er from each other strikingly in details 

 that are only remotely or not at all related to the formation of gametes or to 

 their conjugation. The flowers that are often so highly specialized in the struc- 



408 



