^^^^ ZOOGENESIS '^M 



many plants depend on air to carry the pollen from 

 one flower to another. We see this best in such trees 

 as oaks, chestnuts, alders, willows and birches in 

 which the male or staminate flowers are in long cat- 

 kins. From such trees we sometimes see the pollen 

 blown away in a thick cloud by a sudden gust of 

 wind. 



The spores of molds and fungi and of more or less 

 similar types of plants and the spore-like stages of 

 many of the microscopic animals, especially the 

 protozoans and the rotifers, and also of a number of 

 the minute organisms that cause disease in man, in 

 animals and in plants, are almost constantly present 

 in the air floating about in the same way as, and 

 together with, minute particles of dust. 



Air in stronger motion is essential for the distribu- 

 tion of those plants which have wind transported 

 seeds. Such seeds may, like the seeds of orchids, be 

 of such extremely minute size as to act like particles 

 of dust. More commonly, however, they are pro- 

 vided with various devices such as the wings of the 

 seeds of pines, elms and maples, or the tuft of hairs 

 or ' 'coma" of the seeds of the dandelion and the milk- 

 weed, which delay their falling and aid in their 

 transportation. 



Some insects, so far as their capabilities for trans- 

 portation are concerned, are comparable to specks of 

 dust. Such for instance are the excessively minute 

 wasp-like things that live as parasites mostly in the 

 eggs of other insects or of spiders, and certain equally 

 small fungus-living beetles. Some of these little 



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