108 THE STUDY OF PLANT COMMUNITIES • Chapter V 



some pollen. The evidence is to the effect that birch, pine, oak, 

 willow, sedge, and grass pollen are carried in quantities for more 

 than one thousand kilometers over the ocean. 



The amount of pollen in the air and the distance it is trans- 

 ported is of significance to some plants but more so to many 

 people who suffer from hay fever. Recently the kinds and num- 

 bers of pollen grains in the air in many sections of the country 

 are determined daily and made publicly available for the use of 

 hay-fever sufferers. 



In general, wind-pollinated plants grow in the open or in ex- 

 posed places. Even in a forest it is the trees of the upper strata that 

 are characteristically wind pollinated; the flowers are small and 

 inconspicuous, with simple or reduced structure. The corolla is 

 often lacking, and there is an absence of bright colors, odor, and 

 nectar. Stamens and pistils are commonly borne in different flow- 

 ers, the stigmas are usually feathery, and the stamens are long and 

 pendant. In spite of its apparent wastefulness, the system produces 

 satisfactory results. 



Dissemination— Plants migrate from one point to another by 

 means of spores, seeds, fruits, fragments of plants, or entire plants. 

 The agent of transport may be water, animals, or wind, depending 

 upon the various adaptations of the disseminules, which facilitate 

 the movement. 



Dissemination by spores is characteristic of all plants except 

 spermatophytes. Wind-disseminated spores, like pollen, are small 

 and dry and may be transported great distances. Everywhere that 

 pollen is carried, spores are found too. Their transportation over 

 long distances can be of great ecological and economic impor- 

 tance. A spore carried by a freak wind into distant territory may 

 establish a species where it has never grown before, thus extending 

 the range of the species and possibly necessitating adjustments 

 within the community in which it develops. 275 The economic con- 

 siderations are fairly obvious when it is remembered that fungi 

 that produce diseases of both plants and animals are all propagated 

 by spores. The fight against wheat rust is a case in point. When- 

 ever a resistant strain of wheat is developed, it is immediately sub- 

 ject to attack by mutating strains of the rust, whether these strains 

 are of local origin or not. There is evidence that strains of rust 



