BIOLOGICAL FACTORS 193 



community structure in comparison with the drastic changes that 

 may result from infestation with pathogenic fungi or bacteria. 



Epiphytes.— These include a wide variety of plants, all of which 

 depend upon larger plants for physical support only. Algae, fungi, 

 mosses, liverworts and lichens may be found growing on bark or, 

 in some instances, even on leaves. Often their occurrence seems 

 correlated only with the general humidity of the atmosphere in 

 particular habitats, but they are frequently associated with certain 

 communities and not with others, and, within a community, they 

 may be distributed systematically. Some may grow only on the 

 bark of certain trees and, even more specifically, only in patterns 

 related to drainage of water down that bark. 200 Others may be 

 found only at the base, middle, or top of a tree trunk, and this may 

 be correlated with moisture content of the bark. 23 The occurrence 

 of the moss Tortula pagorimi 8 illustrates how specific a habitat 

 may be required by some epiphytes. This moss has been found 

 only in close proximity to man's habitations and then almost exclu- 

 sively on the trunks of elm trees. The epiphytic lichens associated 

 with evergreen forests of boreal and alpine regions are distinctive 

 and characteristic. 



In and near the tropics, higher and less variable humidity per- 

 mits a greater variety of epiphytes to survive, and vascular species 

 increase. In temperate regions, drought-resistant species, such as 

 polypody ferns, are found occasionally, but farther south, first on 

 swamp trees only and then almost anywhere, epiphytic vascular 

 plants become the rule. Orchids, bromeliads, and ferns are espe- 

 cially abundant. Structures that catch or conserve water are char- 

 acteristic of many of these species. Stratification at different levels 

 in the forest, as controlled by light, air movement, and water sup- 

 ply, is common, and succession of epiphytic communities may be 

 observed as organic "soil" is accumulated. 181 Occasionally their 

 weight may increase sufficiently to break down the branches sup- 

 porting them. Such massive growths as are produced by the well- 

 known Spanish "moss" (Tillandsia) of the southeastern United 

 States must reduce the normal foliage and its functioning (see Fig. 

 8). In general, however, the epiphytes and their "hosts" seem sur- 

 prisingly well adapted to their relationship. 



Symbiosis.— The most generally accepted concept of symbiosis 



