946 ECOLOGY 



tion above the water level reaches its maximum, and on this account the 

 soil is more uniformly moist than in other land habitats. Furthermore, 

 the rich supply of humus makes possible a wealth of saprophytic fungi 

 and bacteria, leading to mycosymbiosis and to other symbiotic relations. 



The most luxuriant of mesophytic forests is the rain forest of the 

 tropics, which is characterized by the dense crowding of individual 

 plants, resulting in the maximum occupation of space. Not only are 

 there ordinary trees, shrubs, and herbs, but lianas often are abundant, 

 while epiphytes cover the limbs and even develop on the leaves of many 

 trees. The trees often are slender and smooth-barked, and the leaves 

 are characteristically evergreen. The epiphytes include ferns and 

 orchids, the latter with characteristic absorptive roots and xerophytic 

 leaves or stems ; the leaves of many of the trees appear xerophytic also. 

 In north temperate regions there are extensive mesophytic forests that 

 either are deciduous, as in the eastern United States, Europe, and Japan, 

 or evergreen, as in the northwestern United States. In these forests the 

 tree species are relatively few in number, as compared with the tropical 

 forests, and the trees often are large and rough-barked. The epiphytic 

 vegetation consists chiefly of lichens, liverworts, and mosses. Fre- 

 quently mesophytic areas are treeless, as in some prairies and in alpine 

 meadows, where grasses and herbage dominate the landscape. 



The influence of man upon vegetation. Man is the most destructive 

 of animals. He has cleared vast tracts of forest for lumber, and for the 

 building of cities and the development of farms, and has destroyed other 

 tracts through forest fires. Man also is responsible for distributing 

 through the world most of the "weeds " which burden the farmer and 

 throng the roadsides. Such plants as the Russian thistle, cocklebur, 

 burdock, and Canada thistle once were somewhat restricted in area, 

 and they owe their present widespread distribution directly or in- 

 directly to man. Plants of this sort that inhabit fields and waste 

 places are known as ruderals. Often there are ruderal associations, such 

 as those that develop on cultivated land that is left fallow. The pioneer 

 associations that follow in man's destructive train, such as the ruderal 

 associations of fallow land or the " fireweed " associations of a burned 

 forest tract, usually are comparable to pioneer associations of xerophytic 

 tracts, and often they contain xerophytic species. If man leaves such 

 areas to their natural course, there is a succession of associations com- 

 parable to those previously noted, culminating finally in the plant asso- 

 ciation that originally dominated in the region. 



