SOCIAL LIFE AMONG PLANTS 7 



epiphytically and form sharply circumscribed communities (dependent 

 communities). As leaf epiphytes (epiphylls) a number of Hchens and 

 liverworts gather on the leaf surfaces of tropical plants. 



Parasites and epiphytes, more especially the former, often show a 

 specific selection of hosts and are frequently confined to one particular 

 species. 



Much feebler is the relationship which binds together the repre- 

 sentatives of the third group of dependents: humus plants, climbers, 

 and plants requiring protection. The term "humus plant" is here 

 used broadly and includes not only the true non-chlorophyll sapro- 

 phytes and green hemisaprophytes but also those humicolous species 

 which are clearly dependent upon raw humus. 



As with parasitism and epiphytism, there is a whole series of 

 gradations in the adjustment to saprophytism and more or less strict 

 dependence of species. All plant groups from bacteria, algae, and fungi 

 to ferns and seed plants include saprophytic species. There are 

 saprophytes which seem adapted to the utiUzation of specific organic 

 plant wastes and therefore are dependent in this way upon certain 

 plant species (c/. Romell, 1921, p. 207). 



Humus plants are often confined to the decomposition products of 

 certain plant communities or groups of communities (deciduous forest 

 litter, coniferous forest litter). They participate in high degree in the 

 development of community life. 



Climbers (lianas in the broadest sense) are more independent of 

 their supporting plants, since they respond only to the need for support 

 or for better illumination, by attaching themselves to taller growing 

 plants. So also with plants requiring protection. Under this head 

 may be included all plants which are benefited by the protection of 

 other plants or plant communities. They are not a few; sometimes 

 they unite into ecologic units. Protection from Hght is a requirement 

 of the shade vegetation of the lower layers of forests. If the tree layer 

 is removed, many species succumb to the increased intensity of the 

 light. Upper layers also furnish protection from cold, since they 

 reduce the radiation by night and thus keep the minima above those of 

 the surrounding open country. On exposed ridges every little shrub or 

 tussock gives protection from wind; patches of shrubs or trees are 

 evidently much more protective. On the wind-swept slopes of the 

 Colorado Rockies, at 3,500 m., deformed specimens of Pinus aristata 

 shelter colonies of delicate mesophytes like Polemonium pulcherrimum 

 (Fig. 2). The sugar-loaf dunes of Argania sideroxylon of the wind- 

 swept coast of West Africa south of Agadir harbor many wind-shy 

 companions which without the shielding of the larger plants could not 



