208 Plant Life and Evolution 



plants like the lichens and mosses, but under more 

 favorable conditions many flowering plants and 

 ferns are found among these epiphytes. In the wet 

 tropical forest, the trunks and branches of the trees, 

 and even the surfaces of the leaves, may be covered 

 with a tangle of liverworts, ferns, orchids, and even 

 shrubs, like some of the rhododendrons and vac- 

 ciniums. As these epiphytes are largely dependent 

 upon the atmospheric moisture for their water sup- 

 ply, they are often more or less xerophytic in habit, 

 having small thick leaves, or developing special 

 water-storage organs, like the " pseudo-bulbs " of 

 many orchids, and the water-storing scales of the 

 Spanish moss and other bromeliads. A good many 

 of the epiphytes, especially ferns, collect between 

 their closely set leaf bases masses of humus, which 

 serve both to hold moisture and to provide nourish- 

 ment for the roots which ramify through the humus, 

 and absorb nourishment from it. 



Light Not Always Necessary. While light is a 

 necessary factor for the growth of all green plants, 

 it is not essential for the existence of many forms 

 without chlorophyll. Thus many bacteria normally 

 live in complete darkness, and certain of the organs 

 of green plants, especially the subterranean parts, 

 develop in darkness. 



Light exercises a powerful effect upon the devel- 

 opment of many organs which are not connected 

 with photosynthesis. The fruiting bodies of some 

 fungi are not perfectly developed except in light, 



