THE PLANT AND ITS ENVIRONMENT 



175 



from the rain and dew. Structural features characteristic of 

 these plants are a much thickened cuticle, protected stomata, and 

 fleshy stems and leaves. 



Sijmhiosis. — In the relationships between the plant and other 

 organisms in its environment which we have mentioned, the 

 advantage has been one-sided. There are instances, however, 

 of true symbiosis, an intimate relation between two plants, or 



Fig. 94. — Epiphytes growing on a tree-tiiink. 



between a plant and an animal, where the advantage, to some 

 extent at least, is mutual. A notable example of this is provideil 

 by the whole group of Lichens, each member of which is a com- 

 posite organism produced by the close association of a species 

 of alga and a species of fungus (Fig. 182) and in which both 

 seem to derive a certain amount of benefit from the union. The 

 mycorrhiza, or association between a fungus and the root tip of a 

 higher plant, which we have mentioned in a previous chapter, is 

 evidently another example of the same state, as is probal)ly the 

 connection, between the nitrogen-fixing root-tubercle bacteria 

 and the leguminous plants in which they live. 



Passing higher in the vegetable kingdom, wc find many instan- 

 ces of relationship between organisms which are also apparently 



