328 STRUCTURE AND LIFE HISTORIES 
monly restricted to those cases where there is no phys- 
iological or nutritional relationship between the two. A 
vine climbing up a tree is an epiphyte, as are also the 
Pleurococcus and mosses growing on the tree trunks. Epi- 
phytism is specially common in the tropics where orchids, 
FIG. 235. An epiphytic Clusia. (From photo by G. V. Nash, 
taken in Haiti.) 
ferns, hohenbergias, and great lianas (vines) are found 
growing in profusion on other plants (Figs. 234, 235, and 
236). 
308. Mutualism. The associating together of two 
plants in intimate physiological relationship, to their 
