INFLUENCE OF ENVIRONMENT 277 



branches of trees, and sending down roots which finally 

 reach the ground. These roots as they increase in 

 number and size finally entirely envelop the trunk 

 of the tree on which the fig is growing, and at last 

 actually strangle it, so that the fig is left mounted 

 on a hollow trunk composed of the more or less com- 

 pletely joined roots. 



As true epiphytes have no root system to supply 

 them with water, and are not connected with the earth, 

 various devices have been developed for supplying them 

 with the necessary moisture and soil-constituents. 

 Many epiphytic orchids develop bulb-like enlargements 

 of the leaf-bases, which serve at once for storing food 

 and water, and may be almost completely dried up 

 during their dormant season without injury. These 

 orchids frequently have long, fleshy, aerial roots, 

 which doubtless are important agents in absorbing 

 moisture from the air. Most of the Tillandsias and 

 many epiphytic ferns accumulate vegetable mould in 

 their enlarged leaf-bases, which serve as reservoirs of 

 moisture, and the scurfy scales with which the leaves 

 of many species of Tillandsia are covered are also use- 

 ful in holding moisture. 



The various types of climbing plants may be con- 

 sidered in connection with epiphytes. Like these they 

 reach their greatest development in the moist forests of 

 the tropics, where the struggle for existence is the 

 fiercest. The development of the climbing habit is 

 doubtless associated with the competition of plants for 

 the light. In more northern regions, where vegetation 

 is less rank and the crowding not so great, fewer plants 

 show this habit, but in the dense tropical forests climb- 



