INFLUENCE OF ENVIRONMENT QT7 
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- 
