ROOTS AND THEIR USES 



183 



of trees in tropical forests, where the air is always moist. 



Evidently there is not likely to be much soil nor a large supply 



of water in such places, although some water may accumulate 



in the crotches of the trees and in the hollows of the bark. 



Small amounts of dead material, such as leaves and parts of 



branches, may be caught also among the branches and roots 



of the epiphyte, and by decaying 



there may supply on a small scale 



the substances that other plants 



obtain from the soil. Evidently the 



great problem for an epiphyte is to 



make the best possible use of such 



water and other food materials as 



may come to its roots. So we find 



that the roots of many epiphytes 



are especially well supplied with 



absorbing root hairs. The roots of 



some epiphytes, notably of tropical 



orchids, are surrounded by a special 



tissue composed of several layers of 



cells whose structure enables them 



to absorb rain water and dew very rapidly and in large 



amounts and to pass the water on to the inner cells of the 



roots. 



204. Direction of Growth of Roots ; Influence of Gravity. 

 — If a young pea, bean, or com seedling is removed from the 

 soil in which it has started to grow and laid upon its side, the 

 root soon begins to curve in such a way that finally its tip 

 is turned directly downward. The curvature takes place a 

 short distance back of the tip, in the region where the cells 

 of the root are growing in length. Growth is faster on the 

 side of the root turned upward than on the side turned down- 

 ward, and it continues so until the tip is vertical. Plainly 

 it is of advantage to a plant to have its root grow downward 

 into the soil. But the fact that the plant is helped by this 



Fig. 115. — An epiphytic 

 orchid growing on the bark 

 of a tropical tree; notice 

 the abundant growth of 

 roots. After Kerner. 



