ABSORPTION OF WATER. 



normally epiphytic plant be caused to develope first in soil, 

 it rapidly perishes if it is subsequently exposed to the air. 



FIG. 11 (after Unger). Transverse section of part of the air-root of an Orchid: 

 v, the velamen ; c, the cortex. 



The roots of parasitic plants penetrate the tissues of their 

 hosts and fuse with them ; in this way a communication is 

 established by means of which the parasite obtains its sup- 

 plies of water and of substances in solution. 



A few words may be said here with reference to the dis- 

 tribution of the roots in the soil. It has been already pointed 

 out that roots, as a rule, branch freely, and that in conse- 

 quence of their continuous apical growth they are always 

 entering new areas of soil from which the plant can obtain 

 fresh supplies of water and of nutritive substances. Their 

 distribution may, however, be materially affected by the 

 nature of the medium into which they penetrate. If a root 

 passes out of soil into water it becomes excessively elongated, 

 but if it enters a moderately strong solution of some salt this 

 rapid growth in length is checked, as Sachs first pointed out. 

 Nobbe studied this subject in detail, and found that the roots 

 of Barley and of Buckwheat attained their greatest develop- 



42 



