. CELL-COXTEXTS AXD FORAIS OF CELLS. 251 



bases, represented by histidine and arginine ; 7, pyrimidine deriva- 

 tives, represented by cytosine ; 8, purine bases, represented by 

 xanthine and hypoxanthine ; 9, pyridine derivatives, represented 

 by picoline carboxylic acid.* 



Root Absorption. — Notwithstanding the various agents 

 which are at work tending to break down and dissolve the sub- 

 stances contained in the soil, as soil bacteria, the liquids given to 

 the soil by the roots of the plants themselves, the presence of the 

 so-called humic acids, and the action of water and air, it has been 

 shown that the soil water is an exceedingly weak solution. This 

 is largely due to the peculiar absorptive and fixing power of the 

 soil itself. 



The dilution of the aqueous solution of the soil constituents 

 is a matter of very great significance, for upon this depends its 

 absorption by the root hairs. While other parts of roots have cer- 

 tain' absorptive powers, the root hairs have been defined as the 

 organs of absorption of the plant. They are very delicate in 

 structure and contain protoplasm. Their absorbent function de- 

 pends upon the principle that when a membrane ( animal or 

 vegetable) is interposed between two liquids of unequal density, 

 the less dense liquid will pass through the membrane and mix 

 with the denser liquid. This process is known as osmosis, and 

 when a liquid passes outward through a membrane or cell-wall 

 it is called exosmosis, and when inward it is called endosmosis. 

 The soil is made up of minute earth particles, each of which is 

 surrounded by a thin film or envelope of water, and it is this por- 

 tion of the soil liquid that is absorbed by the root hairs. The root 

 hairs come into close contact with these soil particles ; in fact, 

 appear to grow fast to them, and the cell-liquid in the root hairs 

 being denser than that surrounding the soil particles, the latter 

 passes through the wall into the root hairs. 



If, on the other hand, the water supplied to the roots of plants 

 should contain an excess of soluble material, the plant will be 

 injured. In this case exosmosis ensues and the plant loses some 

 of its own liquids or cell-sap and will show signs of wilting. It 

 is well known that if cultivated plants are supplied with strong 

 solutions of fertilizer the plants will be injured rather than 

 benefited. 



