ABSORPTION OF WATER 185 



growths are most easily produced, and after re-planting her- 

 baceous plants exposure to a dry atmosphere, or to strong light 

 and other influences which promote loss of water from the leaves 

 by transpiration (see chap, xiv.), should be avoided for a time 

 wherever possible. 



The osmotic absorption of water by the root-hairs of plants only 

 goes on when the following conditions are fulfilled, namely : — 



(i) A certain degree of warmth of the surrounding soil; 

 (ii) Access to fresh air ; and (iii) A suitable supply of water. 



Cabbages and many other plants are able to absorb consider- 

 able amounts of water at freezing-point, but at the low tempera- 

 tures of winter absorption generally ceases or is vastly decreased 

 and it is not until the return of warm days in spring that the 

 activity of the roots is manifest. 



The application of water from wells to the roots of tropical 

 and sub-tropical plants growing in pots in warm houses frequently 

 checks their absorptive power by lowering their temperature 

 considerably. 



Sachs showed that the absorption by a tobacco plant at a 

 temperature of 4° or 5° C. was so small that withering com- 

 menced in spite of the fact that the roots of the plant had 

 access to an abundance of water. 



In consequence of the presence of a considerable amount of 

 water which requires much heat to warm it, the temperature of 

 imperfectly-drained soils is usually lower than that at which the 

 roots of ordinary farm and garden plants do their work best. 

 Moreover, such soils do not allow of the free circulation of fresh 

 air within them, and the respiration process carried on by the 

 living protoplasm of the root-hairs is interfered with. 



Without the access of an adequate supply of oxygen, or where 

 there is much carbon dioxide in the soil, poisonous compounds 

 are formed within the roots as the result of imperfect respiration 

 and the plants become unhealthy. Over-watered plants growing 

 in pots commonly exhibit injuries of this character. 



