ROOTS AND ROOT-HAIRS. 43 



If we have a leaf)' and healthy plant, with 

 roots, bearing numerous root-hairs, properl}' 

 established in suitably moist soil in the pot, the 

 roots cease to absorb water if the temperature of 

 the soil falls below a certain minimum, though 

 they recommence to do so if the temperature is 

 raised again : this has nothing to do with the 

 temperature of the upper parts of the plant, or of 

 the air, and the latter may be so high that the 

 plant rapidly droops from loss of water at the 

 leaves, which is not being compensated owing to 

 the inactivity of the roots. 



Similarly we may have the air so cold, at a 

 time when the soil is warm enough to keep the 

 root-hairs actively at work, that the plant becomes 

 surcharged with water, which escapes from the 

 leaves like drops of dew. The temperatures 

 necessar)^ to cause these disturbances in the action 

 of the living root-hairs vary for different plants, 

 and even for different varieties of the same 

 species. 



Similar arrestation of the functions of the roots 

 may be brought about by removing the oxygen 

 from the soil around the root-hairs, and replacing 

 it by carbon-dioxide, or the vapour of chloroform. 

 If not kept too long in such a condition, the plant 

 recovers rapidly on admitting atmospheric oxygen, 

 which is always present in a normal well-drained 

 soil both as gas in the capillary interspaces, and 

 dissolved in the water on the surfaces of the 

 particles. If the access of oxygen is delayed, 

 however, as often happens in rainy seasons and in 



