FUNCTION OF THE ROOTS 9 



naturally regulated. Transpiration takes place chiefly 

 in light; it will occur abundantly in an atmosphere 

 saturated with water, if the plant be only exposed to 

 sunshine. The amount of water evaporated from the 

 surface of a growing plant is very large. Land that 

 has lately borne a crop is always much drier than a 

 bare fallow (p. 27). 



The results of transpiration are most important, the 

 evaporation of water from the leaves being a principal 

 cause of the rise of the sap, and the consequent drawing 

 up of water from the soil containing plant food in 

 solution. 



Function of the Roots.— The roots of a plant are the 

 organs by which it absorbs water from the soil, and with 

 this water a variety of food substances are introduced. 



1. Assimilation of Ash Constihients. — The roots take 

 up the soluble salts, and, indeed, all the diffusible sub- 

 stances (those capable of passing through a membrane) 

 which are present in the water which they draw from 

 the soil. The plant will thus frequently receive more 

 of some substances than is actually required for its 

 nutrition. 



The feeding power of roots is not, however, confined 

 to the taking up of ready-formed solutions, they are 

 also capable of attacking some of the solid ingredients 

 of the soil, which they render soluble and then appro- 

 priate. This important action of roots exists in different 

 degrees in different plants. The action takes place 

 only at the points of contact between the root-hairs 

 and the particles of the soil, and is brought about by 

 the acid sap which the roots contain. This action of 

 the roots plays an important part in the supply of 



