The Root and the Soil. 73 



soil particles. Fig. 30 shows a magnified root-hair of 

 the wheat plant, closely attached to some particles of 

 soil. The root-hairs are able to take up water freely, 

 even from soil that does not appear very wet, because 

 each soil particle is enveloped in a thin layer ot water 

 (90). Still more interesting is the fact, that root-hairs 

 are able to dissolve mineral matters in the soil, by their 

 excretions, most important of which is carbonic acid, 

 thus permitting the plant to use these matters as food. 

 101. Root-Hairs Absorb Water with considerable 

 force. It is the absorptive power of the root-hairs that 

 causes water (sap) to flow so freely from injured stems 

 of grape vines* and some other plants in spring, and 

 from wounds in the trunks of some trees in summer. 

 This force is probably due to the absorptive power of 

 the protoplasm in the very active young root cells. It 

 is affected by the temperature of the soil within cer- 

 tain limits, lessening as the temperature falls, and in- 

 creasing as it rises. Sachs found that the foliage of 

 plants of tobacco and pumpkin drooped when the tem- 

 perature of the soil in which they were growing was re- 

 duced much below 55° F., showing that the roots did 

 not absorb enough water at that temperature to com- 

 pensate for the loss by transpiration (74). When the 

 soil is warm, on the other hand, the absorptive power 

 of roots may be sufficient to force water from the tips 

 of leaves during cool nights when transpiration is 

 slight (62). 



* Hales found tlie absorbing force of the roots of a grape vine equal 

 to the weight o£ a column of mercury thirty-two and one-half inches 

 high. 



