The Root and the Soil. 73 



pores of the soil or enveloping the 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 par- 

 ticle is enveloped in a thin layer of 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 permit- 

 ting 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 the absorbing force of the roots of a grape vine 

 equal to the weight of a column of mercury thirty-two and one- 

 half inches high. 

 6 



