72 Principles of Plant Culture. 



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

 excretions most important of which is carbonic acid, 

 thns permitting the plant to use these matters as food. 



102. 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 certain limits, lessening as 

 the temperature falls, and increasing as it rises. Sachs 

 found that the foliage of plants of tobacco and pumpkin 

 drooped "nhen the temperature of the soil in which they 

 were growing was reduced much below 55° F., showing 

 that the roots did not absorb enough water at that 

 temperature to comj^ensate for the loss by transpiration 

 (75). 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 tran- 

 spiration is slight (63). 



103. Only the Youngest Parts of Roots are Active in 

 Absorption. The part from which the root-hairs have 

 perished absorbs little water, but is chieiiy useful in 

 giving strength to the plant and in conducting the plant 

 fluids. The absorbing part of any given rootlet is, 

 therefore, comparatively short. It follows that the 

 amount of nourishment a given plant can receive will 

 depend upon the number of its root-tips. Our treatment 



* 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. 



