66 PRINCIPLES OF PLANT CULTURE 



from injured stems of grape vines and some other plants 

 in spring (78), and from wounds in the trunks of some 

 trees in summer. Hales found the absorbing force of the 

 roots of a grape vine equal to the weight of a column of 

 mercury 32| inches high. 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 when 

 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 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). 



102. Parts that absorb. — Only the youngest parts of . 

 roots are active in absorption. The part from which the 

 root-hairs have perished absorbs little water, but is chiefly 

 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 treat- 

 ment of the plant should, therefore, be aimed at pro- 

 moting the formation of root-tips. In other words, we 

 should encourage root branching. Root branches must 

 not be confounded with root-hairs. In Fig. 28, branches 

 of the roots appear at e, e, e. The branches bear root- 

 hairs when of sufficient length, but root-hairs never develop 

 into branches. 



