28 LIVING PLANTS 



of roots 



portion of the root. In order to reach such 

 irregularly distributed masses of nutritive 

 substances, it is evidently necessary that the 

 root should develop an irritability to a much 

 greater number of forces than any member of 

 the shoot, and furthermore it is evident that 

 all theforms of irritability thus acquired must 

 be located in the apical portion of the root, 

 Organization ^^le proper directive activity- of v^hich only 

 can result in facilitating the absorptive pro- 

 cesses. The coincidence of several forms of 

 irritability within such narrow limits has ne- 

 cessitated differentiations in another direction 

 from that offered by the shoot. The differen- 

 tiation of the shoot resulted in a tendency to 

 separate the different forms of irritability 

 with their attendant mechanisms. The in- 

 crease of the efficiency of the root has resulted 

 in the acquisition of a constantly increasing 

 number of forms of which the mechanism 

 must necessarily be identical. Still further, 

 this has resulted, of course, in the differentia- 

 tion of the separate parts of the mechanism, 

 and increase of its delicacy of reaction. This 

 may be held to apply to all similar arrange- 

 ments, especially in the ecological adaptations 

 exhibited by certain members of the shoot. 



Thus if an examination of the mechanism 

 of irritabilit}? of the root is made, it will be 

 found that only an extremelj^ small portion 



