THE FORMATION OF THE ROOT SYSTEM 29 



again the position of the water determines this direction. 



The behaviour of the root thus shows it to possess 

 certain tendencies which are based upon inheritance of 

 the accumulated experience of the race to which it 

 belongs, but which are controlled by certain sensitivities 

 which are its own personal possession. These sensi- 

 tivities are no doubt hereditary also. 



The power of appreciating the influence of these 

 various stimulating influences has been found to be 

 confined to a very small region of the root, extending 

 about one-tenth of an inch from the apex. This region, 

 which may be called the root tip, may consequently 

 be regarded as a rudimentary sense organ. There 

 is, however, nothing in its structure to mark it off 

 from the region further back. The part receiving 

 the stimulus is not the part which becomes curved 

 in the act of responding. The latter is the region of 

 active growth, where the cells are undergoing elonga- 

 tion. The cells at the tip only retain their sensitiveness 

 for a short time. When new cells are formed in front of 

 them in the process of elongation these are found to be 

 sensitive, and the original ones, passing into the region 

 of active growth, lose the power of appreciating stimula- 

 tion. There is thus no permanent sense-organ in the 

 root. The protoplasm is sensitive at some particular 

 stage of its development, and, having passed that stage, 

 loses its power of appreciating these stimulating changes. 



The way in which the stimulus received at the tip 

 causes a modification of the growth of the cells some 

 little distance farther back is not at present understood. 

 Something in the nature of a nervous impulse is thought 

 to be transmitted from the one region to the other, 

 passing along the delicate threads of protoplasm which 

 extend through the separating walls of the cells and put 

 all the cells in communication with one another. 



We noticed in studying certain seedlings, especially 



