THE GROWTH OF BOOTS AND STEMS. 159 



these ?) and the need for the root-system to explore the soil 

 thoroughly and to come into contact with as much soil as 

 possible, in order to do its work efficiently. 



The fact that the removal of the root-tip destroys the 

 power of the root to respond to the stimuli of gravity, etc., 

 goes far to prove that this is the sensitive region of the 

 root, but such experiments are not conclusive, because 

 curvatures due to the injury usually take place; the root 

 sometimes forms a new tip, and then responds as before. 



The fact that the root- tip alone is sensitive to the stimulus 

 of gravity has been shown by the following method: 

 Seedlings of Broad Bean are rotated on a clinostat and 

 their roots caused to grow into small L-shaped glass tubes, 

 open at one end, so that the root is sharply bent at about 

 ^ in. from its tip. They are then placed in various posi- 

 tions, each having this small glass boot on its root. If the tip 

 is vertical and the basal part of the root horizontal ( r* ), no 

 curvature takes place the radicle grows in the horizontal 

 direction. If the tip is horizontal (L), the vertical part of 

 the root outside of the tube bends until the tip of the root is 

 brought into the vertical position. 



191. Regions of the Root. From your observations on 

 the growth of roots, it will be evident that the increase in 

 length of a root is almost entirely due to the elongation of 

 the region lying a short distance behind the root- tip, also 

 that root -hairs only arise where the root has ceased to 

 lengthen, and that branches only arise still farther back 

 (why do not root-hairs and branches arise where the root is 

 still growing in length ?). The following regions may be 

 distinguished in a root : (1) growing-point, covered by 

 root-cap ; (2) elongating region ; (3) region bearing root-hairs ; 

 (4) thickening region, which may bear branches (rootlets). 



192. How is the Boot-Tip protected against injury as 

 the root pushes its way into the soil ? 



A little behind the actual tip of the root lies the growing- 

 point, which is covered and protected by the root-cap. The 

 growing-point consists of cells which are constantly dividing, 

 growing after each division, then dividing again, so as to 



