156 THE GROWTH OF ROOTS AND STEMS. 



sawdust around the flower-pot, but do not water except by pouring 

 water into the flower-pot, and filling the latter up as the water 

 evaporates. After a few days, gently pull up a seedling and see 

 whether its root has curved towards the pot. 



187. How does Light affect the Direction of Growth ? 



Have you noticed the appearance of plants growing near a 

 window? The shoot grows towards the light, the stem 

 placing its axis parallel with the direction of the light, while 

 the leaves place their surfaces at right angles to it. 



* (a) Set some seedlings in such a position that the light falls on them 

 mainly from one side, and notice the effect this has on their shoots. 

 After marked bending has been observed, turn the seedlings round and 

 see whether the shoots again respond to the "attraction" of light. 



(b) Take the wide-mouthed glass jar and cover it outside with black 

 paper or cloth, leaving a narrow vertical chink on one side. Fix a 

 Bean seedling horizontally inside the jar, at right angles with the 

 chink. Does the root bend, and does it bend towards or away from 

 the light ? In what direction does the shoot bend ? 



(c) Pass the roots of germinated Cress, Radish, or Linseed through 

 holes in muslin tied over a tumbler filled with water. Set the tumbler 

 close to a window, or in a box with a vertical slit in the side turned 

 to the light, or use a black sheet, as in the preceding experiment. 

 Observe the opposite directions in which root and shoot grow. 



188. How does Contact affect the Direction of 

 Growth ? As we shall see later, some parts of plants are 

 very sensitive to contact, e.g. tendrils. Has the root this 

 kind of sensitiveness? How does the growing young root 

 behave when it comes against an obstacle which it cannot 

 push out of its way ? The root can force itself through loose 

 soil when young and soft, and when it grows old and hard, 

 becoming thicker year by year, it can exert enormous pressure 

 against anything around it. 



* (a) To the inner side of the glass in your box with glass front fix 

 with sealing-wax small pieces of wood, and plant seeds a little above 

 these obstacles. Notice that the roots diverge from their course only 

 so much as is absolutely necessary to avoid the obstacles, and resume 

 their original course as soon as they have passed one. 



(b) Another method is to use a glass funnel filled with moist soil or 

 sawdust ; plant the seeds close to the glass near the top of the funnel, 

 and fix the obstacles an inch or more below them. 



