42 THE VEGETATIVE ORGANS 



attracted by water is called hydrotropism (Gr. hydor 

 = water). Roots are attracted to, and grow best in, the 

 moist layers of the soil ; and the roots of trees and shrubs 

 may be drawn considerable distances in the direction of 

 a suitable water-supply. A dry soil retards root growth. 



In this connexion it is interesting to note how the 

 distribution of roots below ground is related to the shoot 

 system above ground. The leaves of some plants are so 

 placed on the stem that drops of rain falling on them are 

 directed towards the stem ; the drainage from the leaves, 

 therefore, is concentrated on that part of the ground 

 immediately round the stem. Such plants have commonly 

 a long tap-root but no wide-spreading root-branches. In 

 other cases the leaves are so arranged as to throw the water 

 from their tips and the water falls over a larger circle to 

 the outside of the plant. The watershed of such a plant 

 is therefore a large one, and on examining its root-system 

 we find that the young absorbing branches spread exten- 

 sively, and collect water from a corresponding area. 



The stimulus of light. Heliotropism. If we observe the 

 roots of the Ivy we shall find that they very generally 

 turn towards the surface along which the plant is growing. 

 This will commonly be the moister surface, and hence we 

 have a case of hydrotropism. But it is equally true that 

 they are growing towards the shady side and away from 

 the light. That roots can be tempted to grow from the 

 usually exposed surface of the Ivy stem can be shown by 

 the simple experiment of keeping a portion covered with 

 a wet cloth ; the roots will soon be seen to grow under 

 the cloth. 



Suppose, however, that seedlings are grown on gauze 

 over a jar of water so that the roots hang downwards 

 into the water. If we now exclude light from the jar at 

 every point except a narrow slit on one side, through 

 which a strong light can be admitted, the growing roots 



