60 BOTANY 
towards the water through the moist air in the glass 
(Fig. 38). 
A more striking experiment may be made by filling 
a box with sawdust only slightly damp. In the middle 
of this have a porous pot into which water may be 
poured. A flower pot, the hole in which has been 
tightly corked, will do quite well. Now place in the 
sawdust, at varying distances from the pot, germin- 
ating beans, taking care that in each case the radicle 
is pointing downwards. It will be found that the 
rootlets will turn towards the water in the pot 
(Fig. 389), water, as already seen, exerting a stronger 
pull than gravity. 
The distances over which the roots of willow and 
other trees make their way to a stream or pond, and 
the manner in which they get into and block up the 
drains will be familiar to all. 
Heliotropism (Gk. helios the sun) is the turning 
towards the light. That the stem is positively 
heliotropic (7.e., that it turns towards the light) must 
have been observed by all who have grown plants in 
a window. This response to light on the part of the 
stem has the effect of presenting to the rays the broad 
flat surfaces of the leaves, an obvious advantage, seeing 
that it is with the light relation that the leaf is chiefly 
concerned. That the root is negatively heliotropic 
might be inferred from the fact that the adventitious 
roots on the ivy stem always turn away from the light, 
but the following experiment is more conclusive. 
Obtain a large fruit jar, cover it, except for a 
narrow vertical strip, with black paper. Now hang’ 
through a large cork firmly fixed to the top of the jar 
a bean seedling in such a way that its root is opposite 
the slit. It will be found that the root soon turns 
away from the slit and withdraws itself as much as 
possible from the light. After the root has bent. it 
