92 PLANT STUDIES 
As is well known, when a seed germinates the tip that is to 
develop the root turns towards the earth, even if it has 
come from the seed in some other direction. This earth 
influence is known as geotropism. Another directing in- 
fluence is moisture, or the water influence, known as hydrot- 
FIG. 87. A section through the leaf -stalk of a yellow pond-lily (\iiphar), showing the 
numerous conspicuous air passages (s) by means of which the parts under water 
are aerated ; h, internal hairs projecting into the air passages ; v, the much 
reduced and comparatively few vascular bundles. 
ropism. By means of this the root is directed towards the 
most favorable water supply in the soil. 
Ordinarily, geotropism and hydrotropism direct the root 
in the same general way, and so reinforce each other ; but 
the following experiment may be arranged, which will 
separate these two influences. Bore several small holes in 
the bottom of a box (such as a cigar box), suspended as in- 
dicated in Figure 85, and cover the bottom with blotting 
paper. Pass the root tips of several germinated seeds 
