58 BOTANY 
and henee it is reasonable to suppose that gravity is 
the cause of these phenomena. 
An important practical detail must be observed in 
using the apparatus. The jar soon becomes filled with 
carbon dioxide given off by the germinating seeds and 
thus growth is checked. If, however, the experi- 
menter is careful to uneork the jar and turn it upside 
down for a few seconds each day at the time of winding 
the clock this difficulty will not arise. If it were not 
that evaporation on the surface of the blotting paper 
would cause the seedlings to shrivel, there would be 
no need for the jar at all. Indeed, there is a form of 
apparatus, in which, by allowing the seeds to dip, at 
the lowest point of their course, into a dish of water, 
the jar may be dispensed with altogether. 
Hydrotropism (Gk. hydor water). Gravity is not 
the only stimulus or influence to which the root 
responds. It is obvious that the geotropic habit of 
the root adapts it to the gathering of the minerals and 
moisture which are present in the earth. Since the 
deeper regions of the soil usually contain more 
moisture, it is, as a rule, an advantage to the plant 
for the root to travel there. But it is clear that there 
will be cases where, as in the neighbourhood of a 
drain or a small spring, water is available without 
the customary boring. In such eases it would appear 
that there is competition between the pull of the water, 
and the pull of gravity. The following experiments 
will illustrate this. 
Sow bean seeds in a box of moist earth in which 
the bottom is formed of fine wire netting. Tilt the 
box as shown in the figure, and cover with a dark 
cloth to exclude the influence of light. It will be found 
that the roots, after protruding through the wire, turn 
back again into the moist earth (Fig. 37). The 
impetus derived from gravity was sufficient to carry 
them through the wire, but gravity could not continue 
