GERMINATION AND GROWTH é 51 
chanical effect of gravity, due to weight of parts, as when the 
bough of a fruit tree is bent under the load of its crop, but 
a certain stimulus to which the plant reacts by a spontaneous 
adjustment of its growing parts. In other words, geotro- 
pism is an active, not a passive function, and the plant will 
overcome considerable resistance in response to it. (Exp. 38). 
54. Other factors. — The direction of growth is influ- 
enced by many other factors, such as light, heat, moisture, 
contact with other bodies, @ and perhaps by 
electricity. The result of all these forces is an 
endless variety in the forms and growth of 
organs that seems to defy all law. 
Heat, unless excessive, gen- erally stimulates 
‘growth; contact sometimes stimulates it, 
causing the stem to curve away fromthedis- 
turbing object, and sometimes retards it, causing 
the stem to curve toward the object of contact 
by growing more rapidly on the opposite side, 
Fic. 70.—A piece of a haulm of millet that has been laid horizontally, righting 
itself through the influence of negative geotropism. 
as in the stems of twining vines. Light stimulates nutrition, 
but generally retards growth. The movements of plants 
toward the light are effected in this way; growth being 
checked on that side, the plant bends toward the light. 
Practical Questions 
1. Why do stems of corn, wheat, rye, etc., straighten themselves after 
being prostrated by the wind? (51, 54.) 
2. Do plants grow more rapidly in the daytime, or at night? (54.) 
3. Reconcile this with the fact that green plants will die if deprived 
of light. 
