GEOTROPISM 125 
ivy (Hedera helix). This is negative phototropism. 
Too great intensity of light may cause a positively 
phototropic organ to become negatively phototropic. 
A very small amount of light scarcely perceptible to 
the human eye is sufficient to induce phototropic cur- 
vature in some plants. The effective rays of light 
are those of the blue and violet portion of the spec- 
trum. The perceptive region may be some distance 
from the region of curvature. ‘Thus in the seedlings of 
oats the tip of the first leaf is the perceptive region while 
the curvature takes place at a point near the ground. 
186. Geotropism. If a seedling that usually grows 
upright be placed in a horizontal position for a few hours 
the tip of the stem will be found to be curved so as to be 
directed upward, while the tip of the root will have 
assumed a position directed downward, the remainder 
of the stem and roots being horizontal. If the root tip 
and stem tip have been previously marked with cross 
lines at equal distances it will be found that the curvature 
begins and is carried out by those regions of stem and root 
respectively where the growth is usually most rapid and 
the curvature has taken place by the more rapid growth 
on one side than on the other. The main root, then, is 
positively geotropic and the stem negatively so. 
187. If the plant has been allowed to grow until 
horizontal lateral roots have been produced and then is 
placed with the main stem horizontal it will be found 
that not only does the main stem curve upward and the 
main root downward, but that the lateral roots, which 
are now of course some of them directed upward and some 
downward, will curve so as to occupy a horizontal position 
again. Thus it is apparent that some stimulus causes 
certain plant parts to grow toward, other parts away from 
and still others parallel to the surface of the earth. Care- 
