68 PLANT LIFE ON THE FARM. 
to produce tubers ; and it is clear, from the position and 
direction of the branches of a tree, that the influence of 
gravity, direct or negative, varies greatly in different 
cases, so that on the whole it is probable that the direc- 
tions in question are more especially due to varying de- 
grees of intensity of growth in different situations, accord- 
ing to local necessities and the action of light, than to 
gravitation pure and simple. 
Influence of Light on Stems,— The remarks made 
under the corresponding heading in regard to leaves, 
apply with the necessary modifications to stems. The 
stems have often a marked tendency to move or grow 
towards the light, but the opposite tendency is shown in 
other instances, as in the ivy, the runners of the straw- 
berry, and other cases, where this peculiarity favors the 
application of the stem to the surface of the ground, of 
a wall, or of any means of support, as in many climbing 
plants. 
The action of light in retarding growth, already referred 
to, seems opposed to many of the phenomena just recorded 
—such as the bending of the stems towards the light, 
the fact that stems grow by day as well as by night, the 
circumstance that the tissues of plants grown in the dark 
are feeble and fil-developed. These apparent contradic- 
tions may be explained by the fact that the retarding 
influence of growth, which is so manifest when the plant 
is grown under artificial conditions, when the influence 
of other agencies is prevented or excluded, is compensated 
for or overcome by other agencies—temperature, moisture, 
etc.—when the plant is grown under natural conditions. 
Again, what is called the ‘after effect” has to be con- 
sidered—the facilities for growth afforded by the absence 
of light, by the agency of heat, or other forces, may con- 
tinue after those influences have ceased to act, and so a 
plant may grow for a time under adverse influences by 
