6 
climbing roots are not sensitive to gravity as they have 
a very definite function to perform in fixing the plant to 
the wall, and are not concerned in the absorption of food 
material. 
Wherever bending, such as described above, takes place 
in growing organs, this is due to differences in the amount 
of growth on different sides of the stem or root. The con- 
cave side grows less than the convex side. Thus when a 
stem, which has been laid horizontally, bends upwards, 
this is due to the greater amount of growth of the side 
nearer the ground. If a stem illuminated from one side 
bends permanently in that direction this is due to the fact 
that light retards the rate of growth and the side away 
from the light growing more rapidly the stem becomes 
convex on this side and bends towards the light. The 
fact that light retards growth and therefore causes plants 
to be short and “stocky” is of course a well-known 
phenomenon, while the lack of illumination acting like 
darkness causes more rapid growth and we get long 
“leggy” plants, when they are insufficiently lighted as 
when grown in the shade, in deep frames or pits, or not 
close up to the lights in greenhouses. 
There are one or two other factors influencing the 
growth of plants which it may be useful to refer to at this 
juncture. Besides being sensitive to gravity roots are 
also sensitive to contact, and when a root tip comes in 
touch with a solid body such as a stone in the soil it bends 
away from it. This is brought about as in the case of 
other bending movements, by the fact that contact causes 
retardation of growth on this side of the root and this side 
becomes therefore convex, the root-tip pointing away from 
the obstacle met with in the soil. In this way it is possible 
for the root to make its way even through a stony soil, 
avoiding or rather growing round all obstacles with which 
it may be met, bending to right or to left in its progress 
downwards. This sensitiveness to contact is however not 
only possessed by main roots which grow downwards; it 
is equally important to lateral roots. 
It has been found in the somewhat infrequent cases 
occurring in nature that where the soil is drier on one side 
of a plant than on the other the root system develops 
more abundantly in the moister soil. Experimentally, too, 
we can prove that a root will grow towards moisture just 
as the stem of a plant will grow towards the light. 
