5 
life, and one very difficult of explanation, is the fact that 
when the tap root or the main stem of a plant is destroyed, 
a lateral branch will take its Page and assume the vertical 
position. The importance of this position is self evident 
when we consider the functions of the root and of the 
stem.’ The former acts as the absorbing organ, extracting 
from the soil the water and valuable salts necessary for 
the growth of the plant; it is obviously essential therefore 
that the root should grow downwards in search of moisture. 
It is also important that the lateral roots should not grow 
in the same direction as the main root, so that they can 
search out other regions of the soil in their quest for food 
material. The spreading habit has the further advantage 
that it anchors the plant more firmly in the soil and pre- 
vents it from being easily uprooted by the wind. 
The stem on the other hand growing away from the 
soil is in an advantageous position for exposing the leaves 
it bears to the full rays of the sun and thus enabling them 
to fulfil their main function in the life of the plant. This 
function is to absorb as much light as possible and by con- 
verting the light rays into energy to build up the organic 
material upon which the formation of flowers and fruits 
depends. ti ht being therefore of such vital importance 
to plants we find that in the course of their evolution they 
have acquired the power of responding in their growth to 
the stimulus of light. It is a familiar fact that stems of 
plants will bend towards the light if instead of being 
illuminated from all sides they receive the light from one 
side only. Growing in the open the main stem is erect 
and the leaves are usually expanded horizontally, that is 
at right angles to the direction of maximum illumination 
which is from above. When shaded on one side the stem 
inclines towards the light and the leaves are placed 
obliquely, often indeed horizontally, when a plant is grown 
in a window, in which case it is more or less shaded on 
three sides. Roots as they are underground do not nor- 
mally exhibit in their growth any response to light, but if 
a young seedling plant is grown with its roots in a glass 
jar it will be found that if illuminated from one side only 
the roots will bend away from the light. In nature this 
can be observed in the roots which grow out from the stem 
of the ivy when clinging to a wall. They are: generally 
roduced from the shaded side of the stem and always 
ee away from the light and towards the wall. These. 
