5 



life, and one very difficult of explanation, is the fact that 

 when the tap root or the mam stem of a plant is destroyed, 

 a lateral branch will take its place 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 mam root, so that they can 

 search out other regions of the soil in their quest for food 

 material, ihe spreading habit has the further advantage 

 that it anchors the plant more firmly m 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. Light 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 

 produced from the shaded side of the stem and always 

 bend away from the light and towards the wall. These 



