74 THE VEGETATIVE ORGANS 



is it performed ? Do changes in any of the conditions affect 

 the behaviour and growth of the shoot ? Are the differences 

 we find in the form and structure of plants growing in 

 different habitats correlated in any way with the differences 

 in their environment ? These are problems we must now 

 endeavour to solve. 



CHAPTER VII 



WORK OF THE SHOOT 



We have seen in the experiments with germinating seeds 

 that the main shoot grows upwards towards the light, in 

 the direction opposite to that of the main root, the stimuli 

 producing this result being gravity and centrifugal force. 

 We must now extend our knowledge by making one or two 

 further observations with older plants. 



Perception of and response to stimuli. Lay on its side 

 a plant of Geranium (Pelargonium) or Balsam in a hori- 

 zontal position, as in Fig. 43. Note at intervals the beha- 

 viour of both stem and leaves (a-e) ; the stem-tip turns 

 upwards and continues to grow vertically. Take another 

 actively growing plant, lay it on its side for an hour, then 

 place it in a normal upright position and observe its mode 

 of growth. Does the tip show signs of turning while lying 

 horizontally ? Does any subsequent bending take place ? 

 We see that the stem, did not bend during the short time it 

 lay in a horizontal position, but bending does occur later, 

 even after the plant is placed upright. This experiment 

 shows that a stimulus was received, though response was 

 not immediate ; and it is clear that the stimulus persists 

 for a time, as bending occurs even in the altered position of 

 the plant. 



