7 6 



THE VEGETATIVE ORGANS 



A few trials will enable you to determine the time required 

 for the stem to perceive the stimulus, and also that required 

 for curving. If a plant is fixed in a klinostat (Fig. 44), and 

 caused to revolve in a horizontal position, the stem, like 



the root, does not curve, 

 since it receives the stimulus 

 equally on all sides. 



Force exerted by a growing 

 stem. Just as roots exert 

 much force in their down- 

 ward growth in response to 

 the stimuli of gravity and 

 centrifugal force, so in the 

 reverse direction do shoots, 

 and their lifting power is 

 considerable. Obtain a 

 spring-balance, attach a 

 weight, and arrange it as in 

 Fig. 45, over the stem of a 

 Bean seedling so that the 

 weight is raised as the stem 

 elongates. Fix the weight 

 firmly to the hook so that 

 it is not readily tilted, and 

 determine the lifting power 

 of the shoot. In the experi- 

 ment illustrated, the shoot 

 in three days supported a weight of seven ounces. 



Stimulus of light. Heliotropism. Consider next the 

 heliotropism of the shoot. 



Place a plant in a window so that light falls on it on one 

 side only, and note the behaviour of both stem and leaves 

 (Fig. 46) . In what direction has the stem turned ? Which 

 is the longer side of the stem, the one facing the light or the 

 one which has grown in the shade ? Has the light hastened 



Fig. 45. Experiment to de- 

 termine the Lifting Power of 

 a Shoot. 



