62 PLANT LIFE ON THE FARM. 



their stalks make to right themselves when by any means 

 their normal position is interfered with. At first sight it 

 would seem that these movements must be due rather to 

 the influence of light than of gravitation ; but as they take 

 place in darkness as well as in light, and as they do not 

 take place when plants are so grown as to be exempt from 

 the influence of gravitation, it is clear what the true cause 

 of these movements is (Van Tieghem). 



Action of Light on Leaves Heliotropism. The 



chemical changes which result from the exposure of the 

 leaves to light have already been alluded to under the head 

 of nutrition. It remains here to mention the power that 

 they have of turning to the light, now called " heliotropism," 

 and especially of so placing their upper surface as that it 

 shall form a right angle to the direction of the light. It 

 had been surmised that the horizontal position of leaves, 

 and especially the position with regard to the direction of 

 light, was due to the conjoint action of gravitation of 

 geotropism, of heliotropism, and of the greater relative 

 force of growth on one or the other surface. The par- 

 ticular direction assumed by the leaves was supposed to 

 be due to the balance between these forces ; but by means 

 of experiments made with a view of annulling or counter- 

 acting the effects of gravitation and of unequal growth, 

 Mr. Francis Darwin has shown that the power which leaves 

 have of placing themselves at right angles to incident light is 

 due to a special sensitiveness. This sensitiveness is capable 

 of regulating the action of other forces, whether external 

 to the plant, as that of gravitation, or internal, such as that 

 controlling the direction and amount of growth. The move- 

 ments of the leaves towards the light are different from 



