62 PLANT LIFE ON THE FAIIM. 



the soil. The special form and characteristics of the 

 root in the particular crop it is wished to cultivate tap- 

 rooted, fibrous-rooted, fleshy, surface-rooting, or deep- 

 rooting, etc. must also be taken into consideration in 

 the same relation. 



Action of Gravitation on Leaves. The tendency of 

 leaves during their growing period so to place themselves 

 that their upper surface looks to the heaven, their lower 

 to the earth, is a matter of every-day observation. 

 Scarcely less familiar are the turns and twists which the 

 leaves or 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 ''helio- 

 tropism," 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 sur- 

 face. The particular 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 counteracting the effects of gravitation, 



