62 PLANT LIFE ON THE FARM. 
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 ®f 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, 
