GEOTROPISM 31 



A more accurate measurement of the force exerted by the 



i/ 



root may be made by confining it so it cannot bend, and letting 

 it push down on a spring. In this \yay it is found that the root 

 of the Windsor bean can push with a pressure of about ten 

 ounces. 



Making fine equidistant cross marks with ink along the upper 

 and the lower surface of a root that is about to bend downward 

 at the tip readily shows that those of the upper series soon 

 come to be farther apart,- -in other words, that the root is 

 forced to bend downward by the more rapid growth of its upper 

 as compared with its under surface. 



41. Geotropism. The property which plants or their organs 

 manifest, of assuming a definite direction with reference to grav- 

 ity, 1 is called geotropism. When, as in the case of the primary 

 root, the effect of gravity is to make the part, if unobstructed, 

 turn or move downward, we say that the geotropism is positive. 

 If the tendency is to produce upward movement, we say that 

 the geotropism is negative; if horizontal movement, that it is 

 lateral. It was stated in the preceding section that the direct 

 cause of the downward extension of roots is unequal growth. 

 We might easily suppose that this unequal growth is not due 

 to gravity, but to some other cause. To test this supposition, 

 the simplest plan, if it could be carried out, would be to remove 

 the plants studied to some distant region where gravity does not 

 exist. This of course cannot be done, but we can easily turn a 

 young seedling over and over so that gravity will act on it now 

 in one direction, now in another, and so leave no more impres- 

 sion than if it did not act at all. Or we can whirl a plant so 

 fast that not only is gravity done away with, but another force 

 is introduced in its place. If a vertical wheel, like a carriage 

 wheel, were provided with a few loosely fitting iron rings strung 

 on the spokes, when the wheel was revolved rapidly the rings 

 would all fly out to the rim of the wheel. So in Fig. 25 it will 



1 Gravity means the pull which the earth exerts upon all objects on or 

 near its surface, 



