TROPISMS AND RELATED PHENOMENA 



147 



3. GEOTROPISM 



It is well known that even in the dark the tips of the main roots of 

 many plants show a tendency to grow vertically downward, while the 

 tips of the main stem show the opposite tendency. If such plants 

 are put into a position other than vertical, the tip bends until the 

 vertical lines strike symmetrical points at the same angle. In such 

 cases we call the roots positively, the stems negatively, geotropic. 

 Knight has shown by putting plants on a rotating disk that these effects 

 are due to gravitation. In a centrifugal machine the tips of the root 

 grow toward the periphery, the stems toward the center of the disk. 

 In grasses the curvature occurs in the nodes, while in other forms it 

 occurs in the growing region near the tip of the root or the stem. 



While chemistry furnishes sufficient data for the assumption of 

 photochemical effects in organisms, we do not know of any direct effect 

 of gravitation upon chemical reactions. Eight years ago I pointed out 

 that such an effect might occur in this way ; namely, that in the cells, 

 or in certain cells, of geotropic organs, nonmiscible substances (e.g. 

 solids and liquids) might exist, and that by the change in the position 

 of the organ a change in the relative position of these phases might 

 be brought about.* This change in position might 

 be connected with an acceleration of the reactions on 

 the one side, and the reverse effect on the opposite 

 side of the organ. I was led to such an assumption 

 by the observations made on the resting muscle in a 

 stretched and normal condition. If the excised muscle 

 of a frog is stretched passively by a weight, it produces 

 more lactic acid than in the unstretched condition.f 

 I am inclined to attribute this effect of the stretching 

 merely to a change in the form of the muscle. It 

 might be possible that the stretching increases the 

 surface of certain (the anisotropic?) elements in the 

 muscle, whereby the area of contact (with the isotropic 

 substance?) and therefore the reaction velocity might 

 be increased. Something similar might happen in 

 geotropic organs, when they are put into a hori- 

 zontal position. Suppose that in the normal (upright) condition of 

 the stem certain solid or viscous substances (e.g. nuclei) of a higher 

 specific gravity than the other constituents of the cell lie at the base of 



* Loeb, Pfluger's Archiv, Vol. 66, p. 439, 1897. 



t Gotschlich, Pfluger^s Archiv, Vol. 56, p. 355, 1894. 



FIG. 32. 



