436 TRANSFORMATION OF ENERGY 



Turgor extension is unequal on the two antagonistic sides. The difference 

 would appear to depend on the fact that the osmotic pressure increases on the 

 convex and decreases on the concave side, but that is by no means the case ; 

 on the contrary, the pressure on the concave side appears to remain constant, 

 while that on the convex side is reduced. Since the rate of growth of the 

 cell-wall does not depend directly on the amount of osmotic pressure, there is 

 nothing very astonishing in this. Unequal turgor extension of the two sides 

 must depend on an alteration in the elasticity of the cell-walls. We have 

 already seen that there are no reliable data available as to the causes for such 

 alterations in elasticity any more than there are on the general question of the 

 mechanics of growth in the cell-wall. On that account it is needless for us to 

 enter into such controversial points, although geotropic curvatures have 

 frequently been brought forward in support of different views as to the 

 mechanics of growth in the cell-wall. Unfortunately, measurements made of 

 curving organs often do not at once determine whether the concave or the 

 convex side, or both, are actively concerned in the bending. In many cases, 

 such as those where the concave side is directly shortened, there can be no doubt 

 that it behaves passively, and that the curving is the result of vigorous stretching 

 of the convex side aided by rigidity in the axial region. If, as in grasses, 

 the concave side be thrown into folds, the passivity of that side makes itself 

 apparent at once. It does not always behave in this manner, however. Certain 

 experiments of SACHS (1873 a), where the several tissues were removed during 

 the bending, tend to show that the axis (medulla in the case of the stem) is 

 not directly concerned in the process, but this is true only of the uninjured 

 plants and not of the longitudinally spilt internodes mentioned on p. 435. 



Assuming then that curvature in general depends on unequal growth 

 on opposite sides, that, in positive geotropism, growth is retarded on the 

 side of the organ facing the soil, while it is accelerated on the upper side, 

 and that in negative geotropism the distribution of growth is reversed, we have 

 next to ask how it is that gravity influences growth, and especially how it is able 

 to influence different organs in different ways. As a matter of fact, the question 

 has been in a sense already answered by our describing geotropism as a stimulus 

 reaction ; the significance of this terminology being that gravity is to be regarded 

 merely as a releasing force and not one which acts in a purely mechanical manner, 

 and this conception of the phenomena must obviously be looked upon as the 

 correct one, when we remember that gravity induces diametrically opposite 

 reactions in positively and negatively geotropic organs. The history of the 

 science, however, shows (as to the history of geotropic investigations compare 

 SCHOBER, 1899) that this conception was only arrived at as the result of con- 

 siderable labour and was by no means self-evident from the very first. As a 

 matter of fact, even as late as the seventh decade of the last century, an in- 

 vestigator of the rank of HOFMEISTER (1863) attempted to show that gravity 

 acted in a purely mechanical way. This author believed that the softness of 

 the root accounted for its capacity for bending, which in turn was induced 

 by the weight of the apex. Into his explanation of the negative geotropism 

 of the stem we need not enter, for it has only an historical significance. The 

 theory based on the plasticity of the root apex is also of interest only from 

 an historical point of view, although it is full of lessons for us even nowadays, 

 as showing how thinking men under the dominance of a preconceived notion 

 may go blindly in opposition to facts ; it reminds us also that the fact is the 

 chief thing, the theory only, as it ought to be, the ever-changing expression 

 of the aggregate of experience. Had HOFMEISTER not been imbued with a 

 preconceived idea he must have seen that the apex of the root really more 

 closely resembles a piece of glass than a stick of hot sealing-wax, JOHNSON 

 (1828) had long previously shown that the weight of the root apex may 



