534 POPULAR SCIENCE MONTHLY. 



distinguishing up from down. So that if it were an apogeotropic 

 plant it would need to develop the instinct of relatively accelerated 

 growth on the side D, on which the pressure is greatest. 



What is here roughly sketched is the groundwork of the theory of 

 graviperception suggested by Pfeffer and supported by Czapek, which 

 I shall speak of as the radial pressure theory, and to which I shall 

 return later. 



It is obvious that there is another consideration to be taken into 

 account, namely, that cells do not contain cell-sap only, but various 

 bodies — nucleus, chloroplasts, crystals, etc. — and that these bodies, dif- 

 fering in specific gravity from the cell-sap, will exert pressure on the 

 physically lower or physically higher cell-walls according as they are 

 heavier or lighter than the cell-sap. Here we have the possibility of 

 a sense-organ for verticality. As long as the stem is vertical and the 

 apex upwards the heavy bodies rest on the basal wall, and the plant is 

 not stimulated to curvature; but if placed horizontally, so that the 

 heavy bodies rest on the lateral cell- walls, which are now horizontal, 

 the plant is stimulated to curve. This is known as the statolith 

 theory. 



It seems to me quite certain that the stimulus must originate either 

 in the weight of solid particles or in the weight of the fluid in the 

 cells, or by both these means together. And for this reason. Take 

 the statolith theory first. There undoubtedly are heavy bodies in cells ; 

 for instance, certain loose, movable starch-grains. Now, either these 

 starch-grains are specialized to serve the purpose of graviperception or 

 they are not. If they are so specialized, cadit qucestio; if they are not, 

 there still remains this interesting point of view ; the starch-grains fall 

 to the lower end of the cells in which they occur; therefore, shortly 

 before every geotropic curvature which has taken place since movable 

 starch-grains came into existence, there has been a striking change in 

 the position of these heavy cell contents. Now, if we think of the 

 evolution of geotropism as an adaptive manner of growth we must con- 

 ceive plants growing vertically upwards and succeeding in life, others 

 not so behaving, and consequently failing. There will be a severe 

 struggle tending to pick out those plants which associated certain cur- 

 vatures with certain preceding changes, and therefore it seems to me 

 that, if movable starch-grains were originally in no way specialized as 

 part of the machinery of graviperception, they would necessarily be- 

 come an integral part of that machinery, since the act of geotropism 

 would become adherent to or associated with the falling of the starch- 

 grains. 



This argument must in fairness be applied to any other physical 

 conditions which constantly precede geotropic curvature; it is, there- 

 fore, not an argument in favor of the statolith theory alone, but equally 



