RESPONSIVE CURVATURES — NEGATIVE GEOTROPISM 495 



.1 1 1. 



Fig. 204. Diagrammatic Representation 

 of a Multicellular Organ 



On the upper side the statoliths act on the 

 inner, and on the lower side on the 

 outer, tangential wall (after Francis 

 Darwin). 



water-column the weight of certain relatively heavy bodies, 

 such as starch-grains, differentially exercised upon the lower 

 tangential walls. In the case of multicellular plants, laid hori- 

 zontally (fig. 204), EE and E'E' may be regarded as regions in 

 which stimulation is caused by the weight of the particles. 

 The effects produced on 

 the upper and lower c 

 halves are evidently an- 

 tagonistic, and in spite of 

 this we obtain in the case 

 of shoots a resultant cur- 

 vature upwards. This 

 shows that the stimulation 

 of one half must be greater 

 than that of the other. 

 The inequality must be 

 due to this difference, that 

 the statoliths in EE rest 

 on the inner, and in E'E' 

 on the outer, tangential wall. It would thus appear that one 

 of these must be less excitable than the other. 



Mechanics of responsive movement. — From these 

 hydrostatic or statolithic differences of weight, bearing on the 

 ectoplasm of the cell, it is understood that gravi- perception 

 arises, that is to say that the plant perceives the direction of 

 gravity. But there is no explanation as to how stimulation 

 is produced ; nor is there any satisfactory explanation as to 

 the mechanics of the responsive curvature. It is generally 

 supposed, as has been said, that this curvature is not the 

 direct and necessary result of some environmental change, 

 but that it is an instance ' of a plant reading a signal and 

 directing its growth' accordingly. It is supposed further 

 that in an apogeotropic cell the curvature takes place by the 

 development ' of relatively accelerated growth on the side on 

 which the pressure is greatest.' 



This view, that the curvature induced in some unknown 

 way in a geotropic organ, by gravitation, depends upon the 



