RESPONSIVE CURVATURES— NEGATIVE GEOTROPISM 507 



also, the response is by contraction and concavity of the 

 excited surface. The convexity of the outer is thus to be 

 taken as the inevitable result of the concavity of the inner. 



The fact that in the acellular organ response actually 

 takes place by the concavity to stimulus of the surface acted 

 upon, is further seen in the response of such organs to stimulus 

 of light. In the case of geotropic stimulus it is the internal 

 surface of the lower side of the organ which is irritated by 

 the differential weight of the cell-contents, and becomes con- 

 cave to the stimulus thus acting upon it. In the case of light, 

 on the other hand, stimulus acts from the outside, and it is 

 thus the outer or external surface, say, of the same side, 

 which becomes concave. Thus stimulus, acting on the same 

 side in one case from within, and in the other from without, 

 induces responsive curvatures in opposite directions. 1 



1 The explanation of the responsive curvatures of acellular organs has hitherto 

 offered many difficulties. In a multicellular organ, acted on unilaterally by stimu- 

 lus, there is a difference induced in hydrostatic pressure as between the two 

 opposite sides. The diminished turgidity of the proximal, and increased turgidity 

 of the distal, explains the induced curvature. In an acellular organ, however, 

 there cannot be this difference of hydrostatic pressure on the two opposite sides. 

 But the considerations which I shall now offer may perhaps be found to meet the 

 difficulties of the case. The fundamental effect of stimulus is, as we know, to 

 induce protoplasmic contraction. Hence unilateral stimulation acting on an 

 acellular organ may be expected to induce contraction and concavity of the 

 proximal side of the ectoplasmic layer, the result of which will be a curving over 

 of the organ. As a result of this, the ectoplasmic layer of the distal side will be 

 subjected to tension, which is, as we know, an influence that accelerates growth. 

 Hence the retardation of growth on the proximal, due to contraction, and its 

 acceleration on the distal under increased tension, will combine to produce growth- 

 curvature. 



A problem of somewhat greater complexity arises in the case of stimulus of 

 light traversing a transparent acellular organ. Let us suppose such a vertical 

 organ to be acted upon horizontally by rays of light from the right-hand side. 

 We have in this case to consider the separate effects of stimulus of light on four 

 different surfaces: (1) the outer ectoplasmic layer of the proximal side Vo; (2) 

 the inner ectoplasmic layer of the proximal side Pz ; (3) the inner ectoplasmic 

 layer of the distal side Dz ; and (4) the outer ectoplasmic layer of the distal side 

 Do. The contractions of the outer surface of the proximal Vo, and the inner 

 surface of the distal Dz, would induce a curvature to the right ; those of the inner 

 surface of the proximal Pz and the outer surface of the distal Do a curvature to 

 the left. But it is evident that as light passes through the organ there must be 

 loss of intensity by absorption. Hence the sum of effective intensities at Vo and 



