5o6 PLANT RESPONSE 



be seen from the following experiment. I took three long 

 scapes of Uriclis Lily and laid them horizontally, packed in 

 ice. As long as they were in the ice there was no responsive 

 curvature. After an hour they were taken out of the ice 

 and held erect, and by the time that they were restored to 

 the temperature of the room it was found that the top of the 

 scape in each case was bent from v^^ to i'5 cm. in the 

 direction of what had been the upper surface when they 

 were horizontal. This was evidently due to the fact that the 

 cold brought about an arrest of growth ; but the geotropic 

 stimulus remained latent, to express itself in a responsive 

 movement later, when growth was renewed, under the action 

 of a favourable temperature. 



Responsive curvature of acellular organs. — There is 

 one point, in connection with the induction of gravitational 

 curvature, which might at first sight appear anomalous. In 

 the case of the negative geotropic curvatures of multicellular 

 organs, the fact that it is the upper side which is relatively 

 effective, and that the curvature is the result of its responsive 

 contraction and concavity, is evident from the experiments 

 already described on the local application of cold. When 

 we take an acellular apogeotropic organ, however, such as the 

 stalk of the sporangium of Mucor, we find that it is the 

 irritated lower side, differentially acted on by weight, whether 

 of sap or statoliths, that becomes convex. This looks at first 

 sight as if the effect of irritation were, as it is generally 

 supposed to be, to induce acceleration of growth, and 

 attendant convexity. 



We must in this case, however, bear in mind the position 

 of the surface on which the stimulus acts. In our experi- 

 ment on the irritation produced by the pressure of magnetic 

 particles it was the outside surface of the organ that was 

 acted on by stimulus, and it was that side that became 

 concave. In the case of the organ with a single row of cells 

 as described, however, it is the internal surface which is so 

 irritated, and if we take as our object of observation that 

 internal surface, we shall find that, as in other cases, so here 



