§ 2] EFFECT OF EXTENT OF MEDIUM ON" SIZE 473 



the modified position is, however, possible. The stimulating 

 action of light may interfere with the sensibility of the plant 

 to gravity. An early experiment made by Czapbk seemed to 

 indicate that this is the case. A vertical plant was illumined 

 upon one side and then placed horizontally with the former 

 illumined side facing the nadir. The geotropic curvature was, 

 as might be inferred from the experiment described on p. 444, 

 greatly retarded. The retardation of geotropism by a pre- 

 vious photic stimulation occurs, however, only in species which 

 are much more phototropic than geotropic. That the result 

 just described is not due to a diminution in geotropic sensitive- 

 ness follows from two considerations : first, a plant previously 

 geotropically stimulated is not retarded in its subsequent pho- 

 totropic response, as we should expect if the irritated state 

 interfered with the reception of a new stimulus ; secondly, the 

 retardation in response to light after geotropic stimulation 

 is most directly explained on the ground that a response 

 which is being actually worked out interferes with an incipient 

 response. 



The conclusion may, consequently, be drawn that there is no 

 reason for believing that the modified response resulting either 

 from the simultaneous or successive action of two. tropic agents 

 is due to an alternation of geotropic or phototropic sensitive- 

 ness, but rather to a modification of the response. The modifi- 

 cation of the response is due to the interlocking effect or the 

 mutual interference of the phototropic and geotropic reactions 

 to stimuli. 



§ 2. Effect of Extent of Medium on Size 



The quantity to which an organism shall grow — the size 

 that it shall attain — is a specific character which is, within 

 limits, independent of the amount of food consumed by the 

 organism. In how far this character is dependent upon other 

 environmental conditions is an interesting question. It is clear 

 that size is relative, and among other things it is related to the 

 extent of the space in which the organism can move. An ant 

 can lose its way in a cage in which an elephant can hardly find 

 room to move. The ant is small, the elephant large, in relation 

 to their common room. 



