532 LECTURE XX. 



away from the source of light. Positive and negative pho- 

 totaxis find their parallels in positive and negative helio- 

 tropism. But there is a difference between the two cases, 

 that the positive or negative character of the phototaxis de- 

 pends entirely upon the intensity of the light, whereas the 

 character of the heliotropism of growing radial organs is not 

 so determined. The positive or negative heliotropism of a 

 growing radial organ is a specific property which is not 

 altered by external conditions ; a negatively heliotropic root, 

 for example, cannot be made positively heliotropic by any 

 variation in the intensity of the light to which it may be ex- 

 posed ; nor can a positively heliotropic shoot be made nega- 

 tively heliotropic, though, as we have learned (pp. 425, 429, 

 443) exposure to intense unilateral illumination may cause 

 a radial shoot to become dorsiventral and at the same time 

 diaheliotropic. It is true that the direction of movement of 

 these masses of protoplasm does not appear to be affected by 

 the action of gravity as is the direction of growth of growing 

 organs, but, on the other hand, we find them exhibiting in a 

 striking manner the phenomena of thermotropism, hydro- 

 tropism, and rheotropism. 



We pass now to the consideration of the movements of 

 mature cellular organs. The irritability of mature organs is 

 manifested in essentially the same manner as that of growing 

 organs, and the nature of the response is essentially the 

 same in both cases. But it is convenient to treat of them 

 separately, inasmuch as the result is different. When a grow- 

 ing organ, namely, performs a movement in consequence of 

 stimulation, the movement is irreversible ; when, for instance, 

 a root or a shoot curves geotropically, the curvature is rendered 

 permanent by deposition of substance. When, however, a 

 mature organ performs a movement in consequence of stimu- 

 lation, the position which it takes up is not thus rendered 

 permanent ; when the action of the stimulus has ceased, the 

 organ resumes its normal position, and is again susceptible of 

 stimulation, and the movements may be repeated an indefinite 

 number of times. The movements of mature organs have, on 

 this account, been designated as "movements of variation" 



