444 EFFECT OF LIGHT UPON GROWTH [Ca. XVII 



unknown ; the subject has not been investigated. Is it at the 

 tip or at the responding region, or do these regions exactly 

 coincide ? The essential identity of phototropism in sessile 

 animals and plants is striking, and indicates how closely simi- 

 lar needs are met by similar capacity for response in the two 

 groups. 



3. General Considerations. - - a. Persistence of Stimulation. 

 If a seedling, after momentary illumination on one side, be 

 placed in the dark before any turning has occurred, phototro- 

 pism will follow after the same interval as would have elapsed 

 had the plant remained in the light. Even if the irritated 

 seedling be placed in the dark in a horizontal position, no geo- 

 tropic curvature will interfere with the working out of the 

 stimulus already given. This persistence of an effect wrought 

 by light has been called by WIESNER photomechanical induc- 

 tion : it is, however, only a particular case of persistence of 

 an effect, of which we have seen other examples. As a result 

 of this phenomenon a seedling, intermittently illuminated for 

 one second and kept in the dark for two seconds, will respond 

 phototropically as completely and as quickly as if it had been 

 kept continuously in the light. 



b. Acclimatization to light is a process closely related to 

 the foregoing. As early as 1827 MOHL observed that plants 

 reared in a weak light, or in the dark, became, after a time, 

 phototropically more sensitive than plants which had been con- 

 stantly exposed to full daylight. The observation has been 

 several times confirmed (cf. DARWIN, '81, Chapter IX) ; so we 

 may conclude that the constant subjection to light diminishes 

 the sensitiveness towards light. 



c. Mechanics of Phototropism. It is clear that phototropic 

 curvature, as seen in the seedling, the mold, or the hydroid, is 

 the result of unequal growth upon the two sides of the cylin- 

 drical organ ; and, indeed, that the positive phototropism is 

 due to a relative diminution of growth on the side next the 

 source of light, and the negative phototropism to a relative 

 increase of growth on that side. Experiments have shown 

 that in positive phototropism growth is excessively rapid upon 

 the convex side of the organ, and excessively slow upon the 

 concave side. These results are reasonably attributed to an 



