GROWTH MOVEMENTS 1075 



indicated on the right in meter-candles, while the arrow indicates the 

 time of illumination with the duration noted above in seconds. 



With these curves representing the light-growth response for different 

 given light "quantities" and for one-thirtieth of these values one can 

 predict the phototropic curvature. Thus in d the growth on the proximal 

 side (heavy line) is retarded more than on the distal side (light line) which 

 should result in a positive bending for this weak light. With a greater 

 quantity of light in c a negative bending would occur, while a further 

 increase will again bring about positive bending, as in b, which gradually 

 becomes less as the quantity is increased, as in a. The author concludes 

 that the curvatures deduced from the light-growth responses of proximal 

 and distal sides correspond with the real curvatures observed by Arisz (1). 

 He also explains the reason why Arisz never observed negative curvatures 

 with low light intensity over long periods of illumination. Graphs in d 

 indicate "long" responses for both proximal and distal sides, thus giving a 

 positive curvature. 



Castle (10, 11) has made an extensive study of the light-sensitive 

 system of sporangiophores of Phycomyces and his conclusions are in 

 accord with those of Blaauw. Both the direct growth response and the 

 phototropic response consist of a series of at least three similar com- 

 ponents, namely, " (a) an exposure period during w^hich photochemical 

 change occurs, (b) a 'latent period' involving products directly consequent 

 upon the photochemical action, and (c) an action time occupying a further 

 interval before the growth acceleration appears." Castle arrives at this 

 conclusion by noting the changes in the latent period with reference to 

 the variable, duration of the light exposure period. He states: "The 

 reaction time of each mode of response is constant for a particular inten- 

 sity of illumination, provided that the duration of the exposure period 

 exceeds a certain value. Below that value the reaction time increases 

 progressively as the exposure time decreases." 



Under constant conditions of growth in darkness, sporangiophores of 

 Phycomyces continue at a uniform growth rate for a time. When illu- 

 minated, an increase in growth is shown; this is followed by a decrease 

 and finally the original rate is reached. If the plant is growing at a 

 constant rate in light and the illumination is cut off, the growth rate 

 decreases; then it gradually returns to its former rate. This latter 

 reaction has been called the dark-growth response. These sporangio- 

 phores can be "adapted" to darkness or to light of a definite intensity. 

 A change in illumination will bring about a response, either the light- 

 growth response or the dark-growth response, depending on the change in 

 conditions. Castle (12) accounts for the kinetics of dark adaptation on 

 the basis of a bimolecular reaction, which may be modified by autocataly- 

 sis. He (13) regards the light- and dark-growth responses as due to 

 similar changes of the opposite signs in the concentration of a growth- 

 regulating substance. 



