PHOTOTROPISM JOHNSTON 



317 



low light intensities over long periods of illumination no negative 

 curvatures are found. 



This work of Dillewijn, as well as that of others, shows that light 

 within a certain range of intensity brings about a decreased growth 

 rate, whereas that of another range results in an increased rate of 

 growth. It has been shown that the greater the " quantity " of 

 light (intensity times duration of exposure), the shorter the re- 

 action time becomes. Thus in a with the greatest quantity of light 



# 



4 



10 22 ^ 40 60 10 ^ 30 40 60 60 90 120 ISO leOii.i. 



Figure 2. — Graphs from Went showing light-growth response of Avena. The abscissa 

 represents time in minutes, the ordinate growth in y. per minute, and the arrow the 

 moment of illumination. 



Upper graph: Illumination of entire coleoptile (12 mm long). 



Middle graph: Illumination of the tip (1.25 mm). 



Lower graph : Illumination of base (9 mm) while the top (3 mm) was kept in 

 darkness. 



the greatest decrease in growth rate is reached in half an hour, 

 while in d with the least light quantity the minimum is reached in 

 about 114 hours. 



In much of the work with the coleoptile its entire length was ex- 

 posed to light. It has been found, however, that the extreme tip 

 is the most sensitive of the entire tissue. Went is of the opinion 

 that there are two distinct growth responses — the tip response and 

 the base response. His data, shown graphically in figure 2, are of 

 interest in connection with other work where positive and negative 

 curvatures are obtained with different light intensities. 



