§1] 



UPON THE RATE OF GROWTH 



421 



10 11 12 



an inch long in tlie daylight. Bacillus ramosus, in one case, 

 grew during five hours, in the dark, 540 /u. ; in the light, 200 /x 

 (Waed, '95). These results demonstrate that the inhibiting 

 or retarding effect of sunlight, and even of diffuse daylight, is 

 probably unconnected with the clilorophyll function, but is due 

 to a more general effect of light upon growing protoplasm. 



Even brief illumination has its marked effect. Thus Vines 

 ('78, p. 137), who has made exact measurements of the hourly 

 growth of the sporangiferous hyphse of the mold Phycomyces 

 nitens, found that growth was diminished whenever the plant 

 was subjected for only 

 an hour to sunlight 

 (Fig. 120). The same 

 is true for phanero- 

 gams (GODLEWSKI, 



'93). 



The great diurnal 

 period of darkness and 

 illumination to which 

 plants are subjected 

 in nature likewise has 

 its effect on growth. 

 The first studies made 

 upon this sub j ect wer e 

 by Trbw in 1727. 

 Numerous observers 

 followed, but it was 

 left to Sachs ('72), 

 by the aid of his aux- 

 anometer, to obtain a 

 continuous curve of growth. This showed at a glance that dur- 

 ing the night the rate of growth gradually increases, reaches a 

 maximum at about daybreak, diminishes to a minimum a little 

 before sunset, and then begins to rise again.* This variation 

 in the rate of growth is -opposed to the diurnal fluctuation of 

 temperature, since this is low at night and high during the day. 

 It is favored, on the other hand, by the circumstance that heat 



31" 



23" 



22 







Fig. 120. — Diagram illustrating the retarding influence 

 of light upon the growth of a sub-aerial hypha of 

 Phycomyces. The thick line represents the course 

 of growth, the thin line that of temperatures ; the 

 unshaded spaces, periods of exposure to light ; the 

 shaded spaces, periods of darkness. The figures at 

 the left indicate tenths of millimeters, those at the 

 right, degrees of temperature ; those at the top, 

 hours. (After Vines, '78.) 



* A similar periodicity has been detected among toadstools and puffballs by 

 Kraus ('83, p. 97). 



