1] 



UPON THE RATE OF GROWTH 



421 



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

 grew during five hours, in the dark, 540 /* ; in the light, 200 /i 

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

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

 probably unconnected with the chlorophyll 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 hyphte of the mold Phycomyces 

 nitens, found that growth was diminished whenever the plant 

 was subjected for only s 9 10 11 

 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 subiect were FlG> 120 - Dia g ram illustrating the retarding influence 



by TREW 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 







23 



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). 



