414 



PHYSIOLOGY OF GROWTH. 



covered. Daring the period of darkness, the growth of the sporo- 

 phores is more active than in the period of illumination (see the 

 following table, and the corresponding graphic representation). 

 It is found that exposure to light retards the growth in this case 

 immediately, while darkness immediately accelerates it. 3 



FIG. 136. The thick line indicates the alternations 

 in the rate of growth of a Mucor tube corresponding 

 with alternations of light and darkness. The fine line 

 is the temperature curve. (After Vines.) 



1 See Sachs, Arbeiten d. botan. Inst. in Wiirzburg, Bd. 1, p. 99. 



2 See Prantl, the same, p. 371. 



See Vines, the same, Bd. 2, p. 133. 



167. The Influence of Illumination on the Germination of 

 Potato Tubers. 



In the autumn, or in the course of the winter, we lay a number 

 of potatoes in a box, and cover it with a sheet of cardboard so as 



