56 



BIOLOGY: GENERAL AND MEDICAL 



of bacteria, such as the formation of pigment, take place 

 only in the dark, as in Bacillus mycoides roseus. 



To the higher plants, light is indispensable and their 

 heliotropic reactions are correspondingly interesting. 



A familiar example of positive heliotropism in the 

 higher plants is found in the sprouting of potatoes in the 

 cellar. If the cellar be very dark, the sprouts are long, 



FIG. 11. Mimosa pudica. A. Entire plant in the daytime with leaves ex- 

 panded. B. The same in the position of contraction assumed at night. (Ver- 

 worn.) 



slender and without color. If there be a distant window 

 from which a dim light is admitted, the shoots on the 

 window side are larger and more vigorous. If the light 

 admitted by the window reach a certain intensity, small 

 leaves appear at the ends of the sprouts, and show pale 

 green color. This growth of the potato is entirely dif- 

 ferent from that seen when the tuber is planted in the 



