THE MOVEMENTS OF PLANTS. 



171 



because usually illuminated from one side. Plants kept in 

 darkness have their leaves irregularly placed. 



Fig. 125. — Leaf mosaic formed by a horizontal shoot of Norway maple. The lengthen- 

 ing of the petioles of individual leaves to avoid shading of the blade is marked. 

 About one-third natural size.— After Kerner. 



Fig. 126. — A rosette of leaves of a bellflower {Campanula fiusiUa), showing length- 

 ening of petioles of lower leaves so' as to carry blades from under upper leaves. — 

 After Kerner. 



EXERCISE XL. 

 To show the effect of direction of light as a stimulus on leaves. 

 Set a potted plant (geranium, sunflower, nasturtium, or mallow) in the 

 dark for 24 hours; then place it before a window, shading it so that 



