136 - ELEMENTARY BOTANY 
box, and then after some days to compare the stem and leaves 
of that part of the plant grown in the box with the growth of 
the part outside. It will be found in all cases that light 
(a) retards growth in length of stem; (0) makes the leaves 

Fig. 149.—Mimosa Lear (Day Fig, 150.—Mimosa Lear (NicHT 
POSITION). | PosITION), 
bigger and diverge further from the stem; (c) promotes the 
formation of the dark green colouring matter of leaves. 
Grow plants in a window in order to note the 
effect of the direction of light. Nasturtium, 
Dead Nettle, Mustard, give good results. 
The stems curve towards the light, and the 
leaves will be found to place themselves at right 
angles to the direction in which the light is falling. 
Leaves place themselves so as to secure as 
much light as possible. 
This principle to some extent explains the arrangement of 
leaves of a certain shape on a stem. (See Chapter V.) 
Seedlings may be put to grow under double wall bell-jars, 
one of which may contain a solution of potassium bichromate 
Experiment B. 
Result. 
Conclusion. — 
