BEHAVIOUR OF PLANTS TOWARD LIGHT 139 
we crush these stems with the fingers, we find those 
grown in the light firmer than those grown in the dark. 
A more accurate test would be to dry the plants thor- 
oughly and then to weigh them. The plants grown in 
the light would outweigh those grown aoe 
Se 
* 
MJ 
in the dark. In other words, they have “ie 
made more plant substance. It will g 


f be remem- 
bered that 
green plants 
Loar a4 
“7 starch in 


aS 
b 
| sunlight. 
SESS < ae The starch 1s used, much 
ae x i “ a So Kx 3 SOS e F r 
iste . hie of it, in making new plant 
substance, especially cell 
walls, which constitute the 
firmer portions of plants. 
This is the reason, then, 
Fic. 169. Same a esr itgaasls in Fig. 164, why the stems erown in 
the dark are more slender 
and less firm than those grown in the light. 
The leaves on plants grown in the dark. While stems 
grow less rapidly in light than in dark, light accelerates 
the growth of the leaves. Plants grown in the dark 
have very small or undeveloped leaves. This is well 
shown in the marrow (Fig. 169). Compare the leaves 

