428 



Plant Physiology 



than to other factors that the errors are perhaps negligible. 

 When the buds of ordinary herbaceous plants with central 

 axes are permitted to develop in the dark there is a marked 

 elongation of the internodes and a suppression of branches. 

 Shoots from tubers or tuberous roots affording constant 

 food-supply will show this characteristic in a striking man- 

 ner. The leaves of such herbaceous plants are usually 



FIG. 120. Sumatra tobacco under" cloth tent, Connecticut Valley. 

 [After Shamel.] 



greatly reduced in size, and sometimes restricted to mere 

 scalelike structures. On the other hand, when the light in- 

 tensity is reduced to from 20 to 40 per cent of normal sun- 

 light, the leaves may be increased to twice their size in di- 

 rect sunlight, as demonstrated by many experiments upon 

 lettuce, tobacco^ and other broad-leaved plants. When 

 produced in the dark, the radicle leaves of such plants as 

 the rhubarb develop in a short time petioles of unusual 

 extent and delicacy, while the leaf blade remains small. 



