146 



RELATIONS OF PLANTS TO LIGHT 



If a large dark chamber is at hand various methods of treat- 

 ment may be used, in which the main axis (fore-shoot) only may 



FIG. 80. Stolon of Solanum extending into a small dark chamber in which a 

 branch has been converted into a tuber. After Vochting. 



be allowed to develop, producing a club-shaped stem 15 to 25 

 cm. long and I and 2 cm. in thickness, with similarly thickened 

 branches at the apex : or all of the stems and branches of a bud 

 may be allowed to remain, with results of value in the analysis 

 of the factors operative in the formation of tubers. 



