234 GROWTH OF PLANTS 



carbon disulphide, ethylene dichloride, xylene, and ethyl bromide. The 

 last was very effective, as McCallum found, but it is rather expensive and 

 must be used as a vapor, since it is not soluble in water. Later, Miller ^^- ^^ 

 found a number of sulphur compounds more or less effective in breaking 

 the dormancy of potato tubers: ammonium dithiocarbamate, thiosemi- 

 carbazide, hydrogen sulphide, ethyl mercaptan, ethyl disulphide, and 



Figure 87. Individual eyes of Bliss Triumph potato tubers cut out with a large- 

 sized cork borer and later soaked one hour in 3 per cent thiourea solution. Note that 

 chemical forced the growth of several buds from most of the eyes. 



several others. Guthrie ^^ synthesized what he believed was thiocyano- 

 hydrin (CH2SCN-CH20H), a compound resembling both of the very effec- 

 tive compounds mentioned above, and found it effective in breaking tuber 

 dormancy. Ethyl carbylamine and glutathione ^^' *^ were also effective. 

 Thornton ''^ found 10 to 60 per cent of carbon dioxide in combination '^\dth 

 21 or higher percentages of oxygen effective in forcing dormant buds of 

 potatoes. Since carbon dioxide was effective with normal or higher than 

 normal oxygen, the action of carbon dioxide was not due to anaerobiosis. 

 None of these compounds shows advantages that would lead them to dis- 

 place ethylene chlorhydrin and the thiocyanates as bud forcers. Dennj^ ^s 

 has also found a combination treatment with ethylene chlorhydrin followed 

 by thiocyanate of advantage in greenhouse tests for virus in potato tubers. 

 This gives approximately 100 per cent germination, which is necessary in 

 these tests. 



Thiourea as a bud forcer. Thiourea has proved to be a fair bud-forcing 

 chemical, but it differs from the other effective chemicals in that it breaks 

 up the growth correlation between the several buds of an eye and between 

 the eyes in a seed piece. In normally after-ripened tubers and in dormant 



