STUDIES IN PLANT REGENERATION 241 
or if the carbon dioxide was absorbed from the atmosphere in 
which the parts were kept. Entirely white shoots of several 
species of plants — presumably from their inability to manufacture 
food — proved unable to form roots even though primordia might 
be present. 
16. Since certain parts fail to regenerate one or the other kind 
of organs, when each of the conditions previously assigned as the 
causes of regeneration has been fulfilled, it seems necessary to 
postulate the existence of specific substances which are respon- 
sible for the formation of those organs. These substances have 
been assumed to take the form of different enzymes which are 
not present in all the cells of the plant, but are localized in 
definite places. It is perhaps possible to obtain evidence on the 
grounds of this assumption by experiments, of which a series has 
been already started. 
New York BOTANICAL GARDEN. 
