238 STUDIES IN PLANT REGENERATION 
here supplies the material, and the organ, no longer hindered by 
encompassing tissue, makes its appearance. While, as was sug- 
gested above, the shoot-forming enzyme is usually restricted in its 
distribution, it can also sometimes be formed anew when cells 
which have by differentiation lost this substance, upon injury, 
resume more or less of their embryonic characteristics, in the for- 
mation of a callus. 
Our entire knowledge of enzymes and their mode of action is 
almost in its infancy, but some of the more recent discoveries as 
to their history and condition of activity may apply here. For 
example, it is known that an enzyme may be present in a cell not 
as such but in the form of a pro-enzyme or zymogen which changes 
into the enzyme upon certain kinds of stimulation. Such might 
well be taken to be the case in the root-forming enzyme, where 
the stimulating factor could be the exposure of the cells containing 
the zymogen to moist air, or this condition in combination with 
others, such as a change in the pressure relations and the chemical 
effect of aggregated food. Again, the presence of a substance 
called a nase is known to enhance the action of an enzyme, and 
it may be that such'a substance, in addition to the enzyme is 
present at the nodes and vegetative points. Finally, as a third 
possibility, cases are known where, in company with an enzyme, a 
compound of an opposing nature, termed an “ anti-enzyme "' exists, 
and according as one or the other gains the ascendant the particu- 
lar effect of the enzyme is manifest or obscured. Some such 
explanation might account for the individual variations in the re- 
generation of the same part under like conditions, as for example, 
the differences observed in the behavior of the potato leaf. How- 
ever, while the possibility of the existence of such kinds of sub- 
stances should be considered, it must be admitted that in the 
present status of our knowledge, they do not very materially aid 
in the solution of the problem. The detection of such substances 
is even more difficult than that of the enzyme proper and the 
expectation of success on their identification or effects in the types 
of activity manifested by plants must be remote. 
As to the enzymes themselves, experiment seems to afford 
some opportunity for securing information. The method by which 
evidence as to their presence or absence might be arrived at, is by 
