1899-1900.] Dr J. S. M‘Kendrick on Enzymes in Tissues. 69 
Many other instances could be cited, which show that an inter- 
stitial digestion is being carried on in the cell structure of the 
plant, presumably by enzymes of a nature identical with those 
that exist in the digestive juices of the animal. The zymolysis 
then of plant life is the process of conversion of stored up food 
stuffs into new substances. These new substances are formed by 
the activity of the soluble unorganised ferments or enzymes. The 
zymolytic processes in plants have been investigated by Green,* 
Hansen,f Wortmann, and others, and it is now generally believed 
that in most plants there are at work enzymes of proteolytic, 
amylolytic, and inversive powers. The papaw plant contains a 
proteolytic enzyme, papain, wdiich is very similar in its action to 
trypsin, and moreover the action of the enzyme compares favourably 
as regards activity with those of the proteolytic ferments of animal 
origin. 
Again, it is generally admitted that the inversion of cane sugar 
(as, for example, beetroot sugar into inverted sugar), during the 
inflorescence of the plant, is due to an inversive enzyme. Many 
examples could be cited which show the presence in plants of 
enzymes similar in their nature and action to pepsin, ptyalin, and 
invertin. 
The question presents itself — since we are aware that in plant 
life a zymolytic interstitial digestion is constantly at work — is it 
not possible, and indeed probable, that in animal tissues as well, 
enzymes are in action : they may be of the same or a different 
nature, taking an active part in the metabolic processes occurring 
in the individual cell? If such were the case, it might account for 
the conversion of glycogen into sugar in certain circumstances — 
the conversion depending upon the activity of a soluble enzyme, 
liberated, it might be, from a parent zymogen existing in the pro- 
toplasm of the hepatic cell. Again, it might account for the 
abnormal sprouting of a parent tissue, depending upon the in- 
creased activity of an enzyme in that tissue. When a sarcoma or 
a carcinoma grows, is it not possible that an interstitial digestion 
is at work, altering the nutrition of the parent tissue? This 
* Science Progress , London, vol. i. p. 342 ; vol. ii. p. 109 ; vol. in. pp. 68, 
376 ; vol. v. p. 60. 
t Bot. Ztg ., 1886, S. 137. 
