Green— On Vegetable Ferments. 131 
transform. That acids and enzymes do unite we know from a 
consideration of the relations between pepsin and hydrochloric 
acid, the ferment being absolutely inoperative without the 
acid. Biernacke has shown that when pepsin is heated in the 
presence of *2 per cent, of HC1 to 6o° C. it is destroyed, and 
that the same destruction is reached 5° C. lower if no acid be 
present. He noted a similar relation between trypsin and 
an alkali. Chittenden found that the trypsin of the pine- 
apple will stand a higher temperature in neutral than in acid 
solution. In this case the compound, if it exists, is less stable 
than the enzyme alone. 
Reviewing the actions of these various ferments, it is 
apparent that they may be divided into two classes. One 
set act only intracellularly and do not during their activity 
leave the cells in which they are secreted. As examples of 
these we have the ferment of the lupin, the invertase of yeast, 
the urea-decomposing ferment of Torula Ureae , and probably 
the diastase of translocation. The others are secreted in 
particular cells and are excreted by them to work upon 
substances contained elsewhere. Such are for example the 
ferments in the epithelium of the scutellum of the germinating 
barley-grain, the glucoside-ferments described by Guignard 
and by Marshall Ward, the ferments of Drosera and other 
carnivorous plants. 
The power of diffusion which these enzymes possess is very 
slight. When extracted from the plants and subjected to 
dialysis in ordinary vessels with parchment septa they cannot 
pass through the wall of the dialysers ; they are able however 
to make their way through the cell-wall of the cells in which 
they are secreted. This need not be a matter of surprise 
when we consider the extreme tenuity of the film composing 
the wall, which cannot be approached by any membrane we 
can use in laboratory experiments. The diffusion in the cases 
mentioned may not however be an ordinary physical process, 
as we have evidence that in many cases, especially in endo- 
sperms, the cell-wall is perforated by very delicate strands of 
protoplasm. The most recent theory of the composition of 
K 2 
