130 
the enzymes being formed in one set of cells, and excreted into the parts 
where the absorption of food takes place. 
The composition of the enzymes is not known, some inclining to the 
view that they are of a proteid nature, while others think that they are 
nucleo-proteid, but as yet it is not definitely settled, as the enzymes have 
not been obtained in a pure condition, their reactions being connected with 
those of the substances with which they are associated. On account of 
this lack of knowledge as to their composition the enzymes have been 
classified according to the substances which they decompose. 
The proteolytic enzymes are those which decompose proteids into less 
complex substances. There are two classes of these, the peptic and tryptic 
enzymes. The peptic enzymes decompose proteids to peptones, while the 
tryptic enzymes go farther, effecting the decomposition of the peptones to. 
amides. In plants the amides are formed as antecedents to proteids, help- 
ing in the reconstruction of proteids as well as aiding in their osmosis by 
decomposition. When the carbohydrate, which was united with the aniide, 
forming the proteid, is used up, the amide unites with a fresh carbohy- 
drate, again forming a proteid. The enzyme in the plant, which causes. 
this decomposition, is often in such minute quantity that it is almost im- 
possible to determine its presence.* 
Only a very few of the vegetable proteolytic enzymes have been inves- 
tigated, and nearly all of these are from the higher plants. Those which 
have been investigated minutely have been found to be of a tryptic nature. 
A few of the fungi have also been found to secrete a proteolytic enzyme 
among these yeast.’ It is claimed that if yeast be deprived of both oxygen 
and food material that it breaks up its own reserve proteid, and also, that 
if yeast be pressed, and the extract collected and heated to 45° C., a bulky 
coagulum is formed, which disappears in a few days, the extract in the 
meantime being kept under antiseptic conditions. The digestion of the 
reserve proteid and the disappearance of the coagulum indicate the pres- 
ence of a proteolytic enzyme. 
Proteolysis by yeasts has been noted but indirectiy, except in the 
case of the pressing from the yeast of an extract by Biichner.? Jérgensen,® 
in describing S. J6rgensenii and S. membranefaciens states that the 
* Kerner and Oliver. Natural History of Plants. Vol. I, part 2. 
*Green,J.R. The Soluble Ferments and Fermentation, 1899, pp. 215-217. 
* Biichner, E. Ber. d.deut. chem. Gesell., 1897. 
“Jérgensen, A. Micro organisms and Fermentation, 1893. 
