258 VEGETABLE PHYSIOLOGY 
It is not quite certain that representatives of the first 
class are to be met with in plants. It is for the present 
probable, however, that the enzyme of some insectivorous 
plants is a peptase. It acts only in the presence of a weak 
acid, as does the pepsin of the stomach, but the products 
which it forms have not been accurately investigated. It 
is apparently only secreted when the gland has been stimu- 
lated by the absorption of nitrogenous matter. 
Several varieties of vegetable tryptase have been dis- 
covered and their properties investigated. The earliest 
known enzyme belonging to the group is the papain which 
hag been extracted from the Papau (Carica Papaya). It 
appears to exist in greatest quantity in the pulp of the 
fruit, but is present also in the sap which can be expressed 
from the stem and leaves. It is apparently associated in 
the juice with a peculiar proteose or albumose, and it is 
most energetic in a neutral solution, though it can act 
also in a faintly alkaline one. It is easily destroyed by a 
very small trace of free acid. 
Another tryptase, which has been named bromelin, has 
been extracted from the fleshy pulp of the Pine-apple 
(Ananassa sativa). Like papain it is associated with a 
proteose. It acts most energetically in neutral and faintly 
acid solutions, alkalies in very small traces being preju- 
dicial to it. Its activity varies a good deal according to 
the acid which is present, and to some extent according to the 
protein which it is digesting. 
Other vegetable tryptases have been extracted from the 
germinating seeds of the Lupin, the seedlings of several 
plants, the fruit of the Kachree gourd (Cucumis utilissimus), 
the juice of the Fig-tree (Ficus carica), and the leaves of 
certain species of Agave. How far these are identical, or 
whether they present specific differences, appears at present 
uncertain. They are all active in faintly acid solutions, 
but the most favourable concentration appears to vary. 
The enzyme of the Kachree gourd is most effective when 
the medium is faintly alkaline, whereas that of the lupin 
