Proceedings of the Linnean Society of London - University Library

LIXNEAN SOCIETY OF LONDON. 23
matter of fact, effect proteolysis, though much less active)}' thau
trypsin. If this be established, the result will be that all these
proteases will have been found to differ not in kind, but only in
degree. They \^-ill form a series in which trypsin, acti^^e alike in
peptonisation and in proteolysis, will occupy a central position:
on the one hand will be pepsin, with its active peptonisation and
slight proteolysis ; and on the other will be erepsin, with its
active proteolysis and slight peptonisation.
It may well be asked, what is the use to plants of the proteases
distributed in their tissues ? The importance of these substances
to insectivorous plants is sufficiently obvious ; and it is easy to
imagine how they may be of service to plants like the Fungi
which are parasitic or saprophytic in habit. In both these cases,
so far as is known, they serve to supply the plant with organic
nitrogenous food from without. But what is the physiological
signiticance of these substances in the case of an ordinary plant
which does not require to be supplied with organic nitrogen ?
The reply to this question is briefly as follows. Normal green
plants in their nutritive processes build up, from the simple
materials of their food, organic nitrogenous substance which is
stored in their tissues in the form of proteid matter that is often
insoluble, and in any case is not readily diffusible. Consequently,
when these stores of proteid are to be drawn upon for the purpose
of growth, it is necessary that there should be some means by
which they can be converted into substances which are both
soluble and diffusible. This conversion is effected by the proteo-
lytic enzymes. Their importance is strikinglj^ illustrated in a
germinating seed, where the reserve materials, whether deposited
in the cotyledons or in the endosperm, have to be made available
for the nutrition of the growing embrj'^o. It is also clear in a
germinating bulb or tuber, where the growth of the new shoots is
dependent upon the reserves which these organs contain. But it
is not limited to such cases as these. It is quite as great under the
ordinary circnmstauces of the plant : for it is at all times necessary
that the elaborated organic nitrogenous substance should be
readily distributed throughout the body. Just as diastase, first
discovered in seeds, has been found to occur in all parts of the
plant-body where starch has to be converted into sugar, so
the proteases are to be found wherever insoluble or indiffusible
proteid has to be converted into the soluble and diffusible amidic
acids, such as leucin, tyrosin, and asparagin.
When the digestive activity of certain of the insectivorous
plants, such as Xepenthes and Drosera, was first discovered, it was
difficult to imagine how these plants should have developed the
peculiar faculty of secreting proteases. But in the light of
the subsequent discoveries of which I have endeavoui'ed to give
you some account, the explanation is simple. If leaves generally,
or at any rate commonly produce a protease, it ceases to be
remarkable that this should take place in the leaves of insectivorous
plants. The peculiarity of these plants is now limited to