The Seed and its Germination. 
499 
On submitting seeds to the influences favourable to the 
process of germination, of which moisture and warmth are the 
chief, the embryo or young plant is found to grow and develop, 
while the reserve materials disappear pari passu with such 
growth. The problem involved in germination is, therefore, to 
convert the insoluble and indiffusible materials, stored as de- 
scribed in the cells, into such related forms as are capable of 
being dissolved in the water or sap which permeates the ger- 
minating seed, and are also capable of passing through the walls 
of the cells. The latter we have seen to consist of two 
parts — a cellulose limiting layer, and a lining of protoplasm 
in close approximation to this. Materials capable of passing 
through the former are not necessarily able to pass the latter also. 
This problem is very much like the one which we meet with 
in the animal organism, when we consider the way in which 
the materials which the latter takes in as food are converted 
into forms suitable for absorption into the tissues of its body. 
The food which is thus taken consists of compounds belonging 
to the same classes as the reserve materials of the seed, and, 
as its ultimate end is the same — viz. the nutrition of the living 
substance — it seems likely that the processes of digestion and 
assimilation will be very much alike in the two cases. In the 
case of the animal organism it has long been known that the 
necessary changes are brought about through the agency of 
peculiar bodies known generally as iinorganised ferments or 
enzymes. Such are the pepsin secreted in the stomach, the 
starch-transforming body (ptyalin or diastase) found in the saliva, 
and the similar one occurring in the secretion of the pancreas. 
Another well-known instance of a body of this kind is the 
rennet which can be extracted from the mucous membrane 
lining the stomach of the calf, and which has the property of 
converting the casein of milk into curd or cheese. 
What a ferment is exactly is not at present known ; the 
only proof it gives of its existence is its activity. Certain 
characters, however, which all ferments have in common may 
be briefly summarised as follows : (1) They are present in very 
minute quantities. (2) Their activity depends upon their being 
exposed to a certain limited range of temperature. (3) They 
are totally destroyed by a high temperature, such as that of 
boiling water. (4) They do not become changed or destroyed 
by their activity. (5) They do not themselves enter into the 
composition of the bodies which they produce. (G) Then- 
activity is much impeded, and at last stopped, by the accumu- 
lation of the products of their action. The mode of their action 
seems to be to cause a disruption and rearrangement of the 
