THE CELL. 3 
pounds of carbon, hydrogen, and nitrogen into a ma- 
terial like itself. It can assimilate a mixture of sugar 
and an ammonium salt, or oil and an ammonium salt; 
but it cannot assimilate mixtures entirely inorganic, 
for it cannot decompose carbonic acid. This power 
is possessed alone by chlorophyll (c,, H,, N, 0,,); 
which is a secretion of protoplasm found abundantly 
in plants, forming the green colouring matter of the 
leaves, and very rarely in animals. Chlorophyll has 
the power of manufacturing starch by decomposing 
the carbonic acid of the air under the influence of 
sunlight. The starch, which is insoluble, changes 
into sugar or oil, and is then transformed into 
albuminoids (that is, compounds containing nitrogen) 
by the action of the ammonia absorbed by the roots 
of the plant. 
Trritability—The addition of a weak solution of salt 
produces irregular changes of form in living proto- 
plasm, although it does not affect it when it is dead. 
Corpuscles of protoplasm also absorb particles of 
nourishing material by surrounding them, probably, 
therefore, they feel them. 
All organic substances do not show the phenomena of life, 
but all have been formed through the agency of living 
matter. | 
Growth and formation of the Cell.—Living proto- 
plasm is not only constantly assimilating, but it is also 
constantly secreting — that is, constantly preparing and 
separating other substances out of itself. These secre- 
tions are in two forms, internal and external. The 
internal secretions are chlorophyll, starch, fat, &e The 
external secretion takes the shape of a surrounding wall, 
which encloses the corpuscle of protoplasm, and thus forms 
