6 VEGETABLE PHYSIOLOGY 
plastids. The bulk of the living substance, to distinguish 
it from these specialised portions, is usually called the 
cytoplasm. It is not of the same consistency throughout, 
its outer portion, which is in contact with the cell-wall 
and is somewhat denser in character, being known as the 
ectoplasm. A similar firm layer may frequently be detected 
round the vacuole. These are not, however, to be confused 
with the cell-membrane or cell-wall, being particular layers 
of the cytoplasm. 
The exact chemical composition of protoplasm cannot 
be ascertained, as analysis involves its death, and this is 
attended by changes in its substance. It contains carbon, 
hydrogen, oxygen, nitrogen, and probably sulphur and 
phosphorus, but we are quite unable to say in what different 
combinations they exist within it. Enclosed in it are 
always varying quantities of organic substances such as 
proteins, carbohydrates, and fats, and small quantities of 
various inorganic and organic salts. The substance of the 
protoplasm has been thought either to be arranged in 
the form of a network, these various bodies occupying the 
meshes, or to have a foamy structure much like that pro- 
duced by vigorously stirring a mixture of oil and water. The 
various substances alluded to as occurring in close relation- 
ship to it are connected with the nutritive and other vital 
processes of the cell, or its metabolism, and hence differ 
greatly in nature and amount from time to time. 
In the case of the free-swimming protoplasts, with 
which we began the study of protoplasm, we saw they were 
in active motion. As the protoplasts become enclosed in 
cell-walls this motility is, of course, less and less obvious ; 
indeed in most cells it cannot be distinguished at all. There 
is reason to suppose, however, that protoplasm, wherever 
existing, is in active, though imperceptible, motion. In 
many of the constituent cells of some of even the higher 
plants this motility can be observed, particularly whore the 
protoplasm has a granular appearance. In certain of the 
cells forming the leaves of many aquatic plants, e.g. Vallis- 
