6 VEGETABLE PHYSIOLOGY 
layer may frequently be detected round the vacuole. Some- 
times these limiting layers are spoken of as plasmatic 
membranes on account of their specially dense character ; 
they 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 produced by 
vigorously stirring a mixture of oiland water. The various 
substances alluded to as occurring in close relationship 
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 protoplast, 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, particu- 
larly where the protoplasm has a granular appearance. 
In certain of the cells forming the leaves of many aquatic 
plants, eg. Vallisneria, Nitella, Elodea (fig. 7), and 
others, a streaming movement of the granules the proto- 
plasm contains can be detected under a high power of 
the microscope. In other plants of terrestrial habit, e.g. 
