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 

 proteids, 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 oil and 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 ihe case of the free-swimming protoplasts, with 

 which we began the study of protoplasm, we saw they were 

 in active motion. As the protoplast becomes 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 even some of 

 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, e.g. 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. 



