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 where the 

 protoplasm has a granular appearance. In certain of the 

 cells forming the leaves of many aquatic plants, e.g. Vallis- 



