16 INTRODUCTION TO GENERAL PHYSIOLOGY 



protoplasm itself, and will vary in its composition, according to 

 the chemical processes going on in the cell. Further, we must 

 not forget that if the liquid outside the cell contains dissolved 

 substances, these will assist in the formation of the membrane. 

 This concerns especially the tissues of the higher organisms, which 

 are bathed by solutions of a complex composition. 



This phenomenon of adsorption is met with in a great number 

 of cases, both in living organisms and in other heterogeneous 

 systems. A familiar instance is the use of charcoal for remov- 

 ing colouring matters from solutions of other things (E., p. 168). 

 The colouring matter is not destroyed by the charcoal, but 

 deposited on its surface, whence it can be removed by appropriate 

 means. 



The Permeability of the Membrane 



Having seen how the membrane is formed, we must next find 

 out what are its properties, especially in view of what has been 

 pointed out above as to the escape of matters from the cell. There 

 are some things that it allows to pass, others not. Its "perme- 

 ability " has to be investigated. 



In the first place, it must allow water to pass through quite 

 freely, because we can see cells swell up under some conditions. 

 What is the cause of this swelling is a rather difficult question, 

 which must be discussed presently. But does the membrane allow 

 anything which may be dissolved in the water to pass through ? 

 Unless the solute (that is, the^substance in solution) is coloured, we 

 cannot see directly whether it has gone in or not. But we can 

 test the behaviour to coloured substances, such as aniline dyes and 

 other pigments (E., p. 168). It is scarcely necessary to remark that 

 we must not make use of anything that injures the cell, because the 

 membrane would not then be in its normal state. Some aniline 

 dyes can be used ; aniline blue and congo red will be found not to 

 stain the cell protoplasm. We see, then, that there are some solutes 

 to which the cell membrane is impermeable. 



In the cells of the root of the red beet, there is a pigment to 

 which their membrane is impermeable (E., p. 168). But we can 

 influence the membrane in such a way that it will allow the pigment 

 to escape. Killing by heat does this. Certain chemical agents also 

 do so. Moreover, some of these agents, if carefully applied, do not 

 permanently injure, so that we can get the membrane to recover. 

 This is important, because it shows the possibility of changes 

 during life, so that at one moment a cell membrane may allo\v a 

 substance to enter or escape, at another moment it may refuse 

 passage to it, according to the state of the cell itself. The cane 



