22 PHYSIOLOGY 



cell wall. The latter, which plays a great part in the building up of vegetable 

 tissues, is formed by a process of secretion from the living protoplasm and 

 is situated altogether outside the superficial Plasmahaut. The cell wall 

 differs considerably in its chemical composition from the protoplasm out of 

 which it has been formed. In most plants it consists of cellulose, a substance 

 belonging to the carbohydrate group, and with a composition represented 

 by some multiple of the formula C 6 H 10 5 . In other cells the wall may be 

 built up from calcium carbonate or other lime salts, from silica, from chitin. 

 In many cases it is perforated to allow the passage of communicating strands 

 of protoplasm between adjacent cells. It is generally freely permeable to 

 all kinds of solutions, and in this case plays no part in regulating the inter- 

 changes of the cell with the environment. 



The superficial layer of protoplasm represents that part of the living 

 substance which stands in immediate relationship to the environment. 

 Every change in the latter can only influence the living cell through this 

 layer, and it is through this layer that substances must pass on their way into 

 the cell for assimilation, or out of the cell for excretion. The retention of an 

 individuality by the cell must be determined by chemical and physical 

 differences between this layer and the surrounding fluid. Since it differs from 

 the rest of the protoplasm in the changes to which it is subject, it must also 

 differ in its chemical composition, apart altogether from the factors which, 

 as we saw above, determine molecular differences between the surface and the 

 interior of any colloidal solution. On this account one must assume the 

 existence of a definite boundary layer of the protoplasm, even where it is 

 impossible to see any differentiation between this layer and the deeper 

 parts under the highest powers of the microscope. 



A (living) cell, which leads its life in a liquid environment, must take up 

 the greater part of its food material in the form of solution, and it is the 

 permeability of the superficial protoplasm which will determine the passage 

 of food substances from the surrounding medium into the body of the cell. 

 The immiscibility of the protoplasm with the surrounding fluid shows that 

 the permeability of the membrane must be a limited one. The qualitative 

 permeability can be easily studied in vegetable cells. These present within 

 a pellulose wall a thin layer of protoplasm (the primordial utricle), enclosing 

 a cell sap. If the root hairs of tradescantia be immersed in a 10 per cent, 

 solution of glucose or in a 2 to 3 per cent, solution of salt, a process of 

 plasmolysis takes place. The cell sap diminishes in amount by the diffusion 

 of water outwards so that the primordial utricle shrinks (Fig. 7). On im- 

 mersing the cells in distilled water, water passes into the cell sap until the 

 further expansion of the protoplasmic layer is prevented by the tension of 

 the surrounding cell walls. This behaviour can be explained only on the 

 assumption that the protoplasm is impermeable both to sugar and to salt, 

 but is freely permeable to molecules of water, i.e. it behaves as a semi- 

 permeable membrane. Similar experiments can be made on animal cells. 

 The most convenient for this purpose are the red blood corpuscles. These 

 also shrink when immersed in salt solutions with a greater molecular 



