THE STRUCTURAL BASIS OF THE BODY 23 



ment, this layer acquires a prime importance for the life of the cell, 

 and we may therefore consider here at greater length some of the 

 properties of this layer, the Plasmahaut, as it has been called. 



The superficial layer of the protoplasm is not to be confounded 

 with the 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 com- 

 position 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^HjoOg. 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 interchanges 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 excre- 

 tion. 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 exist- 

 ence 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 cellulose 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 



