Growth and Colloidal Reactions. 7 



tation of permeability will be obtained by a study of these features. 

 Meanwhile no general agreement as to the nature of the ''membrane" 

 or its action is to be expected until many widely current assumptions 

 are discarded. The external layer of a protoplasmic unit is in every 

 case a product of the surface energy of the mass or systems of living 

 material internal to it and of the medium, and has no other permanent 

 or morphological value. Its constitution must necessarily vary widely, 

 as does that of the living protoplasm.^ 



This aspect of the external layer is one which finds recognition among 

 writers on biophysics in various ways. Mathews assumes conditions 

 in the protoplasm which are not valid in plants when he says : 



"Thus it is suggeste4 that in the surface of contact of protoplasm with 

 water, lipin substances will accumulate and thus make a kind of intermediate 

 layer of a lower surface tension and of a fatty nature. But, inasmuch as the 

 whole substratum of the cell is of a fatty or lipin nature, it is difficult to see 

 how the surface tension of the junction of fat and water could be changed by 

 the passage of more lipin into tl;ie film; and, as a matter of fact, there is no 

 good evidence that there is such a layer about the protoplasm."* 



McClendon recognizes a wider range of facts,^ as follows : 



"The composition of the plasma membrane remains a mystery. It seems 

 logical to assume that its building stones are selected from the chief constitu- 

 ents of cells, proteins, fats, lecithin, cholesterin, and carbohydrates. It is a 

 very unstable structure, as will be shown later." 



An admirable presentation of the matter is to be credited to D'Arcy 

 W. Thompson, the keynote of which lies in the sentences:^ 



"The adsorbed material may range from the almost unrecognizable pellicle 

 of a blood-corpuscle to the distinctly differentiated 'ectosarc ' of a protozoan, 

 and again to the development of a fully formed cell-wall, as in the cellulose 

 partitions of a vegetable tissue. In such cases, the dissolved and adsorbable 

 material has not only the property of lowering the surface tension, and hence 

 of itself accumulating at the surface, but also has the property of increasing 

 the viscosity and mechanical rigidity of the material in which it is dissolved 

 or suspended, and so of constituting a visible and tangible 'membrane'." 



In addition to the external layer of the highly hydrated protoplasm, 

 this living material is usually separated from the surrounding medium 

 by walls or coats of specialized character, variously formed, and which 

 may be composed in part or altogether of material originating outside 

 of the masses which they inclose, which may modify the diffusion of 

 liquids into the colloidal mass in a very important manner. 



If the swelling is one of simple hydration, the entrance of additional 

 water would result solely in an increased dispersion of both phases of 



^ See Stiles and Jorgensen, Quantitative measurement of permeability. Bot. Gazette, 65 : 526. 1918. 



* Mathews, A. P. Physiol. Chem., 2d ed., p. 211. 1916. 



' McClendon, J. F. The physical chemistry of vital phenomena, p. 95. 1917. Princeton Univ. 

 Press. 



* Thompson, D'Arcy W. Growth and form, pp. 281 and 282. 1917. Cambridge. 



