MacDOUGAL— HYDRATION AND GROWTH. 349 



of the mass. In actual practice the mixtures are soHdified too 

 quickly for this to occur. 



The separate colloidal masses where they do exist have, of 

 course, definite boundary layers, as are formed wherever two col- 

 loidal phases meet. Protoplasm may not be regarded however as 

 altogether a mechanical admixture of minute strands of material of 

 different composition. Much of it, including the more fluid portions, 

 must consist of molecules of carbohydrates, proteins, salts and even 

 lipins aggregated to form submicrons in the disperse phase or in 

 the denser more solid fibers, mesh or honeycomb of the structure. 

 The external layer formed might well be in a sense a mosaic, but 

 it is to be noted that no actual proof of such a condition is at hand. 

 Both absorption or imbibition and osmosis including differentiated 

 diff'usions would be aft"ected by the composition and relations of the 

 two phases of the colloids in this outer layer, and it seems highly 

 probable that an adequate interpretation of permeability will be ob- 

 tained by a study of these features. Meanwhile no general agree- 

 ment 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 it has no other permanent or morpho- 

 logical value. Its constitution must necessarily vary widely as does 

 that of the living protoplasm. 



The experimental studies described in the present paper have been 

 devoted chiefly to the action of protoplasmic masses in which the 

 pentosans in colloidal combination with proteins and salts determine 

 the volume or mass of the living material of the cell. The other 

 soluble carbohydrates, including the hexoses, sucrose, dextrose, do 

 not occur in the cell in such concentrations as to affect the enlarge- 

 ment of the protoplasmic mass directly, but in the vacuoles they may 

 exert an osmotic effect additive to that of the amino-acids which 

 may accumulate in these cavities. It is to the osmotic activity of 

 these substances in the vacuoles that turgidity is due and a by no 

 means unimportant part in the maintenance of the rigidity of organs 

 and other features is to be ascribed to these turgor stresses and ten- 

 sions. The inadequacy of osmotic phenomena and of the concep- 



