60 CARNEGIE INSTITUTION OF WASHINGTON. 



tion, imbibition and osmosis being controlled or affected by the 

 hydrogen-ion concentration, salts and amino-compounds of the cell 

 sap, and also by the composition of the medium or substratum and 

 other environmental conditions, such as temperature. 



The Hydration of Biocolloids simulating certain Features of Protoplasm, 

 by D. T. MacDougal. 



Extensive experimentation with various biocolloids has given ample 

 support to the conclusion that a mixture of carbohydrates and proteins 

 may be made which will exhibit hydration relations similar to those 

 of protoplasm. 



Agar which has been specially purified for this purpose by E. R. 

 Squibb & Sons has been found a useful pentosan, and others are cherry 

 gum, acacia gum, and the mucilage of Opuntia. The last-named products 

 are prepared by precipitation with alcohol, but the degree of purity 

 of such preparations is not as great as that of the agar. These pento- 

 sans may vary not only in the proportions of the pentoses and hexoses 

 present, but also in their acidity, which may be due to the presence of 

 amino-acids. 



The replacement of some of the agar in an agar-albumin mixture by 

 one of these gums or mucilages modifies the swelhng reactions, as illus- 

 trated by the data in the table on page 61. 



The pH values were calculated from colorimetric tests after the 

 method perfected by Dr. B. M. Duggar. Soy albumin was tested in 

 a 0.5 per cent solution, gelatine in an 8 per cent solution, Opuntia 

 mucilage, cherry gum, acacia gum, and agar in a 1 per cent solution. 



Among other facts, it is to be seen that the hydration capacity of 

 agar-albumin is less in water than other biocolloids in which some of the 

 agar is replaced by another pentosan group. 



Next it is obvious that the hydration in water, which was renewed 

 at regular intervals, is not conditioned directly upon the hydrogen- 

 ion concentration of the components. 



The maximum swelling is greatest in the biocolloid containing the 

 mucilage of Opuntia, which is at the same time the most "sensitive" 

 of all the combinations, as its swelling in hydroxides, salts, and acids 

 is very low, especially in the acids, a fact which corresponds to the 

 inferred action of acidity in the living cacti. 



The single salt used determines a hydration of gelatine less than in 

 water, of agar and agar-protein half or less than in water, and when 

 some of the agar is replaced by another pentosan the swelling capacity 

 is lessened. The foregoing reactions are obviously of primary signifi- 

 cance in the mechanics of growth. 



The swelling of dried sections of the above mixture also yields some 

 information as to the influence of deposition and other historical 



