20 Hydration and Growth. 



no plate was made in which this was entirely eliminated. Thus a plate 

 which would yield increases in thickness of 6 to 800 per cent in trios of 

 sections in distilled water reached a length as much as 50 per cent 

 greater than the original under the same conditions. 1 



A unilateral action such as that described is one which appears to 

 rest upon the supposed honeycomb structure of the colloid. Dehydra- 

 tion would lessen the volume of the mass, and as the sheets or strands 

 of denser material are held in a plane parallel to the surface, the spaces 

 containing the more discontinuous, more liquid element would be 

 deformed and their vertical diameter decreased. Accession of liquid 

 or of water would be followed by the partial resumption of the original 

 form and dimensions. Experience in dealing with a large number of 

 plates leaves the impression that the swelling does not bring the sec- 

 tions back to the thickness of the cooled gel as it was originally in the 

 mold. 



The fact that colloids such as those present in living matter may 

 retain a shearing strain was recognized by Butschli and was the subject 

 of some experimentation by Hardy 2 in a study of coagulation phe- 

 nomena, who concludes that " shearing a colloidal mass, fluid or solid, 

 actually does produce heterogeneity or, simply, structure, which is 

 fixed by the process of coagulation." Such sheared masses of colloid 

 are doubly refractive. Klocke demonstrated the acquisition of such 

 double refraction by sheets of gelatine which were dried enframes 

 covered with tin-foil. It is to be noted that the plates of gelatine 

 which were dried without superficial shrinkage in my own experiments 

 when hydrated showed some extension, while those of the agar-protein 

 mixture did not. The hydration in both cases presumably removed 

 the strain as the structure produced by the stress disappeared. Great 

 cytological interest attaches to the simple experiment by Hardy, in 

 which a small quantity of a colloidal solution is drawn along a glass 

 slide by the point of a needle, after which it is " fixed" by the methods 

 of the cytologist, with the result that the mass appears to consist of a 

 number of fibrillae " * * * so striking that they look as if one might 

 isolate them by teasing." 



Much interest also attaches to some recent work of Miss C. L. Carey 

 of Barnard College upon the structure of agar films. 2.5 per cent 

 gels of this substance were prepared by a method similar to that 

 described on page 16 for preventing superficial shrinkage. When such 

 plates were dried at 45 to 70 C. and again placed in water the 

 rehydrated plates yielded drops of water so readily that an examination 

 of thin sections under the microscope was made, revealing cavities 



^or the original notice of increase in thickness and not in length, see MacDougal and Spoehr, 

 Growth and Imbibition, Proc. Am. Phil. Soc., 56 : 343, 344. 1917. Philadelphia. 



2 Hardy, W. B. On the structure of cell protoplasm. Journal of Physiol., 24 : 158. 1899. 

 See especially pp. 187-190. London. 



