134 D. T. MacDOUGAL and h. a. spoehr 



to the action of distilled water in dissolving out the contents of 

 cells and of the components of colloidal plates, and to estimate 

 the rate and amount of acids dissolved out. 



We are indebted to Prof. H. M. Richards for determinations 

 of the acidity of the water in which fresh slices of the Opuntia 

 were swelled and it was found this might be expressed as fol- 

 lows: 10 cc. solution from dish in which set of fresh sections 

 were swelled in water = 0.44 cc. N/20 KOH. Dried slices of the 

 above material when swelled in water twenty-four hours gave a 

 solution the acidity of which might be expressed as: 10 cc. of 

 solution = 0.10 cc. N/20 NaOH. When such sections were im- 

 mersed in citric acid 0.01N, the strength of the solution was 

 increased so that at the end of twenty-four hours the acidity 

 was expressible as: 10 cc. of solution = 2.25 cc. N/20 NaOH. 



The rapid penetration of cell-masses by acids is a well-known 

 reaction and is accompanied or followed by the extraction of 

 electrolytes, and it has been held by some authors that when 

 the preliminary swelling which takes place in acids changes to a 

 shrinkage that death is implied. 4 This last distinction is one 

 without special meaning in connection with the present paper. 



The hydration of colloidal plates, of dried sections of plants, 

 and of fresh sections presents parallel reaction in hundredth 

 normal organic acids. The chief features of such swellings con- 

 sist in an initial rapid expansion followed by shrinkage due to 

 solution or dispersion of the sections. 



The swelling of the pentosan, agar, which has been used so 

 widely in the imbibition measurements in connection with 

 growth would be accompanied by the solution or dispersion of 

 material of the external part of the sections and by the diffusion 

 of whatever salts or acids might be present in the interior of the 

 mass. A test was carried out to determine what such loss might 

 be in sections of agar and in a simple biocolloid. The agar was 

 cast in plates which came down to 0.25 mm. in thickness from 

 a 2.5% solution. Sections swelled as follows at 17 to 18°C. 



4 MacDougal, Richards and Spoehr. The basis of succulence. Bot. Gaz., 

 67: 405. 1918. 



