242 D. T. MACDOUGAL 



various solutions might be poured. Triangular pieces of thin 

 glass were laid on these pieces. The swinging vertical arm of an 

 auxograph rested in a socket in the middle of this plate. When 

 the entire preparation was in readiness and the pen at the other 

 end of the compound lever was marking properly on the ruled 

 paper of a revolvmg cylinder, the solution was poured into the 

 dish. The rate, course and amount of expansion was recorded 

 by an inked line. (See Mem. N. Y. Botan. Garden, 6, pp. 

 5-26, 1916, for a description of instrument.) 



A mixture of agar 90 parts and glycocoll 10 parts gave the 

 following swelling coefficients: 



A cell which reacted as did these plates of "plant-colloid" 

 would be notably affected by acidity or alkalinity, especially in 

 the imbibition of water from protoplasts with which it might be 

 in contact. 



Acidity and alkalinity are conditions encountered very nearly 

 all the time in the cell, but these are not the only factors affect- 

 ing imbibition. Salts, especially of potassium and calcium, are 

 practically always in solution in cell sap. It is customary to say 

 that some salts increase the swelling power of gels, but most 

 of such assertions rest upon the results of experiments with 

 gelatine. 



Mixtures of agar and any protein substance swell less in any 

 salt solution than in distilled water. The swelling of a mix- 

 ture of 90 parts agar and 10 parts glycocoll in salts is illustrated 

 by the figures in table 1. 



"V^Tien plant-colloids are affected in this differential manner 

 by acids, alkalies and salts, it is ob\dous without further citation 

 of experimental results that these and other features may play 

 a part in the possibilities of parasitism. Combinations are 



