TUBGIDITY 67 



dye has accumulated in the vacuole, there becoming much 

 more concentrated than was the original external solution. 

 This phenomenon will be discussed under e). 



Another method by which direct determinations of per- 

 meability may be made is to analyze the plant or its juice 

 after the culture has been grown some time in a medium of 

 known content. In this way von Mayenburg 1 found that, 

 out of a series of substances used in his culture fluids for 

 Aspergillus niger, only glycerin was absorbed in sufficient 

 quantity to be worthy of consideration as an osmotically 

 active solute within the cells. 



c) Absorption test. If the concentration of the sur- 

 rounding medium is carefully determined and the organisms 

 whose permeability is to be tested be allowed to grow in it 

 for a time, decrease in concentration of the medium may be 

 interpreted to mean that absorption has taken place. This 

 has been shown to occur in the case of a number of inorganic 

 salts, but is especially well marked with solutions of glucose 

 and glycerin. Demoussy 2 determined in this way the relative 

 rate of absorption of potassium and calcium ions by wheat, 

 maize, etc., while Laurent 3 was able to prove the somewhat 

 unexpected fact, that roots of maize can absorb measurable 

 quantities of glucose from a solution in which they are 

 grown. This is probably not a general phenomenon, for if 

 the protoplasm is permeable to sugar in one direction it is 

 difficult to see how it could fail to be permeable in the oppo- 

 site one also, and such a condition must allow outward 

 diffusion of glucoses and other sugars which are found so 

 commonly in plant cells. 4 



1 0. H. VON MAYENBURG, " Losungsconcentration und Turgorregulation bei den 

 Schimmelpilzen," Jahrb.f. wiss. Bot., Vol. XXXVI (1901), pp. 381-420. 



2 E. DEMOUSSY, "Absorption elective de quelques 616ments min6raux paries 

 plantes," Compt. rend., Vol. CXXVII (1900), pp. 970-73. 



3 J. LAURENT, " Sur Tabsorption des matieres organiques par les racines," ibid., 

 Vol. CXXV (1897), pp. 887-9. 



* Cf. PFEFFER-EWART, Physiology of Plants, Cambridge, 1900, p. 99. 



