84 Diffusion and Osmotic Pressure 



niger, when grown in strong nutrient solutions of sugar, etc., 

 escapes plasmolysis by an enormous increase in concentra- 

 tion of the cell sap, this being produced in part by true 

 carbohydrates, but mainly by some unidentified substance, 

 which is, however, probably closely related to the latter 

 group. Maquenne' found by the freezing-point method 

 that the expressed juice of seedling peas six days old had an 

 osmotic pressure such that the average molecular weight of 

 the solutes must be in the neighborhood of 239. This shows 

 that the active substance has a much larger molecule than 

 glucose (mol. wt. 180). The juice failed to show the pres- 

 ence of glucose or cane sugar by tests made with Fehling's 

 solution and with phenylhydrazin acetate. Helianthus seed- 

 lings, however, showed the presence of glucose, and the sap 

 had an average molecular weight of only 136. Here, then, 

 the pressure is largely produced by some molecule much 

 smaller than that of glucose. Stange'^ found that the cells 

 of growing root tips of Pisum, Lupinus, etc., which had been 

 grown in strong KNO3 solutions, showed no accumulation 

 of that salt, although there was a marked increase of it in 

 the parenchyma farther up. The turgor pressure was the 

 same in the root tip as elsewhere in the plant. This argues 

 that the increase in turgor in the growing region, which 

 prevents plasmolysis where the culture is in a strong solu- 

 tion, must be due to some other substance or substances. A 

 collection of analyses of various plant parts, which may be 

 taken as at least some indication of the nature of the active 

 substances in the sap, is given by Mayer.' DeVries* made 



1 L. Maquenne, " Sur la pression osmotique dans les graines germfies," Compt. 

 rend.. Vol. CXXIII (1896), pp. 898 ff. 



2B. Stange, "Beziehungen zwischen Substratooncentration, Turgor vmd 

 Wachsthum bei einigen phanerogamen Pflanzen," Bot. Zeitg., Vol. L (1892), p. 253. 



3 A. Maybe, Lehrbuch der AgricuUurchemie, Heidelberg, 1895, pp. 307 ff. 



*H. DeVeies, "Eine Methode zur Analyse der Turgorkraft,"Jaftr6./.M)t&!.£oi., 

 Vol. XIV (1884), pp. 427-601. 



