68 DIFFUSION AND OSMOTIC PRESSURE 



That ordinary leaves can absorb inorganic salts was shown 

 in this way by Dandeno. 1 He found that drops of solution 

 placed upon foliage leaves were completely absorbed if too 

 rapid evaporation was prevented. When the drop disap- 

 peared no trace of solute crystals remained upon the leaf 

 surface. 



d) Test by toxicity. To all protoplasmic poisons must be 

 accredited power of penetration in a greater or less degree ; 

 if there were no penetration the substance could not bring 

 about its toxic effect. True 2 proved a slight toxicity for 

 KNO 3 and Nad upon Spirogyra; these substances must 

 therefore penetrate the protoplasts of this plant, though 

 probably this occurs with difficulty. More recently Coupin 3 

 has prepared a catalogue of the poisonous effects upon wheat 

 of certain salts in various concentrations. His tables are 

 useful for comparison. Of course, the fact that a rather 

 high concentration of a given solute is needed to affect the 

 plant may mean either that the protoplasm is only slightly 

 permeable, or that the substance is only slightly toxic. 

 From Pfeffer* we have the fact that mercuric chlorid and 

 iodin penetrate many vegetable cells and exert a marked 

 toxic effect. 



There are many other proofs that various mineral and 

 organic substances are able to penetrate the plant protoplast. 

 Where a noticeable and specific effect is produced upon the 

 organism by the presence of a given substance in the 

 medium, there can be no doubt that the substance pene- 



ij. B. DANDENO, "An Investigation into the Effects of Water and Aqueous 

 Solutions of Some of the Common Inorganic Substances on Foliage Leaves," Trans. 

 Canad. Inst., Vol. VII (1901), pp. 238-350. 



2 R. H. TRUE, " The Physiological Action of Certain Plasmolyzing Agents," 

 Bot. Gaz.i Vol. XXXVI (1898), pp. 407-16. 



3H. COUPIN, "Sur la toxicit6 des composed du sodium, du potassium, et de 

 Vammonium a 1'egard des v6g6taux supSrieurs," Rev. gen. 6ot, Vol. XII (1901), 

 pp. 177-94. 



4 W. PFEFFER, Osmotische Untersuchungen, Leipzig, 1877, p. 140. 



