68 Diffusion and Osmotic Peessuee 



That ordinary leaves can absorb inorganic salts was shown 

 in this way by Dandeno.' 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'' proved a slight toxicity for 

 KNO3 and NaCl upon Spirogyra; these substances must 

 therefore penetrate the protoplasts of this plant, though 

 probably this occurs with difficulty. More recently Coupin' 

 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 Pfefifer* 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- 



1 J. B. Dandeno, "An Investigation into the Effects of Water and Aqneous 

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

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



2E. H. Tette, "The Physiological Action of Certain Plasmolyzing Agents," 

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



3H. CoupiN, "Sur la toxicitS des composSs du sodium, du potassium, et de 

 I'ammonium h I'Sgard des vfigStaux sup6rieurs," Rev. gen. bot., Vol. XII (1901), 

 pp. 177-94. 



*W. Pfeffeb, Osmotlsche Untersuchungen, Leipzig, 1877, p. 140. 



