110 DIFFUSION AND OSMOTIC PRESSUEE 



protoplasm, yet plays no leading r6le in the lifting of water 

 up tall stems. As far back as 1853 T. Hartig l showed that 

 a poison (ferric pyrolignate) would pass up the stem of a tree 

 for over 12 meters. He bored five radial auger-holes in the 

 tree trunk near its base, all meeting at the center. These 

 holes were filled with the poison solution and then plugged. 

 When the tree was cut down, the star-shaped stain of the 

 poison was found in a cross-section over 12 meters above the 

 holes. Strasburger 2 performed the same experiment more 

 thoroughly. Trees were cut off and set into tubs of poison, 

 such as aqueous solutions of CuSO 4 and picric acid. The 

 poison ascended to the leaves, a distance of twenty-one meters 

 in the tallest tree. Of course, if these violent protoplasmic 

 poisons ascend the trunk, they must kill all cells lying in 

 their path. Therefore the living cells of the stems cannot be 

 necessary for the rise of sap. 



But after the leaves had been killed the stem ceased to 

 absorb more solution, or absorption took place very slowly. 

 This may be explained by the fact that most leaves collapse 

 and dry upon being killed. The cause of the rise of sap is 

 perhaps the evaporation from the surface of the leaves, and in 

 order that it should rise the leaves must be in their normal 

 turgid condition. Evaporation may thus result in concen- 

 tration of the solution on the surface of the walls of the 

 parenchyma, thus causing an outward osmotic flow of water 

 from the cells, the solutions of which in turn become more 

 concentrated and extract water from cells lying still farther 

 within the plant. This process may be thought of as con- 

 tinuing until water is finally extracted from the zylem. 

 Here the osmotic withdrawal of water would probably become 



IT. HARTIG, "Ueber die endosmotischen Eigenschaften der Pflanzenhaute," 

 Bot. Zeitg., Vol. XI (1853), pp. 309-17. 



2 E. STRASBUEGEE, Ueber den Bau und die Verrichtungen der Leitungsbahnen 

 in den Pflanzen, Jena, 1891; IDEM, " Ueber das Saftsteigen," Histologische Beitrage, 

 Vol. V, Jena, 1893. 



