108 DIFFUSION AND OSMOTIC PRESSURE 



experimentation, an exact knowledge of the manner in which 

 water is raised in tall stems has not yet been reached. 

 Various hypothetical explanations of the observed phenomena 

 have been offered, but no one of them has been thoroughly 

 established. Imbibition, capillarity, the lifting power of 

 evaporation exerted upon a cohering water column, physical 

 osmosis, and undefined " vital activity," have all been invoked 

 to explain the phenomena of the ascent of sap in trees. It 

 is not intended to take up here the discussion of any of 

 these hypotheses excepting those which deal with osmotic 

 pressure and diffusion. 



It is well known that the xylem is the conducting region 

 for water. Since the tracheae, which mainly compose it, are 

 dead and contain no protoplasmic lining, there cannot be 

 attributed to them any active part in lifting the water which 

 they contain. It has been proposed 1 as a partial explana- 

 tion of this rise of water that the exudation pressure which 

 is made externally apparent where a plant is cut or broken 

 may be normally active within the xylem and may thus fur- 

 nish a part of the needed force. This is a very plausible 

 theory if not pushed too far. 



It can hardly be supposed that if this pressure is con- 

 cerned in raising water in the xylem it is exclusively applied 

 at the base of the stem or in the roots. It is much easier to 

 suppose that the various groups of cells exerting exudation 

 pressure act as a set of relay pumps, each group taking the 



i This theory, as far as I know, was first clearly put by G-ODLEWSKI in his paper 

 entitled " Zur Theorie d. Wasserbewegung in den Pflanzen," Jahrb.f. wiss. Bot., Vol. 

 XV (1884) , pp. 569-630. WESTERMAIEB had somewhat the same idea a year previous to 

 this, but did not develop it as well. His article is " Zur Kenntniss der osmotischeu 

 Leistungen des lebenden Parenchyms," Ber. d. deutsch. bot. Ges.,Vol. I (1883), pp. 371-81. 

 But the best exponent of the pumping action of parenchyma was JANSE, whose ideas 

 are expressed in a paper entitled " Die Hitwirkung des Markstrahlen beider Wasser- 

 bewegung im Holze," Jahrb.f. wiss. Bot., Vol. XVIII (1887), pp. 1-69. The last-named 

 author elaborated Godlewski's theory and supported it with experiment. Many more 

 citations might be made ; the literature is very voluminous. For a very complete dis- 

 cussion of this subject, see E. B. COPELAND, " The Rise of the Transpiration Stream," 

 Bot. Gaz., Vol. XXXIV (1902), pp. 161-93, 260-83. 



