1 68 BOTANICAL GAZETTE [September 



of trees in spring better than does the preceding one. The two 

 are closely related, and it would be difficult to distinguish them 

 by experiment. Both have the source of energy in the concen- 

 tration of the sap. Either provides for only a limited period of 

 high pressure, while the supply of stored, dissolved, or soluble 

 material in the root is carried away by the sap. In its essential 

 features this explanation of the mechanism of root pressure was 



offered by Brucke in 1844. He illustrated (pp. 194-6) the 

 proportionality of the concentration of the sap to the pressure 

 observed, and said: **VVirwerden in der Folge in einer andern 

 Abhandlung naher betrachten, wie sich die Zellen vermittelst 

 dieser loslichen und aufquellenden Substanzen zuerst strotzend 

 mit Wasser anfiillen, und wie dann, in dem sie immer noch mehr 

 Wasser anziehen, das was sie in ihrer Hohle nicht mehr beher- 

 bergen konnen, mit einem Theile der gelosten Substanzen als 

 Saft in die benachbarten Spiralrohren hineingepresst wird.'* I 

 cannot find that the treatise promised here ever appeared. 



The parallel between pressure and concentration is illustrated 

 again by the w^ell-known use of the sap of birch and maple, trees 

 with high root pressure, as sources of sugar. Clark (1874^188) 

 found a root pressure in a Betida le?iia seventy feet high sufficient 

 to lift water 84.77 f^^t, the pressure in a root being higher yet. 

 Acer saccharinwn, with 3.57 per cent, sugar in its sap, registered 

 a pressure of 3173 feet of water. Clark also reports a pressure 

 of 49.52 feet — 6.5 feet above Hale's figure — by grape, the sap 

 being sweet to the taste. The curve given by Schroder (1865) 

 for the concentration of the sap of birch during the season and 

 Clark's for the pressure are parallel, though ihe inorganic solute 

 decreases constantly. The same author (Schroder, 1869:280) 

 finds 3.44 per cent, sugar in the sap of Acer platanoidcs. The 

 bleeding sap of walnut contains 4 per cent, sugar (Hartig, 

 1862:88). In all bleeding trees the concentration of the sap 

 and the pressure are supposed to decrease together. In the 

 herbs also (Ulbricht) the concentration of the sap decreases as 

 bleeding go^s on. Almost all publications on the concentration 

 of the sap are in such terms that it is impossible to compute the 



