OF SAP IN PLANTS. 25 



The larse surface of the tree with its hundreds of shoots causes an 

 evaporation even before the buds svi^ell ; the vacuum resulting 

 from this, causes any fluid which may be brought into free com- 

 munication with the interior of the tree, at any spot, to be driven 

 in by the vis a tergo. The power of suction thus produced 

 increases in equal proportion as the surface of the shoots is in- 

 creased by the presence of leaves, and the capacity of the air to 

 retain moisture by the heat of summer, as Hales (Vegetable 

 Statics) and, more recently, Dassen (Wiegmann's Archiv, xiii. 

 2. p. 311) have proved; under the most favourable circum- 

 stances it sustains a column of mercury 12 inches high. It will 

 not be thought strange that the height of the column of mercury 

 did not attain 18 inches, as has been observed in curved glass 

 tubes under similar circumstances. The bark of the shoots of 

 a tree does not form a solid envelope like the glass, for even with 

 a smaller weight of the column of mercury, the air itself pene- 

 trates through the bark into the interior, and thus puts an end 

 to all further ascent. Hence the loose-barked vine absorbs but 

 weakly; the denser Prunus domestica raised a column to ^th of a 

 Flemish ell (6f inches) ; Betula nana, 0*240 (13J inches) (Das- 

 sen). Hales observed the column rise to 4 inches in the vine, 

 and 12 inches on an apple branch (equal to 13 feet 8 inches of 

 water) . 



It is remarkable how accurately the fluids ascending or de- 

 scending in this way are retained in that side of the stem which 

 corresponds to the absorbing branch (or root) ; this is explicable 

 by the straight, uninterrupted, little-branched course of the 

 tracheae ; the anatomical examination of the preceding cases also 

 proved this directly. The absorbing branch was cut off on the 

 11th of March and anatomized, with the following results. In a 

 cross section 1 line above the lower end, which had been dipped in 

 the fluid, all parts, bark, liber, wood and pith, reacted deep blue ; 

 but even at a distance of 2 lines the pith was found uncoloured ! 

 Four inches further up, that is, close above the upper boundary 

 of that portion of the branch w^hich had been immersed in the 

 fluid, the heart-wood in the vicinity of the pith was also found 

 uncoloured, as was the case everywhere above this. From this 

 point the pitted and striped trachea of the softer external por- 

 tion of the wood, with the liber, had taken on the conduction of 



