144 • CIECULATION OF THE SAP. 



Dutroohet found that with a membrane of 40 millimetres in 

 diameter, a tube of 2 mUlinietres, and a solution of sugar, the density 

 of which was 1-083, the fluid rose 39 millimetres in the space of an 

 hour and a half; with syrup, of density 1'145, the rise was 68 milli- 

 metres ; and with syrup, of density 1'228, the rise was 106 millimetres. 

 Syrup, of density 1'3, produced a current capable of raising a column 

 of mercury of 127 inches, which is equal to a pressure of 4 J atmo- 

 spheres. Thus the velocity and force of the rise depend in this 

 instance on the excess of density of the enclosed liquid over that of 

 the water outside. Different' substances act with varying intensity 

 in producing endosmose. The following ratio expresses the variable 

 intensity of endosmose in different cases in which the density of the 

 solution was the same : — Solution of gelatin, 3 ; of gum, 5-17 ; of 

 sugar, 11 ; of albumin, 12. In order that endosmose and exosmose 

 may take place, the liquids must have an affinity for the interposed 

 membrane, and an affinity for each other, and be miscible. The 

 interposed membrane, whether animal or vegetable, is very actively 

 concerned in the intensity and direction of the endosmotic current. 

 Graham assigns a chemical character to osmose, accompanied with a 

 constant decomposition of membrane. In the living plant the renewal 

 of the membrane forming the septum is constantly taking place, and 

 thus the osmotic action is kept up. 



The fluid matter,?, absorbed by the roots, are carried upwards 

 through the cells and vessels of the stem, as ascending sap ; they pass 

 into the leaves, where they are exposed to the influence of air and 

 light, and afterwards return through the inner bark as descending or 

 elaborated sap, and a portion ultimately reaches the root, where it is 

 either excreted or mixed with the new fluid entering from the soil. 

 The presence of light is essential for the elaboration of the sap. 

 Vegetable growth cannot progress unless the vegetable circulation be 

 perfectly accomplished. This act of vegetable vitality may, however, 

 be effected while the plant is removed from the action of light, but 

 the oxygenation of the juices cannot be perfected without their free 

 exposure to its influence. 



Numerous experiments have been performed in order to show the 

 course of the fluids in exogenous stems, such as making incisions or 

 notches in the bark and wood of trees at different heights, and noting 

 the points where the sap first made its appearance at different periods 

 of the year, more especially in spring ; also in plunging plants, with 

 their roots, entire into certain coloured solutions, and marking the 

 course of the coloured fluids. These experiments led to the con- 

 clusion that the sap ascends chiefly through the alburnum or newer 

 wood, proceeds to the leaves, and returns by the bark to the root. 

 If incisions are made into the trunk of a tree at different heights 

 early in spring, it is found that the flow of ^ap (called bleeding) 



