CIRCULATION OF THE SAP. 125 



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

 Avhich was 1'083, the fluid rose 39 millimetres in the space of an 

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

 metres; and with syrup, of density 1'228, the rise was 106 milli- 

 metres. 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\ atmospheres. Thus the velocity and force of the rise depends 

 on the excess of density of the interior liquor over that of the water 

 outside. Different substances act with various intensity hi pro- 

 ducing endosmose. The following ratio expresses the variable in- 

 tensity of endosmose, in different cases in which the density of the 

 solution was the same: Solution of gelatine, 3; of gum, 5*17; of 

 sugar, 11; of albumen, 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. Accord- 

 ing to Matteucci and Cima, the interposed membrane, whether animal 

 or vegetable, is very actively concerned hi the intensity and direction 

 of the endosmotic current. The different surfaces of membranes also 

 act variously, and it is probable that the physiological condition of the 

 membrane has an important effect. 



256. The fluid matters absorbed by the roots are carried upwards 

 through the cells and vessels of the stem, under the form of ascending 

 or crude sap ; they pass into the leaves, where they are exposed to the 

 influence of air and fight, and afterwards return through the bark hi the 

 form of descending or elaborated sap, and a portion of them ultimately 

 reaches the root, where it is either excreted or mixed with the new 

 fluid entering from the soil. 



257. Ascending or Crude Sap. In order to show the course of the 

 fluids in exogenous stems, numerous experiments have been performed 

 by Walker, De la Baisse, Burnett, Schultz, and others. These consisted 

 in making incisions or notches in the bark and wood of trees at dif- 

 ferent heights, and noting the points where the sap 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 conclusion that the sap ascends chiefly through the alburnum 

 or newer wood, proceeds along the upper side of the leaves, and re- 

 turns by their lower side to the bark and root. If incisions are 

 made into the trunk of a tree at different heights early in spring, it is 

 found that the discharge of sap takes place, first from the lower parts 

 of the incisions, and chiefly from the alburnum ; while at a later period 

 of the year the discharge, or the bleeding, occurs on both sides of the 

 incision, chiefly from the new wood on the lower side, and from the 

 bark on the upper side. If a plant be plunged into a Aveak solution 

 of acetate of lead (Avhich is capable of being absorbed), the metal may 



