IX 



METHODS OF EXTRACTING SAP 



179 



of a vice. As the vice was screwed up five or six drops 

 of sap were pressed out and caught in a capsule ; then 

 the vice was opened and the same leaves re-arranged and 

 pressed again. The sap exuding on this occasion was 

 collected and kept separate from the first sample : similarly 

 a third sample was prepared. Successive pellets of leaves 

 were dealt with in the same manner ; and so, from the same 

 set of leaves, three samples of sap were obtained. These 

 were called 1st, 2nd, and 3rd pressings. For each the 

 depression of freezing-point A and, in some cases, the 

 electrical conductivity C, were determined. The latter 

 measurements were always made at 0. 



Table 23. 



Iledeva helix: leaves. 



Concentration of Sap in Successive Pressings. 



These figures show very plainly the increase of concen- 

 tration in the later samples, and by inference the still 

 higher concentration of the sap remaining behind in the 

 pressed leaves. Hence, the concentration of the expressed 

 sap may be expected, in all cases, to be less than the 

 average concentration in the vacuoles of the tissues before 

 the application of pressure. 



The explanation of this increased concentration is not 

 hard to find. When the pressure is first applied, almost 

 pure water is extruded from the intact cells, for the proto- 

 plasmic membranes are sensibly semi-permeable, permit- 

 ting water to pass out under pressure, but resisting, more 

 or less completely, the passage of dissolved substances. 



N 2 



