424 



Scientific Proceedings, Royal Dublin Society. 



These observations were made primarily with other objects in view. But 

 even then the possibility that the sap pressed from the untreated leaves was 

 not so concentrated as that remaining behind in them presented itself. 

 However, it seemed more probable that the greater concentration of the sap 

 derived from the chloroformed, heated, and desiccated leaves was attributable 

 to changes due to the treatment in eacii case, and we deferred the investigation 

 of the discrepancy to a later date. 



Recently a short paper of Marie and Gatin (15) directed our attention 

 again to this point. These writers when investigating the cryoscopic value of 

 the sap of alpine plants note that the sap expressed first from a plant-organ 

 has a smaller depression of freezing-point than that pressed subsequently. 

 They content themselves, however, witli adding the successive samples 

 together, and take the freezing-point of the mixture as the freezing-point of 

 the sap of the plant. 



This progressive concentration of the sap pressed from plant-organs had 

 been, we found, very convincingly established some years previously by 

 Andre (1, 2, 3, and 4), who also showed by exhaustive chemical analysis of 

 the plant organs which he examined that, while the concentration of the sap 

 expressed by increasing pressures rose, the proportion of the constituents 

 remained the same. 



The following experiments of our own illustrate this progressive concen- 

 tration of successive pressings from tlie same leaves. The leaves experimented 

 upon were made up into a pellet, wrapped in two folds of fine linen and 

 pressed in the jaws 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 iu 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"" C. 



Hedera Helix : leaves. 



