178 TRANSPIRATION AND ASCENT OF SAP ch. 



In Experiments 436 and 437 the specific electrical con- 

 ductivities of the saps at C. were also determined, and 

 were found to be respectively 0*00485 and 0*00623. This 

 shows that the quantity of electrolytes in the sap pressed 

 from the desiccated leaves has increased approximately 

 proportionally with the other dissolved substances. 



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 concen- 

 trated as that remaining behind in them presented itself. 

 However, it seemed more probable that the greater con- 

 centration of the sap derived from the chloroformed, 

 heated, and desiccated leaves was attributable to changes 

 due to the treatment in each case, and the investigation 

 of the disci epancy was deferred to a later date. 



Progressive concentration of pressed sap. 

 Finally a short paper by Marie and (latin again sug- 

 gested the necessity of investigating 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 contented themselves 

 however, with 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, it was found, very convincingly 

 established some years previously by Andre, who claimed 

 to show by exhaustive chemical analyses 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 illustrate this progressive con- 

 centration of successive pressings from the same leaves. 

 The leaves experimented upon were made up into a pellet, 

 wrapped in two folds of fine linen and pressed in the jaws 



