OSMOTIC PRESSURE IN ANIMALS AND PLANTS 571 



the sap, the depression of freezing-point due to non-electro- 

 lytes is obtained approximately. The latter value could 

 also be arrived at by estimating the sugars — usually sugars 

 are the main non-electrolyte constituents — and calculating 

 the depressions caused by them severally. Some rough 

 estimates in cases in which only hexose sugars were present 

 gave fair agreement with the values afforded by the other 

 method. 



By this method a systematic examination has been made of 

 the tissues of a number of plants, and in some cases the seasonal 

 changes have been traced. 



A word must be added as to the method by which the sap is 

 obtained. At first it was expressed from leaves, etc., wrapped 

 in linen and pressed between silver discs in a vice. It was, how- 

 ever, found that the sap which came out was to a considerable 

 extent diluted with more or less pure water pressed out through 

 the protoplasm of cells which had not ruptured. This intro- 

 duced an error often very great, and varying from tissue to tissue 

 and from species to species. To avoid this error Dixon and 

 Atkins adopted the plan of rendering the protoplasm permeable 

 by immersion in liquid air. The sap given after this treatment 

 is genuinely that of the vacuole and protoplasm. It is, more- 

 over, richer in enzymes than that obtained by direct pressure 

 of unfrozen tissues, and is consequently more liable to undergo 

 change after expression. For instance, sap thus obtained from 

 yeast contains zymase, and will ferment a sucrose solution since 

 it also contains invertase. 



It has already been mentioned that sugars are the chief 

 non-electrolytes found in plant cells. The most abundant are 

 sucrose, or cane sugar, and its products of hydrolysis the two 

 hexose sugars, dextrose (d-glucose) and laevulose ( J-fructose) . 

 It is as yet a disputed point whether sucrose is the first sugar 

 to be formed in photosynthesis or whether dextrose precedes it. 

 The excessive accumulation of these sugars is prevented by their 

 transformation into the colloid, starch, which exerts no osmotic 

 pressure . 



A fourth sugar, maltose, is frequently met with. It is en- 

 tirely a down-grade product of starch, under the action, in the 

 cell, of diastase . It is further hydrolysed by the enzyme maltase 

 giving two molecules of dextrose for each molecule . In addition 

 certain pentose sugars are occasionally met with in small 



