OSMOTIC PRESSURE IN ANIMALS AND PLANTS 573 



by hydrolysis of the leaf starch. Thus one does not find a large 

 and rapid fall in pressure beyond a certain amount . In addition 

 to the above-mentioned method of regulating the sugar supplies, 

 there remains the pressure exerted by the electrolytes . Further- 

 more, in certain leaves which were allowed to remain on the 

 branch, but shut off from light, it was found that quite a con- 

 siderable osmotic pressure was maintained while the rest of the 

 leaves were actively assimilating. In dull wet weather, however, 

 the pressure of these leaves fell. This furnishes additional 

 evidence that translocation takes place from regions of high 

 osmotic pressure to regions of lower pressure irrespective of 

 whether this motion is up or down the bast. 



Moreover, the supply of electrolytes to leaves and cells of 

 other tissues influences the osmotic pressure. Especially is 

 this the case with leaves, from which large quantities of water 

 are lost by evaporation ; the electrolytes in the water supply 

 consequently accumulate . Thus old leaves have always a higher 

 electrolyte content than younger ones on the same plant. The 

 conductivity is nearly entirely due to salts of inorganic and 

 organic acids. As a rule very little is to be attributed to organic 

 acids. For example, the conductivity of the juice of an orange 

 or lemon is less than that of many leaves and roots which are not 

 noticeably acid, and very far below that pressed from celery 

 stalks. 



In salt marshes considerable quantities of electrolytes accu- 

 mulate in the cells of the flora and consequently the cells are not 

 plasmolysed even when evaporation causes a very large rise in 

 the concentration of the solutes in the surrounding water. The 

 Neapolitan physiologists Cavara, Trenchieri, and Nicoloso- 

 Roncati have investigated this class of plant-association by 

 cryoscopic methods with special reference to the marshes of 

 Cagliari. Sand-dune vegetation had been studied by the 

 plasmolytic method by Drabble and Lake previously, and the 

 alfvar vegetation of Oeland has since been examined by Falck. 

 Desert flora too gives examples of the existence of very high 

 pressures, up to about ioo atmospheres in some cases. 



The strand-flora of East Indian islands and that of mangrove 

 swamps have also been studied by the plasmolytic method. 

 Conductivity measurements on sap from these classes of plant 

 would be of interest, as they would show what proportion 

 of the osmotic pressure is due to electrolytes. It seems highly 



