574 SCIENCE PROGRESS 



probable that a very large percentage is to be accounted for 

 by them. 



A few measurements were made by Dixon and Atkins of 

 the freezing-point of sap pressed from brown algse. It was 

 found to be slightly below that of the surrounding sea-water. 

 It seems probable that the slight excess pressure of the algal 

 cells was due to organic crystalloids, though this has not been 

 tested. It was shown that dilution of the sea-water led to a 

 considerable reduction in the depression of freezing-point of the 

 algal sap within a few hours. The difference existing between 

 the internal and external osmotic pressures was, however, then 

 much greater than at the start, but it seems that sufficient time 

 was not allowed for equilibrium to be reached under the new 

 conditions. 



In the foregoing pages a few of the principal causes in- 

 fluencing osmotic pressure in plants have been mentioned. 

 But there are many problems in plant physiology which can be 

 studied by the cryoscopic method, or better by the combined 

 use of it and conductivity measurements. 



Osmotic Pressure Studies in Special Physiological 

 Problems 



Allusion has already been made to the use of these measure- 

 ments in studying the physiology of foliage leaves. It must, 

 however, be remembered that the osmotic pressure only gives 

 a measure of the total number of molecules of solute present, 

 irrespective of the kind of molecule . An interesting case illustra- 

 ing how large changes can take place in the relative proportions 

 of the sugars of a sap, while the total osmotic pressure remains 

 constant, is afforded by some analyses recorded by Brown and 

 Morris in their classical work on the Chemistry and Physiology 

 of Foliage Leaves. Dixon and Atkins found but little change 

 in the osmotic pressure of detached leaves stored overnight. 

 Yet Brown and Morris had shown that under such conditions 

 quite considerable changes had taken place, sucrose having been 

 hydrolysed, the quantities of dextrose and lsevulose altered, 

 and maltose having made its appearance from the leaf-starch. 

 On calculating out the number of gram molecules of each sugar 

 it was, however, found that this quantity had also remained 

 practically constant. One is therefore tempted to consider that 



