57 o SCIENCE PROGRESS 



they occur. Moreover, instead of the osmotic pressure of all 

 the cells of an organism being very nearly equal, as is the case 

 in animals owing to the circulation of the blood and the action 

 of the nephric system, in plants very great differences are met 

 with in the osmotic pressures existing in the different tissues. 



Substances producing Osmotic Pressures 



In animal body liquids by far the greater part of the osmotic 

 pressure met with is due to electrolytes. Small quantities 

 of sugar are found in the blood, very minute indeed, but in the 

 milk lactose abounds, being presumably elaborated in the milk 

 glands. Very small amounts of urea and other down-grade 

 products can also be detected . This comparatively insignificant 

 content of non-electrolytes in animal blood is partly due to the 

 fact that a small supply to each cell, maintained at a constant 

 level by the movement of the blood, is equivalent to a larger 

 supply. Soluble waste products, too, are continually being 

 removed, so do not accumulate. Moreover animal cells do not 

 take carbon dioxide in to form sugars as do those of green plant 

 tissues, and much of their nourishment is supplied in the form 

 of complex nitrogenous products of high molecular weight con- 

 veyed by the blood-stream only or by the ccelomic liquid in the 

 lower groups. The writer is not, however, aware of any exact 

 measurements having been made as to the percentage, un- 

 doubtedly a very small one, of the total osmotic pressure of 

 animal blood which is due to non-electrolytes. 



In plants, however, a large, in many cases a quite pre- 

 ponderating, proportion of thepressure is due to non-electrolytes, 

 of which sugars are by far the most important. This has been 

 shown by Dixon and Atkins by determining both the freezing- 

 points and electrical conductivities of sap pressed from plant 

 organs or obtained from the water channels of the wood with 

 the help of a powerful centrifuge. The freezing-point of solu- 

 tion of potassium chloride, having the same conductivity as the 

 sap under investigation, was then found. Since the presence 

 of the non-electrolyte has only a small indirect effect upon the 

 conductivity, unless present in very large quantities, it is per- 

 missible to consider the freezing-point of the equivalent potas- 

 sium chloride solution as being that of the electrolytes of the 

 sap. By subtracting this value from that found directly for 



