OSMOTIC PRESSURE IN ANIMALS AND PLANTS 569 



cells in osmotic equilibrium with the blood this is rather to be 

 expected. Over a thousand samples examined in Paris gave the 

 mean value just quoted, which was identical with that found by 

 the writer by testing something under a hundred samples in 

 Dublin. The utility of the test lies in its application to the 

 detection of fraudulent watering, which would bring the 

 freezing-point nearer to that of pure water practically in direct 

 proportion to the amount of water added . Usually the question 

 of adulteration of milk is largely decided by an estimation of its 

 fat content. If, however, this is for any reason low, and the 

 sample gives a normal freezing-point, it is genuine milk beyond 

 any question, unless faking has been carried out by means quite 

 beyond the knowledge of the average dairyman . The cryoscopy 

 of milk has been adopted as an official test in at least one 

 municipal laboratory, that of Amsterdam I believe. 



General Discussion 



On the whole, it may be said that the osmotic pressures met 

 with in animal cells do not exceed y 5 atmospheres, if one excepts 

 those of organisms living in the sea ; in these the pressure only 

 slightly surpasses that of the medium, if greater than it ; if lower, 

 it is always because the animal has a more or less impermeable 

 membrane which renders it to a certain extent independent 

 osmotically. Within the body, however, each cell of an animal 

 tissue must be considered a water-dweller, being in osmotic 

 equilibrium with the other cells and the inter-cellular solutions. 



Osmotic Pressure in Plants 



In primitive naked plant cells, and in free sperms and 

 unfertilised ova, the osmotic relationships with the surrounding 

 medium are much the same as in the animal kingdom. 



The advent of the comparatively inextensible cellulose wall 

 introduced an entirely new condition. The cell was free to 

 elaborate solutes to a relatively enormous amount, consequently 

 high osmotic pressures are met with in plants, 15-20 atmo- 

 spheres being nothing unusual in the foliage leaves of phanero- 

 gams . Accordingly as cellulose or very similar substances boun d 

 all plant cells except in a few primitive types, differences of 

 osmotic pressure are due to external conditions affecting meta- 

 bolism rather than to the particular groups of plant in which 



