Dixon a.Nd Atkins — Osmotic Pressures in Plants. 



429 



both for the sap of the root of Beta and for the leaf sap of Chamaevop^. In 

 the case of the latter the demonstration is particularly convincing, Sap from 

 the frozen leaf was found to have a depression of 1"517°, while the value of 

 A for that of the untreated organ was 0'599°. Yet the former caused no 

 plasmolysis in the cells of a section of the leaf mounted in it, even after 

 twenty minutes. The difference in concentration indicated by these two 

 freezing-points would of course rapidly produce plasmolysis. This clearly 

 shows that no appreciable concentration has been effected by the treatment, 

 and that the sap pressed from the untreated organ is not isotonic with that in 

 the vacuoles of its cells. 



Of course the application of liquid air cannot stop changes taking place 

 while the sap is being pressed, as is evidenced by the production of colour in 

 the sap of many tissues during the process. That this coloration is in reality 

 due to oxidation, in contradiction to our surmise put forward in a previous 

 paper (10), may be shown by the fact that a tissue which when untreated 

 yields a coloured sap will give a colourless sap if injected with a small 

 quantity of a solution of tannin before pressing. The tannin acts as an 

 inhibitor of the oxidases present and checks the formation of the pigment. 



The cells treated with liquid air seem to be rendered completely permeable. 

 This appears from the fact that the sap is so easily pressed from the tissues 

 after the exposure, often without any disruption of the cells. Also the 

 concentration of successive pressings from these frozen tissues remains sensibly 

 the same, e.g. — 



Hedera Helix : leaves. 



Hence we may assume that the sap so obtained is a fair sample of the sap of 



the uninjured tissues. 



It will be noticed that in most instances the difference in conductivity 



between the sap of organs treated with liquid air and that of those untreated 



is not so marked as the difference in freezing-point. Comparison of the 



C X 10' . 



ratio — - — for the pairs of experiments will make this clear. 



This perhaps may be largely attributed to the greater permeability of tli© 

 protoplasm to electrolytes, so that the sap pressed from the untreated organs 

 is relatively richer in them. 



The result, however, was not anticipated, as from Andre's work (1, 2, 3) i<i 

 appeared that the proportions of the solutes present in the sap were not altered 



SOIENT. PROO., B.D.3., VOL. XIH., NO. XXVIII. 



Ss 



