Dixon and Atkin.s — Osmotic Pressures in Plants. 



57 



the total sugars. The sugar present in preponderating quantities is sucrose, 

 as only traces of reducing sugars were found or none at all. For the sake of 

 comparison these figures are recorded in columns side by side in Table V. It 

 must be remembered that while the values of A, A e and A - A e are accurate 

 determinations, the percentages of sugar recorded are only rough measure- 

 ments. 



It is at once evident that there is an enormous influx of sucrose in the 

 spring, the amount being from five to fourteen times as great in February as 

 in October at various levels in the stem. By April the quantity of sucrose 

 has fallen to about half what it was in February. 



The explanation of the rise in the sugar content, and to a lesser degree in 

 the content of electrolytes, appears to be that the storage cells of the wood 

 parenchyma and medullary rays are actively secreting sugar into the 

 transpiration stream as it passes them. Consequently the latter becomes 

 richer and richer as it ascends. It is quite possible that the secretion is really 

 a simple diffusion of the sugar from the cell in which it is stored and formed 

 anew by the hydrolysis of polysaccharides, into the dilute stream which is 

 passing by. 



The fact that sucrose is the most important sugar in the transpiration 

 stream in this and many other trees cannot be passed over lightly. It is not 

 stored as such to any very considerable extent. But starch and hemicelluloses 

 disappear and sucrose is found. Now it is well known that diastase 

 produces maltose from starch, and on further hydrolysis glucose results. How 

 then does the sucrose arise ? One is forced to postulate either a peculiar type 

 of starch hydrolysis, or that the cell synthesizes sucrose from glucose as fast 

 as the latter is formed. The subject is one for further investigation, and is 

 allied to the production of sucrose from polysaccharides in many fruits in the 

 last stages of ripening. 



Table VI. 



litmus campestris, March 3rd. 



In this experiment a tall tree was examined when at the stage of vernal 

 mobilization of carbohydrates. The osmotic pressure rises very markedly 



