Dixon and Ball — Extraction of Sap from Living Leaves. 265 



In one experiment a branch weighing 42 g. was exposed to 20 atmospheres 

 for 20 minutes, i.e. until no more liquid was obtained. The amount of liquid 

 yielded was 225 c.c. The pressure was then reduced and a few e.c. of toluene 

 was introduced into the cylinder. After 1^ hours a pressure of 20 atmospheres 

 was again applied and continued until no more liquid was obtained. The 

 amount yielded this time was 65 c.c. This liquid was brown in colour and, 

 after inversion with inveitase, was titrated with Benedict's solution. About 

 5 per cent, of sugar was found to be present. The branch was then removed 

 and dried. Its dry weight was found to be 17 g. The amount of liquid 

 extracted represented therefore about 35 per cent, of the total amount of water 

 In the branch. 



June, 1923. 



An experiment with Tilia americana gave a similar result to that obtained 

 in September, 1922. 



It seemed possible that the liquid, which was obtained before the cells were 

 rendered permeable with toluene, represented only that which was present in 

 the wood at the time of the experiment, being forced out by the sap pressed 

 from the leaf cells. To obtain this latter with as little dilution as possible 

 the following arrangement was made : — Three small branches supporting 

 together 18 large and 4 small leaves were fixed in the lower easting and 

 enclosed together in the apparatus. From these about 3 c.c. were extracted 

 under a pressure of 20 atmospheres. The volume of the wood, excluding the 

 negligible amount in the veins of the leaves, was estimated by making cross- 

 sections of the twigs at various levels, and found to be about 2 c.c. About 

 50 per cent, of this volume would be represented by the walls of the tracheae; 

 so that about 2 c.c. of the liquid obtained must have come from the cells of 

 the leaves. The first few drops of the liquid which were collected showed the 

 presence of a trace of reducing sugax's, due probably to contamination from 

 the cut surface of the twigs; but the remainder gave no reduction with 

 Benedict's solution either before or after inversion with hydrochloric acid. 



It seems clearly established by these experiments that, even when the cells 

 of the leaves contain considerable quantities of sugar, no part of this sugar 

 can be driven by external pressure from the tracheae of the supporting branch. 

 This may be due to the continued impermeability of the leaf cells under the 

 conditions of the experiment, or possibly the cells in contact with the conducting 

 channels at lower levels extract what sugars pass into these channels from 

 the leaves, and prevent the carbohydrates appearing at the cut surface of the 

 stem. 



In order that a backward transport of organic substances from the leaves 

 to the stem may take place through the wood, two conditions must be fulfilled : — 



( 1) The cells, owing to internal or external causes, must be rendered permeable ; 



(2) a backward flow must take place through the tracheae. It is probable 

 that the second condition . would follow as a result of the first, since then 

 any tension existing in the tracheae would be no longer resisted by the osmotic 

 pressure of the leaf cells. The experiments described above seem to show, 

 however, that the first condition is not of necessity a result of the second. 



At the present state of our knowledge it would be useless to speculate on 

 the xxnderlying causes which, under normal conditions, effect changes in the 

 permeability of plant cells. No matter what theoiy regarding the mechanism 

 of transport be held, it seems to be necessary to postulate the occurrence, either 

 gradually or suddenly, of such changes. It is probable that further progress 

 towards the elucidation of the problem of the transport of organic substances 



