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No. 32. 



ON THE EXTRACTION OF SAP FROM LIVING LEAVES BY MEANS 



OF COMPRESSED AIR. 



By HENRY H. DIXON, Sc.D., F.R.S., 

 Professor of Botany in the University of Dublin, 



AND 



NIGEL G. BALL, M.A., 

 Assistant to the Professor of Botany in the University of Dublin. 



[Bead November 27. Printed Decembek 7, 1923.] 



It has already been pointed out (1 and 2) that there are many reasons which 

 favour the suggestion that the xylera provides the conducting channel for 

 the transport of organic substances in plants. 



According to this theory a backward current of water containing dissolved 

 carbohydrates, etc., must pass, either continually or intermittently, from the 

 leaves to places of growth and storage. 



By the application of a method which was used by one of us (3) a long 

 time ago for the direct measurement of osmotic pressure in plants, it was 

 found possible to cause a baclvward current to flow in the xylem from the 

 leaves towards the lower parts of the plant. It seemed desirable, therefore, 

 to test whether this current, although produced artificially, would be utilised 

 by the plant for the purpose of transporting carbohydrates. 



The apparatus used is represented in the diagram. It consisted of a thick- 

 walled cylinder of specially annealed glass about 40 cm. long by 12 cm. external 

 diameter. (Owing to the bursting of this cylinder it was subsequently replaced 

 by a similar one made of mild steel.) The ends of this cylinder were closed 

 by strong castings secured by three long bolts, the joints between the cylinder 

 and the castings being made air-tight by means of leather washers impregnated 

 with wax. The cut end of tie branch was passed through a tubulure in the 

 lower casting, the joint being secured by thick rubber tubing wired on and 

 covered with glue. Compressed air, supplied through a lead tube of narrow 

 bore, was admitted at the top of the cylinder. In order to obtain the necessary 

 pressure a pump was used in the earlier experiments, but later this was 

 replaced by a cylinder of compressed air. The pressures were measured by 

 means of a Bourdon pressure gauge. 



The experiments were carried out as follows : — a small branch was fitted 

 with its cut end protruding through the lower casting. The cylinder was then 

 assembled and bolted up tightly, the waxed-leather washers being previously 

 rendered soft and limber by heating. The compressed air was then admitted. 

 At a pressure of from 5 to 8 atmospheres drops began to appear at the cut 

 end. By gradually increasing the pressure to a maximum of 20 atmospheres 

 something more than 35 c.c. of liquid could be obtained from a small branch. 



SCIENT. PROC. R.D.S., VOL. XVII, NO. 32. 3 O 



