224 ] 



XV. 



CHANGES PRODUCED IN THE SAP BY THE HEATING OF 

 BPANOHES. 



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

 University Professor of Botany, Trinity College, Dublin. 



[Read January 27. Published March 10, 1914.] 



A LAKGB accumulation of evidence shows that the withering and death of 

 leaves supported by a branch which has been killed by heat are due chiefly 

 to a change in the nature of the sap supplied to them, or, in other words, to 

 the contamination of the sap-supply by substances emanating from the killed 

 branch.' 



A priori such a contamination seems inevitable. When the heat has 

 killed a portion of the stem, the cells adjoining the water-tracts become 

 permeable; and hence the dissolved substances in their vacuoles are set free 

 into the upward current of sap in the tracheae. The vacuoles contain acids 

 carbohydrates, and salts, so that, even in the absence of corroborative 

 observations, we would expect the sap to be enriched with these substances. 

 Furthermore, very probably substances in the cells ordinarily not in solution 

 would be brought into solution, and introduced into the sap by the higher 

 temperature ; possibly, too, some bodies might be precipitated from tlie rising 

 sap by the higher temperature. Yet another change is to be anticipated. The 

 heat will destroy any thermolabile substances in the sap and in the adjoin- 

 ing cells. Coagulation changes may also be expected. 



It is not difficult to test these surmises experimentally; and indeed a 

 colour-change in the sap issuing from heated stems has before now been 

 recorded and commented upon.^ 



The sap extracted from various trees primarily for other experimental 

 purposes incidentally provided material suitable for this investigation. The 

 extraction was effected by means of a centrifuge. Short lengths of the 

 branch to be investigated (9-10 cm. long x 2-2'5 cm. diam.) are placed in 

 gilt buckets of a centrifuge, and the sap yielded after about five minutes' 



' J. B. Overton ; " Transpiration and Sap-flow." Bot. Gaz., 1911, pp. 28 and 102. 



- Henry H. Dixon : "Vitality and the Transmission of Water through the Stems of Plants." 

 Proc. Boy. Dublin Soc, vol. xii, No. 3, 1909, p. 21, and Notes from the Bot. Sob., Trinity College, 

 vol. ii, No. 1, p. 5. 



