Dixon and Marshall — Examination of the Wood of Trees. 367 



at least amounting to 1'637 cm. and 2'574 cm. per minvite to overcome the 

 leak alone. 



From the foregoing records it will be seen that measurements and 

 experiment lend no support to Janse's hypotliesis as to the intervention of 

 the living cells in the ascent of sap in stems — the velocity of protoplasmic 

 streaming in the cells demanded by the hypothesis ranging from 0'76 cm. 

 per minute to 5"82 cm. per minute, whereas the most rapid streaming in 

 closed cells, hitherto recorded, is 02 cin.-0"3 cm. per minute; while Janse 

 himself records 0'03 cm. per minute in the endodermis cells of the root to 

 which he ascribes a similar function. 



No doubt one principal reason why so many investigators have ascribed 

 a direct function in raising the sap to the living elements of the wood, 

 is the fact that no other function has been generally assigned to them. 

 The recognition, however, that the trachese not only conduct mineral 

 solutions and water upwards in trees, but also distribute carbohydrates 

 throughout the plant, immediately shows the necessity of living elements 

 in the wood. The function of the wood-parenchyma and the medullary 

 rays is to transmit carbohydrates from the bark into the wood, to transform 

 and store them there, and finally to secrete them into the trachese. Not 

 only does this secretion in all probability give rise to root-pressure, and 

 so lead to a translocation of carbohydrates upwards in spring, but even 

 in times of rapid transpiration it charges the sap, as it is drawn upwards 

 in the trachese, with carbohydrates, and thus supplies the upper growing 

 regions with the products of assimilation to be used in respiration and 

 growth. This question is discussed more fully in an account of an investi- 

 gation recently carried out on the contents of the sap of the conducting 

 channels of plants (4). 



Literature. ' 



(1) Dixon, H. H.— On the Transpiration Current in Plants. Proc. E,oy. 



Soc, Loudon, 1907, vol. Ixsix B, p. 41. 

 (2) Transpiration and the Ascent of Sap. Progressus Rei 



BotanicEB, 1909, Bd. iii, s. 1. 

 (3) Transpiration and the Ascent of Sap in Plants. (Macmillan) 



London, 191i. 

 (4) Dixon, H. H., and Atkins, W. R. G.— Osmotic Pressures in Plants. 



Part IV. Ou the Constituents and Concentration of the Sap in the 



Conducting Tracts, and on the Circulation of Carboliydrates in 



Plants. Proc. Hoy. Dubl. Soc, vol. xiv, 1916. 



