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XXXI. 



OSMOTIC PRESSURES IN PLANTS. 



IV. — On the Constituents and Concentration of the Sap in the 

 Conducting Tkacts, and on the Circulation of Carbohtdeates in 

 Plants. 



By henry H. DIXON, Sc.D. (Duel.), F.R.S., 



University Professor of Botany, Trinity College, Dublin ; 



and 



W. R. G-. ATKINS, Sc.D. (Dubl.), F.I.C, 



Assistant to the University Professor of Botany, Trinity College, Dublin. 



[Read Deoemeeu 15, 1914. Published February 25, 1915.] 



So long ago as 1858 Tli. Hartig (7) recognized that the soluble products of 

 the reserve materials found in the wood-parenchyma and the medullary rays 

 must utilize the tracheae as their channels of transport to the higher regions 

 of plants. This he demonstrated by the depletion of these stores in ringed 

 branches. He concluded that the materials assimilated in the leaves are 

 passed down in the bark and stored in the wood-parenchyma and medullary 

 rays. In spring these store materials are brought into solution, and passed 

 into the tracheae, where they rise with the upward moving current of water 

 from the roots. 



In 1888 A. Pisher (8) demonstrated by chemical means the presence of 

 reducing sugars in the tracheae of a large number of trees at various times 

 of the year. He does not appear to have tested for sucrose. 



From this it might be inferred that tlie conveyance of carbohydrates in 

 the wood described by Hartig, and supposed to occur noticeably only 

 in spring, in reality takes place all the year round, but in spring most 

 markedly. 



Notwithstanding this, it is surprising to find how Sachs' (9) statement 

 that the water iu the tracheae is " an exceedingly dilute solution of these 

 (nutritive) salts, wliich may be compared at once to ordinary drinking-water," 

 seems to liave won the ear of writers : so that the function of tlie tracheae 

 in conveying organic substances upward is either ignored in text-books 

 and omitted from the consideration of plant physiologists, or its continuance 

 throughout the year is discredited or left doubtful ; as, for example, by 

 Jost (5), and in the cautious statement of Haberlandt (6). 



