UPWARD TRANSPORT OF ORGANIC MATTER 45 



SUMMARY 



4. During the dormant season large quantities of carbohydrate, in both 

 sohible and insoluble forms, are found in the xylem regions, principally in 

 wood rays and xylem parenchyma, of branches, trunks, and roots of woody 

 plants. In early spring the water-conducting vessels also of many woody 

 plants have been found to contain considerable quantities of soluble sugar. 

 It has been assumed that much of this carbohydrate that is stored in the 

 xylem is carried through the wood to the developing shoots, where it is used 

 in their early growth. When rings are made just prior to bud break at 

 varying distances from the apical buds, the closer the ring is to the apex, the 

 less is the growth of these buds above a ring. This points to a relation 

 between the amount of food stored above the ring and the amount of 

 growth. Tests for starch demonstrate an early cessation of growth to be 

 associated with an early depletion of starch above the ring, while the starch 

 in the xylem immediately below the ring may still be abundant. These 

 findings strongly suggest that, when the ring is near the apex, carbohydrate 

 was limiting the growth above the ring and that sugars, even when stored 

 in the xylem region, cannot be transported upward past a ring in appreciable 

 quantities. 



5. The disappearance of starch from below a ring made on the trunk of a 

 tree has been interpreted as demonstrating that the sugars from carbo- 

 hydrate stored below must move up through the xylem. A second ring 

 placed below the first, however, prevents removal from between rings, 

 indicating that the carbohydrate below the first ring had not moved up 

 through the wood but had moved down. Two rings placed a short distance 

 apart will effectually prevent movement of carbohydrate out of or into the 

 region so isolated. 



6. Since the disappearance of starch is not always associated with removal 

 of sugar, it has been suggested that many of the experiments do not demon- 

 strate a failure of transport through the xylem, because starch tests were 

 used in many cases as a criterion of the presence of carbohydrate. It is 

 true that a lack of correlation between starch and carbohydrate content 

 has been found to occur in tissues at low temperatures close to 0°C., or high 

 temperatures around 35°C., in tissues that are severely wilted, or at unusual 

 hydrogen ion concentrations; but in the experiments on translocation the 

 parts tested were not exposed to conditions that would be expected to bring 

 about such discrepancies between starch and sugar contents. Furthermore, 

 in every type of experiment where starch disappearance was used as a 

 criterion, a few quantitative sugar tests were also made, and there was a 

 direct relation between sugar amounts as determined by analysis and the 

 amounts of starch as determined bj' the iodine test. This relation was 

 further established through determination of dry weights, a high dry weight 

 per unit volume and per stem being associated with high starch content. 

 Experiments on feeding of sugars from an external source and on removal by 

 leaching also demonstrate a definite relation between starch content and 

 sugar content. 



7. Not only does the carbohydrate that is stored in the xylem tissues fail 

 to be transported upward through the wood when the phloem is ringed, 



