CRITERIA AND METHODS 119 



carbohydrates. The investigators mentioned in the pre- 

 ceding paragraph found the xylem sap of woody plants to 

 fluctuate inversely with the temperature, especially increas- 

 ing to a marked degree at air temperature slightly below the 

 freezing point of water. As was mentioned in Sec. 6, many 

 investigators have found that accumulated starch and other 

 carbohydrates tend to be changed to soluble sugars at 

 these low temperatures. With this formation of excess 

 sugar, possibly also associated with an increase in perme- 

 ability, the cells evidently lose carbohydrates which pass 

 into the water-conducting vessels. As the temperature 

 rises again, starch seems to be redeposited and sugar may be 

 reabsorbed into the Uving cells. The disappearance of 

 sugar from the xylem vessels as the temperature rises may 

 therefore be entirely unrelated to any transport through 

 the xylem. 



The finding of appreciable amounts of solutes during the 

 limited season of bleeding is not very strong evidence of 

 transport during the growing season when bleeding usually 

 ceases. It is true that Fischer (1888) and Dixon and 

 Atkins (1915 and 1916) claimed that there was sugar 

 present in the xylem vessels in midsummer at concentrations 

 ranging up to 1 per cent. The evidence presented, how- 

 ever, is far from conclusive that such amounts of sugar are 

 actually present in the vessels carrying water. Linsbauer 

 (1920) has demonstrated that the reduction of Fehling's solu- 

 tion observed by Fischer was probably not due to sugar in the 

 cells but to a reducing effect of the cell walls. Dixon and 

 Ball (1923), when they determined the sugar content 

 of the sap in the vessels by forcing the sap out by high 

 gas pressures on the leaves, were unable to find sugars 

 present. It seems possible that the rather violent treat- 

 ment of centrifuging may have extracted sap from living 

 cells or from tissues not carrying water. Dixon and 

 Atkins (1916) stated that the question is open as to how far 

 the pigments and oxidases that they found in the sap 

 obtained by centrifuging are normally present in the 

 transpiration stream of uninjured stems, and how far they 



