DOWNWARD TRANSPORT THROUGH THE XYLEM 99 



Fischer (1888), Atkins (1916), and Dixon and Atkins 

 (1915, 1916) have observed sugars to be present in the 

 vessels of woody plants not only in the spring, as have a 

 large number of other investigators, but also in midsummer. 

 Fischer did not, however, find any sugar in the herbaceous 

 stems he tested nor even in woody shrubs. Fischer's 

 conclusions were based on the reduction of Fehling's solu- 

 tion when pieces of woody stem were heated directly in it. 

 Linsbauer (1920) has questioned Fischer's interpretation 

 and gives evidence to show that the reduction of Fehling's 

 solution was due not to sugar but to nonsoluble constituents 

 of the cell wall. Atkins and Dixon by centrifuging pieces 

 of woody stem supposedly threw out sap normally present 

 in the vessels which, in their experiments, contained any- 

 where from little or no traces of sugar up to solutions of 

 0.5 to 1 per cent sugar. In later experiments, however 

 (Dixon and Ball, 1923), sap contained in the vessels was 

 forced out by placing the leafy branch under high air 

 pressure, and this sap contained no traces of sugar unless 

 the tissues were first killed or made permeable by toluene 

 vapor. Therefore he postulated that there must be some 

 mechanism regulating changes in permeability so as to 

 synchronize increases in permeability of cells neighboring 

 vessels with a time when water is flowing backward through 

 the vessels. 



17. Findings and Interpretations of Kastens. — Emma 

 Kastens (1924) has accepted Birch-Hirschfeld's and Dix- 

 on's hypothesis and considers that solutes move in both 

 directions chiefly through the xylem. She offers an ingen- 

 ious explanation of the effects of ringing which, though 

 they probably will not receive wide acceptance, are at 

 least worth mentioning for some of the ideas expressed 

 are hinted at by others though less definitely stated. 

 Haberlandt had suggested that in the potato the phloem 

 produces hormones which are essential for regeneration. 

 Kastens, however, suggests that instead of producing the 

 hormones the phloem merely carries hormones of various 

 types especially '^ cell-division hormones" and "meta- 



