DOWNWARD TRANSPORT THROUGH THE XYLEM 107 



seems incapable of carrying solutes at the necessary rates, whereas the xylem 

 carries introduced solutions with great rapidity in both directions, she 

 suggested that it is possible that the xylem is the normal path of backward 

 transport. 



16. Dixon and his associates summarily dismiss all evidence based on 

 ringing experiments on the grounds that ringing always results in plugging 

 of the xylem. Calculations are presented showing the rates at which 

 sugar solutions must move if the transport of sugar from photosynthesis 

 is restricted to the phloem of leaf petioles, or that moving to the storage 

 tuber of potato or yam is carried through the phloem. These calculations 

 indicate, for example, that if restricted to flow through the sieve tubes, 

 a 25 per cent solution of sugar must move at a rate of 88 cm. an hour to 

 carry the requisite amount of sugar to the storage tuber of the yam. Since 

 such rates of movement appear impossible, because of the narrowness of 

 the sieve tubes, their frequent cross walls, and their content of colloidal 

 material, it is definitely proposed that solutions must move back through 

 the xylem where solutions flow easily. This suggestion is supported by 

 observations of rapid backward flow of colored solutions throughout an 

 entire potato plant, roots, tubers, and tops, when the dye is introduced 

 through a cut leaf. To support the suggestion that a backward trans- 

 port through the xylem is normal, the evidence of backward transport of 

 a hormone causing movements in pinnae of Mimosa is cited. The find- 

 ing of sugar in the sap obtained by centrifuging pieces of stem is cited as 

 proof of sugar being normally present in the transpiration stream during 

 midsummer. 



17. Kastens considers that foods are carried in both directions through 

 the xylem and not the phloem, and that the disturbed growth responses 

 following ringing are to ha explained on the grounds that special hormones 

 controlling behavior are transported through the phloem. The change in 

 food distribution above and below rings, therefore, is partly due to the 

 influence of the hormones on growth and that growth m turn controls food 

 distribution, and partly to the effect of the ring in plugging the xylem 

 through which the foods move. 



18. MacDougal, as a result of anatomical and injection studies, concludes 

 that in conifers water with its contained solutes rises in the inner layers of 

 the wood, and that possibly sugars are carried downward in the outermost 

 layers of wood, which are not directly connected anatomically with the 

 leaves. Injection experiments are claimed to indicate a greater downward 

 flow through the outer layers of wood. These same layers also were often 

 found to have a higher sugar content than the inner layers. This, it is 

 suggested may indicate that sugars are being carried down through these 

 outer layers. Arndt, as a result of injection experiments with the coffee 

 tree, found rapid movement of eosin both upward and downward through 

 the outer xylem layers, even when all effects due to unequal gas pressure, 

 capillarity, and saturation deficits were supposedly eliminated. This led 

 him to suggest that the xylem is normally concerned in transport in both 

 directions, but that the mechanism involved is unknown. 



19. The principal reason why it has been proposed that the xylem is 

 chiefly concerned in backward movement is that it has not been possible to 



