CRITERIA AND METHODS 117 



is not at the time being carried through that tissue. On 

 the other hand, the presence of a substance in a possible 

 conducting channel, though it is often cited as proof of its 

 transport, is not conclusive proof that it is being carried 

 in that channel, or that the channel concerned plays a 

 significant part in the transport. In the first place, as 

 mentioned in the preceding section, the presence of a sub- 

 stance in the walls of the supposed channel, or in a form 

 that would be blocked at constrictions cannot be con- 

 sidered as good evidence for transport. In the second place, 

 a substance, even when present in the lumen in easily 

 transportable form, may not be transported in sufficient 

 amounts if at the time of its presence the vehicle or solvent is 

 not moving or is moving at an inadequate rate. In the third 

 place, if the substance is present in a moving vehicle but is 

 present in small amounts, the channel in consideration may 

 play merely a minor or insignificant role in the transport. 



One of the most serious obstacles to an acceptance of the 

 hypothesis that the phloem is the chief channel for upward 

 transport of solutes is that, in a number of plants, solutions 

 at appreciable concentrations have been found in the xylem 

 vessels. For example, the sugar in the vessels of certain 

 trees, notably the sugar maple (Jones et al, 1903), has 

 during certain seasons been found at concentrations as 

 high as 2 to 8 per cent (2,000 to 8,000 p.p.m.). Many 

 investigators have given data on the composition of the 

 Uquid bleeding from cut stems of many woody plants. 

 Among such reports are those of Schroeder (1871), Jones, 

 Edson, and Morse (1903), Moreau and Vinet (1923), and 

 many others. 



Wormall (1924) and Priestley and Wormall (1925) 

 collected about 110 Hters of sap that exuded from the 

 stumps of 12 grape vines, and, on the analysis of over 

 90 liters, found a total soUd content of 0.156 per cent. The 

 ash content was 0.056 per cent, the total sugar 0.0338 per 

 cent, and the organic acid content 0.056 per cent. It was 

 assumed by these writers that this solution was moving 

 to the apical meristems and that the composition of the 



