DOWNWARD TRANSPORT THROUGH THE XYLEM 101 



18. Findings and Interpretations of MacDougal and of 

 Arndt. — MacDougal (1925), though not so positive in his 

 acceptance of the hypothesis that the xylem is concerned 

 in backward transfer of solutes, nevertheless seems to think 

 it a feasible hypothesis and offers data which, to him, tend 

 to give it support. By anatomical studies and the injection 

 of dye solutions he is led to the conclusion that in conifers 

 water with contained solutes rises in the inner layers of 

 the wood and that the backward transfer of solution from 

 the leaves takes place through the outer layer. Though he 

 specifically states that the transpiration stream does not 

 rise in the outer layer of wood and bases his conclusions 

 on movements of dyes in cut and injected stems, his 

 own data seem not to bear out his conclusion. In a few 

 instances (pp. 13 to 17, 22 to 23) he found the dye to move 

 upward (in cut stems) somewhat farther in the inner than 

 in the outer layers, but in several other instances (pp. 17, 18, 

 19, 24, and 31) he found movement to a greater height in 

 the outermost layer. 



In introducing dyes through holes or slits to determine 

 the direction of movement, he sometimes found greater 

 upward than downward movement in the outer layer. 

 For example, in an experiment with a Monterey pine 

 (p. 31) the dye moved up 10 cm. in the outer layer and 

 down only 4 cm. Again (p. 15) he found the greatest 

 downward movement not in the outer layer but in the 

 internal layers. 



Different quantities of sugars found in the different layers 

 were cited as evidence that these may be moving down in 

 the outer layer. For example, in July (p. 27) the outer 

 layer of wood in the Monterey pine was found to contain 

 sugar to the extent of 0.204 per cent of the dry weight, 

 whereas the next inner layer contained only 0.007 per cent. 

 By September, however, the second layer had nearly as 

 much as the outer (p. 31). When the layers were separated 

 at this time, the outer had 0.146 per cent sugar and the 

 next inner layer 0.128 per cent. The two layers were 

 said to have 0.05 per cent sugar, but this must be a misprint. 



