UPWARD TRANSPORT OF NITROGEN 



61 



Maskell and Mason (1929a) suggest that these same 

 data support the hypothesis that the chief path of upward 

 transport of nitrogen is the wood, for, assuming all the 

 twigs at the beginning had a nitrogen content like those 

 in group 2 with two rings, those treated as in group 3 had 

 increased 3.3 mg. per twig and the checks (group 1) 

 increased 4.66 mg. per twig. The increase in group 3 was 



70.8 per cent of that in the checks. Their hypothesis 

 assumes that the nitrogen moves up through wood to the 

 leaves, from which it subsequently moves back through 

 the phloem as organic nitrogen and accumulates in the 

 bark and wood. According to this hypothesis they suggest 

 that the defoliated region of the twigs, treated as in group 

 3, could receive nitrogen only from the eight leaves at the 

 top, those treated as in group 2 could receive no organic 

 nitrogen from leaves, while the checks (group 1) could 

 receive nitrogen from the entire plant except the eight 

 leaves at the top. The increase in nitrogen of twigs in 

 group 3, receiving organic nitrogen from only eight leaves, 

 since it was nearly 71 per cent of the increase of those 

 twigs receiving nitrogen from the entire plant, suggested 

 to them that this experiment adds support to the hypothesis 

 that nitrogen moves to the leaves through the xylem and 

 is transported from them through the phloem. They seem 

 to overlook the fact that the amount of nitrogen found in 



