200 TRANSLOCATION IN PLANTS 



much like that shown in shoot 1 of Fig. 1. In this case, 

 however, a second ring is placed at R^. In one set they 

 have leaves at the base with the upper part defoliated as 

 here indicated, and in the other the leaves are in the upper 

 part just below the upper ring while the lower part is 

 defoliated. They assume that all nitrogen moves first 

 to the leaves and then moves down through the phloem 

 when the leaves are above. When the upper part is 

 defoliated and the leaves are below, they assume they have 

 reversed the direction of flow of nitrogen through the 

 phloem. 



When the leaves are above, however, and the defoliated 

 region below, it is probable that some of the nitrogen as it 

 passes the ring through the xylem will be immediately 

 absorbed by the living cells of the rays which in this region 

 would be gorged with carbohydrate. From here it may 

 move over to the phloem and up through this defoliated 

 region and not down. Eckerson (1924) and Thomas (1927) 

 have rather clearly demonstrated that tissues of roots and 

 stems may convert nitrate to organic nitrogen. Maskell 

 and Mason's assumption that all the nitrogen in the bark 

 must have come from the leaves above is certainly not 

 well established. Their "increased negative gradient" 

 for several forms of nitrogen in this region, therefore, may 

 be in fact an increased positive gradient of nitrogen moving 

 to the leaves or to the growing apex. I do not doubt 

 that in ordinary stems some nitrogen moves to the leaves 

 through the xylem. My own experiments as well as 

 those of Mason and Maskell have demonstrated that 

 conclusively. There are indications that, when nitrogen 

 is in excess, or when the tissues are starved for carbo- 

 hydrates, rather large amounts may move through the 

 xylem. But it is far from established that all the upward 

 moving nitrogen is carried exclusively through the xylem, 

 and, unless movement through the phloem is strictly 

 unidirectional, it would seem highly probable that some 

 of the nitrogen after it passes the ring will be absorbed 

 by the phloem, especially when it is gorged with carbo- 



