THE METHOD OF MOVEMENT 201 



hydrate. The increase with time in "negative gradient" 

 (that is, higher nitrogen near the base and lower above) in 

 both bark and wood of nitrogen-starved cotton plants, as 

 reported by Mason and Phillis (1934), would to me seem 

 more likely to he due to a retention by the carbohydrate-filled 

 tissues, of nitrogen that has never reached the leaves. Actually 

 Maskell, Mason, and Phillis have no conclusive proof, 

 except perhaps where it is entering bolls or an isolated 

 flap or defoliated region above leaves, as to the direction 

 of movement through the phloem, so it is not surprising 

 that they have difficulty in determining which constituent 

 or constituents are directly concerned in transport through 

 the phloem. They give data (19296) which clearly indi- 

 cate that the conducting phloem is especially rich in 

 crystalloid nitrogen, particularly amino-acid nitrogen and 

 asparagine (see Table 21 and Fig. 11), and if one is not 

 misled by their assumption of a transport restricted to 

 downward movement through the phloem, their data 

 clearly point to these as the translocation forms of nitrogen. 

 This oversight on their part, it seems to me, is a serious 

 weakness attendant upon most of their extensive dis- 

 cussions in which they attempt to interpret their data on 

 concentration gradients for nitrogen as well as for potas- 

 sium, phosphorus, and calcium. Since they have not 

 established beyond a doubt the direction in which these 

 materials are actually moving through the phloem, the 

 data can hardly be handled with confidence. If their 

 assumption, namely, that nitrogen and ash constituents 

 under normal conditions all move directly from the roots 

 to the leaves and then down only through the phloem, is 

 true, their data could be interpreted as they do. If the 

 mechanism of movement proposed by Miinch is correct, 

 one might expect upward movement of salts from the soil 

 to be limited to the xylem and the downward flow would 

 take place exclusively by way of the phloem. In that 

 case, however, concentration gradients of individual con- 

 stituents would be meaningless because the flow would be 

 dependent on a pressure gradient determined by osmotic 



