THE METHOD OF MOVEMENT 199 



reasoning, a mechanism of transport depending on stream- 

 ing protoplasm does not invariably demand transport 

 along a diffusion gradient. 



Although Mason and Maskell's studies of concentra- 

 tion gradients and transport of sugars are rather con- 

 vincing, their attempts to find the form in which nitrogen 

 is transported have been much less successful (Maskell 

 and Mason, 1929a, 19296, 1930a, 19306, 1930c; Mason and 

 Maskell, 1934; and Mason and Phillis, 1934). Their 

 difficulties in this line are due in part, as they suggest, to 

 the fact that the nitrogen compounds may rapidly change 

 from one form to another and may also be carried in more 

 than one form. There is also the complicating factor of 

 static gradients of storage or protoplasmic nitrogen. These 

 same difficulties apply to determinations of gradients of 

 carbohydrate but are greatly minimized in the latter 

 because there are fewer forms, only sucrose and reducing 

 sugars, that seem to be concerned in carbohydrate trans- 

 port, whereas for nitrogen there are many forms that may 

 be concerned. During their studies they have determined 

 total nitrogen, protein nitrogen, crystalloidal nitrogen 

 which has been fractionated to determine amino-acid 

 nitrogen, ammonia nitrogen, nitrate nitrogen, asparagine 

 nitrogen, and residual nitrogen. Although nitrogen will 

 not disappear en route while carbohydrate may disappear 

 through respiration, the amounts of carbohydrate moved 

 are greatly in excess of the nitrogen, so gradients may be 

 more easily determined. 



What seems to me a major difficulty in determining 

 nitrogen gradients appears to have been completely over- 

 looked by Maskell and Mason and this oversight seems 

 greatly to weaken their interpretations (see also Sec. 14, 

 p. 81). They assumed that all nitrogen moves through 

 the xylem to the leaves from where it moves through the 

 phloem to other parts. Thus in a normal stem they assume 

 nitrogen is moving only downward through the phloem. 

 In their attempts to reverse the movement of nitrogen 

 through the phloem (1930a) they treat shoots in a fashion 



