UPWARD TRANSPORT OF NITROGEN 65 



dry weight of the tissues studied certainly increases during 

 the day, it is very possible that the gain in residual dry 

 weight, the more permanent tissue, takes place chiefly at 

 night. Direct experimental data on this point are lacking, 

 yet there are clear indications that permanent cell enlarge- 

 ment and perhaps nuclear divisions, occur largely at 

 night. Furthermore since the residual dry weight makes 

 up such a small part of the total weight of the plant, slight 

 fluctuations in its amount will make a relatively large 

 fluctuation of the nitrogen content when expressed as a 

 percentage of the residual dry weight. 



Considerable care should be taken before drawing con- 

 clusions from the data based on percentage. Chibnall is 

 justified in pointing out the fallacy of interpreting changes 

 in nitrogen, when expressed as percentages of dry weight, 

 as real changes in nitrogen, for the altered percentages 

 may be due largely to changes in dry matter and not to 

 changes in actual nitrogen content. He is also right in 

 suggesting that changes in percentages of water content 

 between morning and night, or night and morning, may 

 be due, at least partly, to changes in dry matter content 

 resulting from photosynthesis or translocation. He seems 

 to imply, however, as have also others, that differences 

 in water content of from 0.1 to 1 per cent or thereabout, if 

 they are due to changes in actual water content, introduce 

 but slight errors when expressing nitrogen or any other 

 constituent on a fresh-weight basis. 



In comparing methods of expressing nitrogen, A as 

 weight of nitrogen per unit number of leaves, B as nitrogen 

 per gram dry weight, and C as nitrogen per unit fresh 

 weight, he gives the following data obtained with plants of 

 Viciafaba (Table 12). All the leaves from 12 plants were 

 picked. 



Using these same data, from which it is obvious that the 

 leaves contained 86.82 per cent water and 13.18 per cent 

 dry matter, let us assume two samples are taken, one in 

 the morning with the composition indicated above, and the 

 other the evening before with the same actual total dry 



