66 



TRANSLOCATION IN PLANTS 



weight and nitrogen contents; that during the night enough 

 water had been absorbed by the leaves to alter the per- 

 centage water content from 85.82 to 86.82. This is a one 

 per cent difference which is seemingly slight. Actually, 

 however, to bring about such a one per cent difference in 

 water content by absorption of water alone, 100 g. of 

 fresh leaves must have absorbed 7.59 g. of water. With 

 no actual change in dry matter or nitrogen content, there- 



Table 12. — Sampling Errors and Methods of Expressing Nitrogen 



IN Leaves of Vicia Faba 



{From Chibnall) 



fore, the nitrogen, which formed 0.753 per cent of fresh 

 weight in the morning, would have formed 0.815 per cent 

 of the fresh weight the evening before. Expressed on this 

 fresh- weight basis, therefore, 7.6 per cent of the original 

 nitrogen would seemingly have disappeared over night, 

 and this apparent change is all due to the slight change of 

 only 1 per cent of fresh weight, a seemingly insignificant 

 figure. Diurnal differences in water or dry-matter con- 

 tents, when expressed as percentages of fresh weight, are 

 often much greater than 1 per cent. 



Chibnall suggests that expressing nitrogen as a per- 

 centage of dry weight is misleading, with which I agree; 

 but if the change in percentage water content was due 

 solely to a loss of dry matter, as he claims, then to change 

 the water content from 85.82 to 86.82 per cent there must 

 have been a loss of 1.15 g. of dry matter, or 8.75 per cent 

 of the original dry matter. (In actual experiments he 



