64 TRANSLOCATION IN PLANTS 



starved plants which contained high carbohydrate, that 

 nitrogen diminished in the older more basal leaves, increased 

 in the upper younger leaves and stems, and increased 

 also in the older part of the stem at the base. Abnormal 

 starvation and release from the parts below the ring and 

 redistribution above the ring may largely account for the 

 findings observed but does not prove that most of the 

 nitrogen and ash constituents are normally carried chiefly 

 through the xylem. 



12. Diurnal Fluctuations in Nitrogen and Ash of Leaves, 

 Indicating Transport with the Transpiration Stream. — It 

 has been tacitly assumed by many that during the day, 

 when transpiration is taking place at high rates, salts are 

 rapidly carried to the leaves in the transpiration stream, 

 that in the leaves the inorganic ions are combined with 

 organic substances, and the resulting compounds are 

 retransported from the leaves to regions of utilization 

 or storage. A diurnal fluctuation in nitrogen and ash 

 contents of leaves showing high contents during the day 

 and diminished amounts at night would tend to support 

 this assumption. 



Although Chibnall (1923) and Maskell and Mason 

 (1929a) give data which they claim prove an appreciable 

 diurnal variation, and although their data clearly show 

 statistically significant increases during the day and 

 decreases during the night, they do not positively demon- 

 strate an absolute diurnal variation, for the data are 

 expressed as variations in percentages of fresh weight 

 or of residual dry weight, and these themselves may be 

 variables. A gain in fresh weight at night, and a loss in 

 fresh weight during the day, would make a seeming loss 

 and gain, respectively, in nitrogen. It is also possible 

 that much or all of the diurnal variation expressed even 

 on the residual dry- weight basis may be merely apparent. 

 With a steady gain in total nitrogen an increase in residual 

 dry weight at night might appear as a loss in nitrogen 

 while a failure to gain in residual dry weight during the day 

 would appear as a gain in nitrogen. Though the labile 



