62 TRANSLOCATION IN PLANTS 



remaining leaves. Furthermore, the deficiency in carbo- 

 hydrate, particularly in the lower part of the stem and the 

 roots, would tend to starve these tissues and increase the 

 leakage of nitrogen from them into the transpiration stream. 

 Schumacher (1930) has demonstrated a marked transport 

 of nitrogen from leaves which have been starved by dark- 

 ening. Thus when a single leaf of an otherwise normally 

 treated plant was darkened, from 70 to 80 per cent of the 

 original nitrogen was transported from the leaf in a week 

 through the phloem. This evidence points toward the 

 inability of a starved tissue to retain its nitrogen when 

 the carbohydrate content is lowered. 



Steward in correspondence has suggested another some- 

 what similar explanation for the disagreement between my 

 findings and those of Maskell and Mason. He points out 

 that these workers used only three of the upper leaves 

 of the cotton plant and that the increase of nitrogen in 

 these younger leaves of the ringed stems over that in the 

 checks may have been due to a redistribution within the 

 parts above the ring. Thus it is possible that the ring, by 

 preventing movement of nitrogen basally from the older 

 leaves, has tended to increase its movement into these 

 younger leaves. Or, as was suggested in the foregoing, if 

 carbohydrates were deficient in the upper leaves, the 

 increased supply due to diversion by the ring might favor 

 an increased accumulation of nitrogen in these same leaves. 

 Mason and Maskell state that the leaves used were three 

 mature leaves taken from near the top of the main axis 

 of each plant. I have failed to find any statement as to 

 what proportion of the total number of leaves these 

 formed. It is also not clear as to whether these ''mature" 

 leaves had or had not reached their maximum growth. 

 In some plants even what might be considered as mature 

 leaves, under favorable conditions, will seem to continue 

 growth or withdraw materials from still older leaves. 

 Although it is possible that the changes in composition of 

 these leaves may be largely due to redistribution within 

 the upper part of the plant, it cannot be assumed to be a 



