86 TRANSLOCATION IN PLANTS 



more total nitrogen, although the difference was not 

 statistically significant. 



In contrast with these findings of Chang's, ringing 

 experiments with woody plants have consistently shown 

 less nitrogen in the leaves above rings than in normal 

 stems. This was evident in both short-time and long- 

 time experiments. If nitrogen had been carried in the 

 transpiration stream, one would expect more nitrogen in 

 the leaves of ringed stems, because the ring would have a 

 trapping effect, preventing export from the leaves above. 

 The fact that there was consistently less nitrogen in these 

 leaves would indicate but slight normal transport through 

 the xylem. With these woody plants, although nitrate was 

 in some cases added to the soil about the plant, undoubtedly 

 the amount was much less than that in the solution about 

 the bean roots. Furthermore, in no cases were the woody 

 plants heavily pruned or defoliated and in no cases were 

 the rings so placed as to prevent an adequate supply of 

 carbohydrate to the roots, so root starvation did not com- 

 plicate matters. 



SUMMARY 



10. Ringing experiments with several species of woody plants have con- 

 sistently shown a reduced absorption of nitrogen by the leaves above the 

 ring, as compared with those of normal stems. This has been true both if 

 the ring is made early in the season before shoot growth has occurred when 

 the ring would prevent formation of new xylem in the region of ringing, and 

 when the ring is made after leaves had developed and the current year's 

 xylem is mostly formed. When the experiment is so arranged as to equalize 

 the transpiration streams passing through defoliated regions isolated by 

 rings and those not so isolated, less nitrogen was found in the wood and 

 bark of those stems that were ringed at the base than in those not ringed. 

 This occurred independently of whether the carbohydrate content was high 

 or low. In practically all cases some nitrogen and minerals were found to 

 pass rings but much less than in normal stems. 



11. Mason and Maskell, working with cotton have consistently found 

 accumulation of nitrogen and ash in leaves and bark above rings and 

 diminished amounts below rings. This contradictory evidence may be due 

 to the fact that all of their experiments were so conducted as to bring about 

 starvation of roots and release of nitrogen to the transpiration stream by all 

 parts below the ring. 



12. Increase of nitrogen in leaves during the day and decrease at night 

 has been claimed to indicate transport to the leaves in the transpiration 

 stream. It is pointed out, however, that diurnal fluctuations may not in 



