82 TRANSLOCATION IN PLANTS 



Furthermore, with but few exceptions I always took pre- 

 cautions to have branches and leaves below rings in such 

 positions as to keep the roots well supplied with carbo- 

 hydrates. Root starvation, therefore, did not complicate 

 matters. 



Chang (1932) has carried oui an extensive series of experi- 

 ments with bean plants, and his findings point to trans- 

 port of nitrogen through the xylem with the transpiration 

 stream. He worked with large numbers of plants care- 

 fully selected for uniformity and, by selecting carefully 

 matched plants or by use of a stamp method, was able to 

 follow changes in nitrogen contents on an absolute basis, 

 which was independent of changes in leaf area, water con- 

 tent, and dry-matter content. 



In order to obtain appreciable changes in composition 

 over 12-hour periods he watered the plants, which were 

 growing in a sandy soil, with Knop's solution of four times 

 normal strepgth. Since herbaceous stems could not easily 

 be ringed, possible transport through the phloem was 

 stopped by scalding the stems for a distance of about 5 mm. 

 The results of such an experiment, showing diurnal changes 

 in normal plants as well as those in plants with scalded 

 stems, are presented in Table 14. He found that normal 

 plants, as a whole, absorbed more nitrogen during the day 

 than at night; the growing shoot gained during both the 

 day and night, but slightly more in the day; the leaves 

 gained markedly during the day and lost nitrogen at night, 

 while the roots gained most at night and much less or not 

 at all during the day. His data clearly demonstrate a 

 diurnal fluctuation in the nitrogen content of leaves. 



It is clear that the plants with scalded stems behave 

 practically the same as normal plants, with the exception 

 that at night the leaves did not lose nitrogen and the shoots 

 gained slightly more. Though the difference was not 

 statistically significant, it was apparent in all four sets of 

 experiments. This difference is probably due to the fact 

 that scalding has prevented removal from leaves to the 

 roots and that starvation of the roots has resulted in par- 



