134 



FUNDAMENTALS OF FRUIT PRODUCTION 



supplies the branches with an amount of elaborated nitrogen about equal to the 

 amount of nitrates taken in. From September on, however, the leaves receive 

 less nitrate in proportion to the elaborated nitrogen which passes back into the 

 branches; consequently the percentage nitrogen content of the leaf is cut in 

 half and only one-third the amount the leaves once contained remains in them 

 when thej^ fall. 



In Branches, Trunks and Roots. — A study of the seasonal variation in the 

 content of various parts of a tree shows what becomes of the nitrogen that passes 

 out of the leaf. Table 19 shows the distribution of nitrogen in a 7-year old apple 

 tree at different seasons. Nitrogen content is expressed in percentages of dry 

 weight. 



Table 19. — Seasonal Changes in the Nitrogen Content of a 7-year Old 



Apple Tree^^ 



(Percentages of dry weight) 



Dormant, 

 Dec. 3 



Buds 

 swelling, 

 Apr. 



20 



In bloom. 

 May 18 



Active 



growth 



over, 



July 12 



Leaves 

 falling, 

 Oct. 12 



Summer's growth. . . 

 1-year old branches . 

 2-year old branches. 

 3-year old branches. 

 4-year old branches. 

 5-year old branches. 



Trunk 



Large roots 



Small roots 



0.80 

 0.63 

 0.42 

 0.40 

 0.39 

 0.23 

 0.41 

 0.79 



1.01 

 0.68 

 0.62 

 0.41 

 0.32 

 0.32 

 0.47 

 0.78 



0.69 

 0.38 

 0.32 

 0.29 

 0.28 

 0.27 

 0.46 

 0.70 



0.64 

 0.40 

 0.32 

 0.27 

 0.24 

 0.23 

 0.22 

 0.28 

 0.48 



0.61 

 0.57 

 0.50 

 0.37 

 0.30 

 0.25 

 0.24 

 0.31 

 0.77 



These figures bring out two important points — first, that the younger the 

 tissue the greater is its nitrogen content and second, that practically all tissues 

 have a minimum when active growth has ceased and a maximum at the time of 

 bud swelling. The increase in all tissues, except leaves, during the fall indicates 

 nitrogen storage. The nitrogen that is stored over the winter evidently comes 

 from the leaves. 



Reference to the last table shows that in two places only is there a decrease 

 in the percentage of nitrogen before bud swelling, namely, in the smaller roots 

 and in the 5-year old branches. The decrease in the roots probably is due to 

 their beginning to function and to renew growth earher in the spring than do the 



In Spurs. — The seasonal changes in the nitrogen content of bearing, non- 

 bearing and barren spurs from mature apple trees is shown in Fig. 12. 

 The variations in non-bearing spurs, or more accurately productive spurs in the 

 off year, are similar to those in the roots, trunks and branches with a maximum 

 in March at the time of bud swelling and a minimum at the end of June when 

 growth is over.^°" Barren spurs have a lower nitrogen content throughout the 



