CHAPTER VIII 



INDIVIDUAL ELEMENTS 



The intake of nitrogen and mineral constituents in inorganic form 

 has been described. Their incorporation into the plant is now considered 

 with particular reference to orchard or fruit plants. In the study of 

 nitrogen content analyses are expressed in percentages of fresh weight or 

 of dry weight or in the absolute amounts present in a certain tissue such 

 as 100 leaves; ash analyses are given in percentages of fresh weight or 

 of dry weight, in percentages of total ash or in absolute amounts. 

 Careful distinction should be made between determinations expressed 

 in these different terms since they are not comparable. For example, 

 during the development of a tissue — say the leaf — some ash constituent 

 may decrease in terms Of percentage of total ash, remain constant in 

 percentage of dry weight and increase in absolute amount. Absolute 

 amounts are particularly valuable data and show the actual changes 

 in the amount of substance present. Percentages of dry weight will 

 indicate the same changes provided there is no increase or decrease 

 in the absolute dry weight. If there is, then these changes must be 

 taken into consideration. Expression of percentage in terms of fresh 

 weight involves in addition changes in the water content. Percentages 

 of total ash show the relative proportions of the various ash constituents. 

 Each of these determinations has its value, but each expresses different 

 relations. 



NITROGEN 



Nitrogen enters the roots from the soil solution as a salt of nitric acid,, 

 such as potassium or sodium nitrate, or sometimes as ammonia. The 

 supply of nitrates in the soil varies with temperature and moisture, 

 usually being greatest in late spring and early autumn, but persisting 

 throughout the summer. 



Synthesis of Organic Nitrogenous Compounds. — Most of the inorganic 

 nitrogen absorbed is carried up the trunk and branches to the leaves 

 where it is elaborated into amino-acids and other nitrogenous organic 

 compounds. The elaboration of nitrates to amino-acids takes place for 

 the most part in the chloroplasts of the leaf mesophyll cells. Light has 

 been shown^^^ to increase nitrogen assimilation, blue-violet and ultra- 

 violet light being particularly effective. Light from the blue end of the 

 solar spectrum is relatively stronger in cloudy weather; light from the 

 other end of the spectrum which is the more important for the photo- 



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