CHAPTER IX 

 NITROGEN ASSIMILATION; THE NITROGEN CYCLE 



In all things there is a kind of law of cycles. 



— Tacitus. 



Protoplasm, which makes up the living material of the plant, 

 contains in addition to carbon, hydrogen, and oxygen, a large 

 percentage of nitrogen. Wheat straw which seems almost pure 

 cellulose has 0.5-1.0% of the dry weight in nitrogen, while as 

 much as 25% of the dry weight of bean seeds is composed of this 

 element. The nitrogen is absolutely essential for the formation 

 of proteins and consequently of protoplasm. In its absence the 

 leaves of plants are generally stunted, and the foliage, which at 

 first becomes reddish, later develops a yellowish, sickly look. An 

 abundance of available mineral nitrogenous compounds seems to 

 favor vegetative growth and to retard the formation of fruiting 

 parts, so that with plenty of nitrogen, there is a rank production 

 of rich green foliage without flowers; but this relation of nitrogen 

 to reproduction will be further considered in Chapter XXXI. 

 Since nitrogen is present in protoplasm, it is to be expected that 

 a deficiency would derange the general metabolism. Molliard 

 (1922) found that respiratory processes were consequently affected 

 and acids were caused to accumulate when plants were grown with 

 little or no nitrogen. Radish plants thus grown contained five 

 times as much acid in the cell sap as those with nitrogen, and 

 sorrel (a very acid plant) was twice as acid. 



This general action of nitrogen may be summed up as follows: 



1. Forms a component of all protoplasm and, therefore, is nec- 

 essary for normal metabolism. 



2. Increases foliage and, therefore, is especially important for 

 leaf crops (spinach, forage plants, cabbage, etc.). 



3. Retards flowering and maturation of plants. 



4. Aids in the production of a healthy, green color. 



5. Produces succulence and crispness of foliage, thus improv- 

 ing the quality of leaf crops like celery and lettuce. 



6. Lowers resistance to diseases, insects, and cold weather. 



87 



