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THE QUARTERLY REVIEW OF BIOLOGY 



gen played a role more important than 

 that of any of the other elements tested. 

 If the dose of nitrogen was increased or 

 diminished the color of the plants in- 

 creased or diminished also. Urban found 

 that the darker beet leaves contained the 

 most nitrogen and the lighter the least. 

 Wlodek observed that the width of the 

 absorption bands correlated with the 

 nitrogen content of the leaves. Sir John 

 Russell in discussing the effect of fertilizer 

 upon plant growth said that nitrogen 

 produced larger leaves and stems. A 

 greater formation of green coloring was 

 induced. The writer has shown in his 

 work on Coleus that nitrogen appears to be 

 one of the limiting factors in chlorophyll 

 formation. It was also shown that after 

 adding nitrate to a plant only a few days 

 were required before a greening showed up 

 in the leaves. Eckerson observed that 

 simultaneously with the appearance of 

 amino acids the chloroplasts of the 

 younger leaves became greener, in from 

 three to five days after the addition of 

 nitrates. The chloroplasts of the deep 

 green leaves of Tropaeolum, as noted by 

 Meyer, were larger than those of the pale 

 green leaves . Size of the chloroplasts and 

 color of the leaves were definitely corre- 

 lated. He looked upon the chloroplast 

 as the birthplace of the proteins, hence it 

 is easy to see why the color of the leaves 

 and the protein content are so intimately 

 related. He observed also that as the 

 leaves yellowed there was little change in 

 the size of the nucleus, the nucleolus, or in 

 the protein content of the cytoplasm. He 

 inferred that the formation of chlorophyll 

 in the chloroplasts follows the develop- 

 ment of protein in the leaves. 



Allison in considering nitrogen as a 

 plant food says that of the ten elements 

 commonly considered as essential plant 

 foods, nitrogen stands out in many ways 

 as the most important. Not only is it 



deficient in many soils, expensive to supply 

 and difficult to retain, but it plays an 

 exceedingly important part in plant metab- 

 olism, including both the constructive 

 changes and also the respiratory reactions 

 resulting in the destruction of carbohy- 

 drates and proteins. These reactions 

 apparently take place within the plant 

 cells, the synthetic reactions being con- 

 fined largely to the leaves and active 

 growing portions of the plants, while the 

 respiratory changes which result in carbon 

 dioxide production may occur throughout 

 the plant tissues. Without doubt the 

 nitrogen compounds present in the proto- 

 plasm of the cells as protein, amino acids 

 or other compounds are the active constit- 

 uents which enable the tissue building to 

 proceed. Without nitrogen there could 

 be no life upon the earth. 



The writer has observed a definite corre- 

 lation in cotton leaves between the size of 

 the chloroplasts, the color of the leaves 

 and the amount of nitrogen which has 

 been added to the soil in which the 

 plants grew. He has also found that in 

 the case of potatoes a high chlorophyll 

 content of the leaves is correlated with 

 high nitrogen fertilization. In the case of 

 cotton, leaves from plots high in nitrogen 

 contained more of the chloroplast pig- 

 ments than did those from other plots. 

 Without forgetting the relation of nitro- 

 gen to the development of chlorophyll, the 

 latter will be considered as a limiting 

 factor in the growth of plants. 



Briggs in working on vegetable assimi- 

 lation and respiration found some very 

 good lines of evidence regarding the 

 chlorophyll factor in plant growth. It 

 appears, he says, that the inevitable con- 

 clusion to be drawn is that increase in 

 assimilation with progress of time cannot 

 be due to increase in chlorophyll content 

 during that time, increase in leaf area, fall 

 in respiration or a combination of these 



