220 HOW CROPS GROW. 



and these were brought to a normal color by addition of 

 iron. 



E. Gris was the first to trace the reason of these effects, 

 and first found (in 1843) that watering the roots of 

 plants with solutions of iron, or applying such solutions 

 externally to the leaves, shortly developed a green color 

 where it was previously wanting. By microscopic stud- 

 ies he found that, in the absence of iron, the protoplasm 

 of the leaf-cells remains a colorless or yellow mass, desti- 

 tute of visible organization. Under the influence of iron, 

 grains of chlorophyl begin at once to appear, and pass 

 through the various stages of normal development. We 

 know that the power of the leaf to decompose carbon 

 dioxide and assimilate carbon resides in the cells that 

 contain chlorophyl, or, we may say, in the chlorophyl- 

 grains themselves. We understand at once, then, that 

 in the absence of iron, which is essential to the forma- 

 tion of chlorophyl, there can be no proper growth, no 

 increase at the expense of the external atmospheric food 

 of vegetation. 



Risse, under Sachs' s direction (Exp. Physiologic, p. 

 143), demonstrated that manganese cannot take the plac^ 

 of iron in the office just described. 



CHAPTER III. 

 1. 



QUANTITATIVE RELATIONS AMONG THE LSGREDIENI - 

 OF PLANTS. 



Various attempts have been made to exhibit definite 

 numerical relations between certain different ingredients 

 of plants. 



Equivalent Replacement of Bases. In 1840, Lie- 

 big, in his Chemistry applied to Agriculture, suggested 



