200 HOW CROPS GROW. 



plant may unfold its buds at the expense of already organ- 

 ized matters, as a potato-sprout lengthens in a dark cellar, 

 or in the manner of fungi and white vegetable parasites ; 

 but the leaves thus developed are incapable of assimilating 

 carbon, and actual growth or increase of total weight is 

 impossible. Salm-IIorstmar showed that plants which 

 grow in soils or media destitute of iron, are very pale in 

 color, and that addition of iron-salts very speedily gives 

 them a healthy green. Sachs found that maize-seedlings, 

 vegetating in solutions free from iron, had their first three 

 or four leaves green ; several following were white at the 

 base, the tips being green, and afterward, perfectly white 

 leaves unfolded. On adding a few drops of sulphate or 

 chloride of iron to the nourishing medium, the foliage was 

 plainly altered within 24 hours, and in 3 to 4 days the 

 plant acquired a deep, lively green. Being afterwards 

 transferred to a solution destitute of iron, perfectly white 

 leaves were again developed, 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 exter- 

 nally to the leaves, shortly developed a green color where 

 it was previously wanting. By microscopic studies he 

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

 leaf-cells remains a colorless or yellow mass, destitute of 

 visible organization. Under the influence of iron, grains 

 of chlorophyll begin at once to appear, and pass through 

 the various stages of normal development. We know 

 that the power of the leaf to decompose carbonic acid and 

 assimilate carbon, resides in the cells that contain chloro- 

 phyll, or, we may say, in the chlorophyll-grains themselves. 

 We understand at onfce, then, that in the absence of iron, 

 which is essential to the formation of chlorophyll, there 

 can be no proper growth, no increase at the expense of the 

 external atmospheric food of vegetation. 



