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 181£3,) 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 carbonie acid and 
assimilate carbon, resides in the cells that contain chloro- 
phyll, or, we may say, in the chlorophyll-grains themselves, 
We understand at once, then, that in the absence of iron, 
which is essential to the formation of chlorophyll, there 
ean be no proper growth, no increase at the expense of the 
external atmospheric food of vegetation, 
