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The Food of Plants. 339 
cells. It is of interest to note, however, that the ability of 
plants to use the gases which have thus penetrated their 
structure, is dependent upon certain important conditions, 
viz :—1st, a favorable temperature, (2) the presence of the 
ordinary green coloring matter of plants—the chlorophyll— 
and (8) the direct influence of sunlight, or at least of its lumin- 
ous rays. Neglecting further consideration of temperature 
which is essential to all functional activity, it should be 
pointed out that plants devoid of chlorophyll, such as mush- 
rooms and other colorless plants, are incapable of obtaining 
carbon from the atmosphere. They are therefore forced to 
obtain their supply of this important element either from 
other plants upon which they feed as parasites, or from the 
organic products of decay, upon which they feed as 
saprophytes. Moreover, the power of green plants to 
appropriate carbon and liberate oxygen is arrested 
under conditions of darkness—as at .night—when the 
mode of growth is precisely the same as in colorless 
plants. 
The whole relation of light to the appropriation of car- 
bon, is one of the most interesting with which the physiologist 
has to deal, but it would lead us too far from our present 
purpose were we to consider it more in detail, though it 
may be as well to point out that, if ordinary white light be 
replaced by such luminous rays, as the orange and yellow, 
this function is not impeded in any way ; while on the other 
hand, the rays of higher refrangibility such as the blue, 
indigo and violet, arrest this function and thus bring 
ordinary green plants under abnormal conditions of growth, 
in which functional disturbance is the unavoidable result. 
In this particular connection, it only remains for us to 
indicate what changes take place when carbon dioxide is 
taken up by the leaves. Under the influence of chlorophyll 
this gas suffers decomposition. The liberated oxygen re- 
turns to the atmosphere, while the carbon, uniting with the 
elements of water already present, becomes transformed 
into starch, sugar and oils,—substances which not only pro- 
vide for the nutrition of growing parts, but, when formed in 
