Popular Science Monthly/Volume 21/October 1882/A Partnership of Animal and Plant Life

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THE fundamental difference in the feeding of plants and animals is conditioned on the presence or absence of chlorophyl. Green plants are competent to assimilate inorganic matter by means of the chlorophyl-bodies in their leaves, while animals require organic substances for food. Were this difference mere comprehensive, it would incontestably be regarded as the most important of all the differences between the two classes of organisms. But there are, on the one hand, plants that have no chlorophyl—the fungi; and, on the other hand, animals which have been known for a considerable time to contain chlorophyl, as the fresh-water sponge (Spongilla), the hydra, several gyrating moners, and many infusoriæ and rhizopods.

The fungi feed, like animals, on organic matters; but it is not yet sufficiently established whether the so-called chlorophyl-bearing animals can be nourished entirely after the fashion of real plants, by the assimilation of inorganic matter; or, in other words, whether, with an abundant access of air and suitable lighting, they can live in filtered water. Before we can approach this question more closely, however, we must decide another equally important one, whether the chlorophyl-bodies present in the animals are really elementary parts, morphologically corresponding with vegetable chlorophyl produced by the animals themselves, or whether they are not unicellular vegetable organisms parasitic in the animals—in other words, it must be decided whether the green bodies in animals are parts of cells or are them-selves cells; whether they are morphologically and physiologically dependent on the tissue in which they appear, or independent of it.

Morphological investigation has been pursued upon hydras, spongillas, a planaria, and a number of infusoria, from which the green bodies have been pinched off and examined with strong magnifying powers. All the examinations of these different objects have given the uniform result that the green bodies of animals are not evenly green like the chlorophyl-bodies of plants, but contain colorless protoplasm besides the green mass, or, at least, a cell-kernel which can be easily distinguished on treatment with hematoxylin. Among them were likewise several cell-kernels, which were regarded as evidences of the beginning of division, for normal chlorophyl-cells never contain a cell-kernel.

Thus the green bodies of animals do not correspond with the chlorophyl-bodies of the algæ, but are independent organisms, or one-celled algæ, which have been named zoöchlorella. Yellow cells are found living under similar relations in actinias and radiolarias, which have been distinguished as zoöxanthella.

The physiological as well as morphological independence of the green cells is also demonstrated by the fact that when separated from the animals they continued to live in this condition for days and weeks, and formed starch in the sunlight. When grafted upon hydras and infusoriæ, which were quite free from chlorophyl, they continued to live upon them.

The conclusion is drawn from these researches that self-formed chlorophyl is wanting in real animals, and that, when it is present in their bodies, it originates in plants that have immigrated to them. The most interesting result from them is the answer they give to the question as to the significance of the green and yellow algæ to the animals in which they occur. In order to examine this matter more closely, colonies of radiolarii containing numerous yellow cells were put into filtered sea-water. They not only continued to live in it, but outlived the specimens that were left with the other organisms. Now, since the radiolarii are real animals, incapable of living on any but organic matter, while in this case air and water afforded them all the support they required, they could have been kept alive only by the yellow cells that lived upon them, working up the inorganic substances that were provided for them, under the influence of light, into organic. Further experiments showed that fresh-water sponges could be cultivated to the best advantage in filtered water, thus demonstrating that the zoöchlorella and the zoöxanthella are fully competent to maintain the animals in which they live. If the animals contain few or no green or yellow algæ, they are fed, like real animals, by the assimilation of solid organic matter; but, when they contain algæ, they may be fed, like real plants, by the assimilation of inorganic matter. In the latter case, the algæ living in animals perform precisely the same function as the chlorophyl-bodies of plants.