The science of life : fully illustrated in tone and line and including many diagrams

THE LOWLY AND MINUTE

mentous algz, with chlorophyll, which are not motionless but sway and glide about like microscopic serpents by some means that is at present entirely mysterious. What then are we to do? If we regard chlorophyll as the definite feature of plants, then mushrooms are animals. If we regard immobility as the criterion, the green flagellates are animals—and what of the streaming protoplasm of many plant-cells and the active animal-like spermatozoa of ferns and mosses ? We are forced, in fact, to abandon the old categorical distinction between animal and vegetable, to recognize that there are transitional forms lying between the two. There is a curious assembly of such formsthe green flagellates, the “ slime-fungi,” and one or two others—which are described in both zoological and botanical text-booksa sort of no-man’s-land (or, rather, bothmen’s-land) between the animal and vegetable kingdoms.

And now we must point out the indefiniteness of a second categorical distinction. We described the structure of amoeba and pointed out that it is in all essentials like a single cell from one of the higher organisms ; upon that fact we based a distinction between single-celled and many-celled organisms. Now, the line between the two, like the line between animals and plants, is not so much a fine black line as a broad grey smudge, not so much a boundary as a neutral zone. Here again there are transitional forms.

We noted that the commonest method of reproduction in protozoa is simple fission. The creature pulls itself into two halves, two cells, which creep away in different directions and live independently of each other. Now, suppose that instead of parting they remained stuck together, suppose that they divided and divided and built up a colony of many adhering cells instead of a population of free individuals, the result would be a step in the direction of the manycelled body of the higher animals. And, indeed, such a process as this actually occurs during the life-history of many flagellates. The single-celled forms may remain together more or less permanently as colonies, and thus they lead up to the many-celled plants, and by a series of stages to the familiar seaweeds. Because of this relationship the whole series, from Euglena to the gigantic seaweeds of Antarctic seas, is grouped together and spoken of as the Alge. Let us examine the various kinds and note this grading in more detail.

The simpler plants include the green flagellates that we have already noted,

together with several other single-celled kinds. There is, for example, the green coating on the shady sides of trees, rocks, fences, and the like, which consists of millions of tiny, globular, thick-walled alge. And there is an enormous group of forms, the diatoms, which are characterized by the siliceous shells in which they live, shells consisting of two valves which fit together like a pill-box and its lid. There are very many species of diatoms, showing a, wide variety of forms ; their shells are patterned with tiny pits and grooves so fine as to be standard objects for testing the high-power lenses of microscopes. They live wherever there are moisture and light, but the richest variety of forms is found in the surface layers of the sea; their skeletons may sink to the bottom, as with marine protozoa, and form oozes and earths. In some places whence the sea has receded layers of fossil diatom shells over a hundred feet in thickness have been found. Such _ siliceous earths may be used commercially ; the kieselguhr, which is mixed with nitro-glycerine to make dynamite, is an example, and diatom skeletons are responsible for the abrasive properties of certain silver polishes and tooth-pastes. Also conspicuous among the microscopic vegetation of ponds and the like are the desmids, algae lacking siliceous shells but showing a variety of beautiful and symmetrical forms.

Now many of these single-celled plants have a habit of associating themselves into chains or masses, sometimes temporarily and sometimes for the whole of their life-history. In this way they lead up to the so-called filamentous alge, which consist of chains of cells stuck

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Top, a single

Fig. 115. individual of the active

green flagellate Chlamydomonas, half animal and half plant. Bottom, less highly magnified, several indwiduals temporarily associated into a colony and living in a_ gelatinous mass.