element. The doctrine of the Greek philosophers, that everything is derived from water, is so far correct for the vegetable world that the first plants that appeared on the earth were water plants. There were probably little microscopic forms inhabiting the barely cooled waters of the primitive seas before there was any land to afford a suitable home for any living beings—formless albuminous masses, like "organisms without organs," which, like some of the bacteria, drew their food from the dead stone. Like their living kindred, the lower Algæ, they were of too tender nature to be preserved in the cavities of the sea slime. The first remains seemingly of vegetable character preserved in the oldest strata of the earth's crust are therefore of relatively large fucoids. Their existence justifies our supposing an already richly developed flora of Algæ such as is now found in the deepest parts of our lakes. Mosses, ferns, and flowering plants are absent. They appear later, and under conditions which prove that they were produced not in the sea but on swampy land. Geological evidence shows us that only the Algæ, and the fucoids originated in water and were water plants from the first. The other water plants, especially flowering plants growing in water, were driven into the water by increasing competition among the growing number and variety of the land plants, and assumed the properties that now distinguish them from land plants during their compulsory emigration, and in consequence of their water life. This process is now going on in our sight in a certain plant—the wandering knotgrass—a relative of the small-flowered, spreading swine grass and of the adderwort. This plant grows on the borders of ditches and ponds, often half on land and half in the water; and it can not escape the attentive observer that it presents a quite different appearance in the water from that upon land. Stiffly haired, and having short-stemmed leaves on land, it is in water bald and smooth, and develops very long leafstalks which terminate on the surface in flat, floating expansions. Here there is a plant which only occasionally, and usually only partly, makes its home in the water, and is in a position to suffer such remarkable changes that it is no longer a wonder that plants which have become entirely at home in the water are very little like those of their genus which remain land plants. Of many of them, in fact, it can no longer be determined from what family of land plants they are really derived. Even such well-defined plants as seaweed and duckweed would not at first sight suggest to any one relationship with the Arum family. So with the water crowfoot, which we shall take as our starting point in the discussion of the properties of water plants. Well known are its little white flowers, which adorn the ponds and even the swift streams in the summer time.
