These little blossoms, thickly massed on the surface of the water, or sitting on long stems, spread out their five petals, which, with their numerous stamens and styles, mark them as relatives of the buttercups. In the other parts we look vainly for resemblances with the ranunculuses. If, for instance, we take one of them which we find floating in running water, out of its element, the whole plant falls together, and we hold in our hand nothing but a bunch of long threads, in which no difference can be perceived between stems and leaves. If we spread a part of the bunch upon a stone, we may discover branching shoots beset with leaf forms; but both organs are widely different from those of their nearest generic relatives. The stems of the ranunculuses of the fields are upright, stiff, skeletonlike, strong enough to defy wind and storm, and able to bear the weight of their leaves, flowers, and fruits. The stems of the water ranunculuses are slack and weak. They are swung around helplessly by the waves, winding hither and thither in the direction toward which the run of the stream carries them. They are stable only in the direction of their length, because in any other case the current would carry them away. In other respects the stem does need cohesive power. The whole plant is pierced with connected air passages, and all its parts are adapted to floating or swimming. The water here takes the burden upon itself which is imposed on the stems of land plants. Floating plants need no skeletons; and dissection and microscopic examination show that all those forms are wanting in their interiors which, like the bones of animals, give stability and tenacity to their structure.
Many water plants lack organs still more closely associated with the life processes. We can not conceive of a higher animal without veins and lymph-vessels. But in water plants we not seldom miss the long and broad ducts of which the vascular system of land plants is constituted. At all events the vessels do not perform so important a part in the vegetable kingdom as the circulation of the life juices in the animal kingdom. Their principal service is to carry water from the roots to the leaves. From this we can understand how organs essential to the life of land plants can be dispensed with in water plants. They do not need a special conducting of water, because they are surrounded by that element on every side. The most marked instance of the absence of internal organs is met in an alga which forms green fields in the deeper parts of the Mediterranean Sea. It has slender, branching, horizontally creeping stems which develop above in the water into leaves and below in the sand into fine thread roots. But the whole plant, often many feet in length, consists only of single gigantic cells. A tough skin incloses its juices, which flow in a continuous stream through the stem, leaves, and roots of the
