Page:Popular Science Monthly Volume 47.djvu/824

806 vital career of the annual water plants closes with the formation of the seed. They die and fall into decay through the agency of the water bacteria, whose activity carries their substance on to renewed life in the circulation of matter. The vegetation of most of our northern perennial water plants is interrupted during the winter. Some of them, like the pond lily, have long rootstocks, in which superfluous food is deposited as in a storehouse in the course of the summer, to serve in the spring for the formation of the new leafy growth; others form special winter buds which, likewise filled with food, separate from the mother plant, sink to the ground, or are frozen in the ice, till the returning warmth of the sun revives them.

Having sought to uncover the mysteries of the household life of the water plants, we now turn to their relations to other families. No group of organisms has ever been able to develop itself independently of all other living beings. Individuals have to acquire the useful properties we admire in them in constant conflict—plants especially, in conflict with the animal world; and the vegetation of the water is as much subject to it as any other. Besides the fishes, there are the water snails and innumerable crustaceans, large and small, turning to water plants for food. Some plants are protected against these creatures by the presence of substances that give their leaves a bitter taste; some have many pointed prickle cells in the interior of their leafstalks which make it impossible for their enemies to bite through them. Most of the seaweeds are furnished with slimy cell walls, on which the water snails try their teeth in vain. The calcareous Algæ of the sea enjoy the best protection in the shape of a knotty or coralline form which has little resemblance to a plant, and through the deposition of carbonate of lime in their cell walls, almost turning them into real stone. Only a few marine animals know how to attack them. Among these is a snail that dissolves the lime by means of a secretion of sulphuric acid. With the same material, as Semon has shown, these snails also make sea urchins and starfish digestible, and are therefore brought in reach of an unusual variety of food, in which they are rivaled only by the lobster with his strong cutting jaws.

There are, besides, animal-catching plants among the water vegetation as the common bladderwort (Utricularia), a yellowflowering plant, with slender stem and finely dissected leaves, which is abundant in still waters in summer. Its leaves bear on and between their points round bladders, about as large as the head of a pin, which serve as animal traps. The most interesting part is an elastic lid, which opens only toward the interior. A wreath of glandular hairs surrounding the entrance of the bladder secretes a slimy material which entices the smaller