Page:Popular Science Monthly Volume 47.djvu/825

Rh water Crustacea to their destruction. They swim up greedily and collect at the mouth of the bladder to enjoy their feast. One of the guests ventures to get upon the lid. He remains there at first quietly held; but upon his making a more vigorous motion, the lid opens suddenly, swallows the little animal, and then closes again. The captive struggles awhile to escape from his prison, but gradually his movements become weaker, and he dies at last. Now the hairs clothing the interior walls of the bladder begin their work of imbibing food from the softer parts of the animal. When we reflect that a length of a finger and a half of a branch of the bladderwort can thus entrap two hundred of the little crustaceans, we can easily comprehend how the plant can do without roots, upon which it would otherwise depend for its supply of nitrogenous food.

In view of certain investigations which have been made of water flora and fauna for special purposes, I add a few words on the place of the water flora in the economy of Nature. The lower vegetation is of very great importance, especially the microscopic plants, innumerable plantations of which inhabit extensive tracts of all, and especially of northern, lakes. They move around in the water in masses or singly, changing the deep blue color of the spots destitute of organisms into green or dirty yellow. Most numerous among these minute plants wandering in lakes are the diatoms or siliceous Algæ, the richness of the forms of which surpasses all imagining. They consist of a nucleus of living substance inclosed as in a box between two siliceous shells. These shells bear markings so fine that they are used, in the same way as the dust of butterflies' wings, as tests for the delicacy of our best microscopes. The diatoms move through the water by the aid of a peculiar propulsory apparatus till they dying sink to the bottom and there go to help form the slime which is of so great importance in the evolutionary history of the crust of the earth. There are also diatoms in fresh water. They are the principal constituents of the brownish-yellow slippery coating of the stones in the beds of brooks.

The work of these and other similar living beings in the economy of Nature is not therefore lost, because they help with their dead remains to build up the dry land and prepare the ground for future generations. Their existence is thus of benefit to their fellow-creatures. As on the land, so there are plants in the sea which elaborate the unenlivened substances of the air and of the mineral kingdom and convert them into matter to become constituents of the bodies of living beings. Inasmuch as they serve as food, or as animals living upon them fall victims to the larger animals, they are the support of the whole animal life of the ocean. When we consider that twelve million