THE LIFE OF WATER PLANTS. 807 



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 crus- 

 taceans, 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 Alga?, the richness of the forms of which 

 surpasses all imagining. They consist of a nucleus of living sub- 

 stance 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 econ- 

 omy 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 individ- 



