8o6 THE POPULAR SCIENCE MONTHLY. 



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 Algce 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 star- 

 fish 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 yellow- 

 flowering 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 



