Page:Collier's New Encyclopedia v. 01.djvu/267

AQUATIC PLANTS 215 AQUEDUCT merous forms illustrate the transition from the former to the latter. The as- cidians are exclusively marine. Some fishes have a limited povirer of life out of the water, the double-breathing dipnoi being in this connecflon especially in- structive. Among many amphibians, the transition from water to terra firma is seen in the individual life-history, when the fish-like gilled tadpole becomes the lunged gill-less frog. The instance of the gilled axolotl becoming, in the absence of sufficient water, the gill-less amblystoma, forcibly illustrates the importance of the medium as a factor in evolution. Among reptiles there are numerous aquatic forms — chelonians, lizards, snakes, and crocodiles, though the absence of any gill respiration marks the progressive general adaptation to terrestrial life. While an emphatically terrestrial amphibian like the tree frog seeks a watery hole for the rearing of the young gill-breathing tad- poles, the habit is reversed in such rep- tiles as the sea turtle, which, having re- turned to the more primitive aquatic home, yet revisits the land for egg-laying purposes. Among mammals the sea cow, the seal, and the whale are familiar illus- trations of very different types which have returned to the primeval watery home and aquatic habit, with consequent change of structure. In the more thoroughly aquatic ani- mals, which have remained in the prim- itive environment, and have not merely returned to it, the blood is usually pviri- fied by being spread out on feathery gills which catch the oxygen dissolved in the water; while in terrestrial forms which have betaken themselves to an aquatic life, the ordinary direct "air breathing" is still accomplished at the surface of the water, or, in some isolated cases of insects and spiders, by means of the air (entangled in their hairs, or even con- veyed into their submerged homes. The genuinely aquatic animals are known to have a body temperature not much higher than that of the surrounding medium, and often survive even the freezing of the water; while the higher warm-blooded vertebrates which have re- turned to the aquatic habit, various modi- fications, such as thick fur and plumage, waterproof varnish, formation of blub- ber, serve as protections against the cold. AQUATIC PLANTS, plants growing in or belonging to water. The presence of water is not only essential to the ac- tive life of all organisms, but is pecul- iarly necessary for plants which are for the most part dependent for food supply on matter dissolved in water, as well as on the carbonic anhydride mingled with the surrounding medium. Numerous plants are, moreover, in the strict sense of the word aquatic, having never ac- quired or having lost all direct connec- tion with the soil. The simplest plants or algae are almost all aquatic, though many occur in damp situations on land, or on other organisms, while others re- main for long periods quiescent in com- parative dryness. Many algae are abso- lutely isolated in the water, while others are more or less intimately fixed to some solid substratum. Some rhizocarps, such as salvinia, are aquatic, with leaves rising to the surface, while others are land or marsh plants, like the higher horse-tails and club-mosses. Among the flowering plants, or phan- erogams, a return to aquatic life is ex- hibited by numerous, though exceptional cases, while a very large number grow in moist situations, and have a semi- aquatic habit. The simple monocotyle- dons, known as helohiese, or marsh lilies, are more or less strictly water-plants. The arrowhead (sagittaria) , and other alismaceas; the butomis of the marshes; hydrocharis, with floating kidney-shaped leaves; the water soldier (stratiotes) , with narrow submerged leaves; and the Canadian pond weed (aimcJiaris) , Among dicotyledons, the white water buttercup (ranmiculus aquatilis) , with its slightly divided floating, and much dissected submerged leaves; the yellow and white water-lilies (nymphiea) ; the sacred lotus flower of the Ganges and Nile {nelumbium) ; the gigantic Victorut regia of tropical South America ; and the insectivorous bladderwort or utricularia, are among the most familiar aquatic forms. AQUATINT, a method of etching on copper by which a beautiful effect is produced, resembling a fine drawing in sepia or Indian ink. The special char- acter of the effect is the result of sprink- ling finely powdered resin or mastic over the plate, and causing this to adhere by heat, the design being previously etched, or being now traced out. The nitric acid (aqua fortis) acts only in the interstices between the particles of resin or mastic, thus giving a slightly granular appear- ance. AQUEDUCT, an artificial channel or conduit for the conveyance of water from one place to another; more particularly applied to structures for conveying water from distant sources for the supply of large cities. Works for supplying com- munities with water must have been con- structed at a very early period. In China there are said to be aqueducts dating back to prehistoric times. In Persia and Assyria there are structures