30 PRACTICAL BOTANY 



air passages, wliicli Icail from 1lic leaf, tlnougli the stem, 

 down into tlu'. roots (Fig. 360). It is supposi^d tliat " cypress 

 knees," curious outgrowths from the roots of the American 

 cypress (Fig. 19), absorb air, which passes down into the roots. 

 A. ■ supply of water is, as already suggested, even more 

 evidently necessary for earth roots than is a supply of air. 

 The appearance during a drought of fields planted with ordi- 

 nary crops is familiar to most people. The dwarfed condi- 

 tion to which plants can be brought by a scanty supply of 

 water is less «'ell known. i\Iany annuals, if given barely 

 enough water to keep them alive, will flower and bear seed 

 after reaching a height of hardly a greater number of inches 

 tlian they would measure in feet under favorable conditions. 

 When the water supply is wholly withheld from ordinary 

 pottt'<l plants they soon wilt aud die, as every one knows. 



31. Water roots. IMost aquatic perennials, like the cat-tails, 

 arrowheads, pickerel weeds, pond lilies, and many grasses 

 and sedges, form mainly earth roots. On the other hand, some 

 plants not aquatics, as many willows, can develop roots indif- 

 ferently either in earth or in water. A row of willows along a 

 brook usually sends great numbers of roots into the earth, and 

 also pr(j(luces a multitude of fibroids roots which dangle in the 

 watei' of the brook. Cuttings of W^audering Jew (Zrhrind), 

 geranium {Pctiin/didiim), and many other common plants, root 

 readily in water, and grow for a long time if supplied only with 

 ( )rduiary ri\'er or well water. The number i if kinds of seed plants 

 which float, and therefoi'e produce only water roots (if they liave 

 roots at all), is rather small. Some of the commonest are the 

 so-called "water hyacinth" and the little duckweeds (Fig. 357) 

 so often seen on the surface of stagnant pools and streams. 



32. Air roots. IJoDts may be produced by portions of the 

 stem above ground, in the case of plants ^^'hich root in the 

 earth. Well-known examples of these are the brace roots 

 of corn, often originating a foot or more above the earth and 

 usually at length extending into the soil, and the tough, 

 fibrous roots by means of Avhich English ivy and poison ivy 



