30 PRACTICAL BOTANY 



air passages, which lead from the leaf, through the stem, 

 down into the roots (Fig. 360). It is supposed that " 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 well known. Many annuals, if given barelj T 

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

 after reaching a height of hardly a greater number of inches 

 than they would measure in feet under favorable conditions. 

 When the water supply is wholly withheld from ordinary 

 potted plants they soon wilt and die, as every one knows. 



31. Water roots. Most 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 produces a multitude of fibrous roots which dangle in the 

 water of the brook. Cuttings of Wandering Jew (Zebrina), 

 geranium (Pelargonium), and many other common plants, root 

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

 ordinary river or well water. The number of kinds of seed plants 

 which float, and therefore produce only water roots (if they have 

 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. Roots may be produced by portions of the 

 stem above ground, in the case of plants which 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 which English ivy and poison ivy 



