30 THE NATURE AND WORK OF PLANTS 



serve the purpose of bracing the stem against being 

 blown over. 



31. Keel or ballast roots. — Some aquatic plants 

 are in the form of a leaflike stem or a rosette of 

 leaves which floats on the surface of the water, and 

 one or more roots are formed which hang downward 

 in the water, serving to keep the floating parts in 

 proper position, and perhaps also as organs of absorp- 

 tion. The members of the duckweed family (Lemna) 

 and the water hyacinth are good examples of this, 

 and should be examined. 



32. Substances of which the soil is composed. — 

 Examine a half cupful of soil taken from the gar- 

 den, by means of a hand lens, or spread it out on a 

 sheet of glass and hold in direct light. When sepa- 

 rated into small portions with a needle it may be 

 seen to be made up of irregular bodies of various 

 sizes, bits of rock, fragments of leaves, stems, seed 

 coats, splinters of bone, feathers, cast-off shards of 

 beetles, as well as a variety of other matter which 

 will depend on the locality from which the soil was 

 taken. Every particle has a faint shiny appearance 

 which disappears more or less completely if the mass 

 is dried over a hot stove. The shiny appearance was 



