PLANT LIFE. 33 



ever, the soil instead of being merely good and moist 

 becomes water-logged, so that all the spaces between 

 the soil particles are filled with water, in place of con- 

 taining some air, the roots begin to suffer because 

 they cannot obtain enough oxygen. The result is that 

 plants not native to such soils die if this condition 

 persists longer than a few days. 



How Some Plants Get Air to the Roots. Certain 

 plants have other contrivances for getting air to the 

 roots and thus can live with their roots in water, or in 

 water-soaked soil. The stems may be hollow and 

 filled with air; the cavities extending down into the 

 roots, or certain roots may grow up into the air as 

 air-absorbing organs, like the cypress knees for ex- 

 ample, and the aerating roots of some of the man- 

 groves. It is noteworthy that the cypress does not 

 form knees unless the soil is very wet. 



Parasites and Saprophytes. There are some plants 

 which, like animals, do not manufacture their own 

 food, but have to take what has been prepared by other 

 organisms. Such plants are called parasites if they 

 feed upon living animals or plants, and saprophytes if 

 they live on dead animal or vegetable substances. 

 They are not green and do not need light, although 

 light is not harmful to some of them. If they have 

 any leaves, they are mostly very small and like little 

 scales. Many of these parasites are the cause of 

 great injury to crops and have great economic im- 

 portance. We need but mention a few of the more 

 common ones: rust of grain; smut of oats, wheat and 

 corn; black heart or wilt of cotton and other crops; 

 leaf blights and spots on the fruits of most of our 

 common fruits; brown rot of peaches; decay of tim- 

 ber; dodder or love vine of clover and alfalfa; etc. 

 These as well as the methods of combating them are 

 discussed more fully in the section on Plant Diseases. 



EXERCISE. How do plants and animals differ? Why is shade in- 

 jurious to some plants? Why are trees at the border of a field un- 

 desirable? 



