SAPROPHYTISM AND SYMBIOSIS 343 



tant because of the enormous financial losses occasioned 

 by the timber-decay which they induce. 



314. Parasitism Means Degeneration. Most parasites 

 among the flowering plants have suffered the loss of some 

 organ or organs, and of one or more functions as a result 

 of the parasitic habit. In fact, parasitism must be re- 

 garded as an acquired habit, and the parasite among 

 plants, as in human society or elsewhere, as a degenerate 

 form of life. Some plants can live only as parasites 

 (obligate parasitism), while others may live either as 

 parasites or as saprophytes (facultative parasitism). 



FIG. 250, a. Cissus laciniala, parasitic on the cactus (Opuntia Blakeana). 

 The parasitism was artificially induced (xeno-parasitism). The host 

 plant has been sectioned to expose the roots of the xeno-parasite. (Re- 

 drawn from D. T. MacDougal.) 



316. Flowers and Insects. The dependence of certain 

 plants upon insects to secure the transfer of pollen from 

 one flower or plant to another, will be mentioned more 

 in detail in Chapters XXVII-XXIX. 



