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 laciniata, 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. 



