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.) 
315. 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. 
