Environment and Adaptation 217 



sites grow upon the roots of other plants, e.g., 

 beech-drops (Epiphegus), canker-root (Aphyllon), 

 and others. Certain parasites may not be entirely 

 dependent upon their host for carbon, being able 

 to assimilate CO 2 . Of these parasites the various 

 species of mistletoe are the best-known examples, 

 and a number of the Figwort family are also known 

 to be root parasites, although these have well-devel- 

 oped roots. Of these green root-parasites Gerardia 

 and Castilleia may be mentioned. 



Saprophytes. There are many saprophytic flow- 

 ering plants, these being especially numerous in the 

 Heath family, and among the orchids. Sapro- 

 phytism may be present in plants having green 

 leaves, like species of rhododendron and many 

 others ; while in others all chlorophyll has been lost, 

 and the leaves and sometimes the roots are 

 rudimentary. The coral-root orchids, the Indian- 

 pipe, and the curious snow-plant (Sarcodes), of 

 the Sierra Nevada, represent this extreme case of 

 saprophytism. In all of these, so far as they have 

 been investigated, there is always associated a 

 fungus, by means of which they seem to be able to 

 utilize the necessary carbon compounds from the 

 humus. This peculiar form of parasitism, symbi- 

 osis, has already been referred to. The exact nature 

 of this association is not always clear, but its con- 

 stant occurrence implies that the association is mu- 

 tually beneficial. In the lichens the association is 

 so intimate that the resulting structure has assumed 



