The Lower Plants 79 



extent parasitic upon the alga and cannot exist with- 

 out it. On the other hand, the alga can, and not 

 infrequently does, grow quite independent of the 

 fungus. However, it is by no means unlikely that 

 the latter furnishes to the alga certain food con- 

 stituents, probably including nitrogen. Moreover, 

 the fungus conserves water in such a way that the 

 alga cells associated with it are able to grow as 

 they could not do if they were exposed to the air. 



A good many flowering plants, especially those 

 which are deficient in chlorophyll and especially 

 those that grow in humus like the Indian-pipe and 

 certain orchids, have associated with them a fungus 

 which in some way, not very clearly understood, 

 furnishes them with certain food constituents from 

 the humus in which the plants are growing and 

 make them available for the use of the plants. Re- 

 cent studies on these forms have shown that some- 

 times the fungi possess the power of fixing free 

 nitrogen, like the nitrogen bacteria, and it is likely 

 that the associated symbiont gets the benefit of this 

 by its association with the fungus, as well as ob- 

 taining the carbon which it cannot fix for itself by 

 photosynthesis. Some ferns and liverworts also 

 show this symbiotic association with fungi. 



Many species of parasitic fungi must be of com- 

 paratively recent origin, as they are restricted to 

 a single host, which in many cases is a highly spe- 

 cialized flowering plant and must be a relatively 

 recent development. 



