I 4 2 TEXT-BOOK OF FUNGI 



Salmon, Ann. Bot., 19, p. 125 (1905). 



Ward, Phil. Trans. Roy. Soc., 196, p. 29 (1904). 



Nordhausen, ' The Biology of Parasitic Fungi, how they 

 gain Access to the Host-plant, etc.,' Pringsh. Jahrb.^ 33, 

 p. i (1899). 



SYMBIOSIS 



After what has been said respecting parasitism it will 

 be seen that every transition between saprophytes and 

 obligate parasites in all probability exists. In every shade 

 of parasitism hitherto discussed the fungus is always 

 dominant to the extent of living entirely at the expense of 

 its host, whereas the latter, so far as is known, receives no 

 benefit from its parasite. In some instances, however, this 

 one-sided condition of things has to a great extent been 

 overcome, and parasite and host respectively benefit. 

 When this point has been reached it is expressed by the 

 term symbiosis or mutualism. 



Symbiosis is most evident in Lichens, every member of 

 which consists of a compound body consisting of a fungus 

 and one or more algae. The alga, by virtue of its chloro- 

 phyll, forms organic carbon compounds of use to the 

 fungus, whereas the vegetative portion of the fungus sup- 

 plies the necessary mineral food. Even in this case the 

 fungus appears to be the dominant partner, as it alone is 

 capable of producing fruit, the alga, with very few excep- 

 tions, remaining barren. 



It has long been known that the rootlets of trees belong- 

 ing to the Cupuliferae are more or less enveloped in a 

 sheath formed of fungus hyphae. Frank considered this 

 union to be of a symbiotic nature, and we now know that 

 such combinations occur in considerable numbers in every 



