396 GEOGRAPHICAL DISTRIBUTION OF FUXGI 



to be governed bv the interaction of interrelated and interde- 

 pendent factors, climatic, edaphic, and biotic. 



The validity of this conclusion becomes apparent if, for ex- 

 ample, an attempt is made to deduce the probable distributional 

 range of certain parasitic fungi from knowledge of the range of 

 their hosts. The student may think that each parasite should be 

 coextensive in range with its host (suscept), only to discover that, 

 although such is the situation among certain species, it is not 

 among others. Such observations lead to the conclusion that 

 endemism exists, that is, certain fungi are indigenous on native 

 species within particular areas where they have become dispersed 

 by natural agencies. These certain species may eventually be 

 spread to areas outside their natural range, but only as a result of 

 "artificial" introduction, chiefly by man. In a very real sense 

 man has interfered in no small measure with the natural factors 

 that influence the distribution of fungi. To retard or prevent 

 artificial dissemination of pathogenic species, quarantines have 

 been instituted, eradication campaigns have been organized and 

 conducted, inspection procedures have become compulsory in 

 connection with shipment of plants or plant parts from one local- 

 ity to another, and researches have been and are being made to 

 produce hosts that are resistant to disease. Problems which have 

 arisen as a result of disturbance of biological balance by man and 

 of his attempts to rectify them, therefore, constitute an interesting 

 and important phase of the geography of fungi. 



For a discussion of the structural features possessed by certain 

 species that aid in their geographical distribution the reader is 

 referred to Chapter 8, Spore Dissemination. In this chapter in- 

 formation will also be found on such natural agencies as air cur- 

 rents, rains, streams, floodwaters, and insects and various other 

 animals as factors in distribution. In a very real sense fungi tend 

 continually to extend their range, some behaving as settlers and 

 others as tourists. 



In the account that follows, greatest emphasis will be placed 

 on the geographic distribution of fungi as modified by man 

 through the introduction and cultivation of exotic plants of eco- 

 nomic importance. The presentation will not follow the logical 

 arrangement based on the distribution of fungi as governed by 

 climatic, edaphic, and biotic factors, but will be artificial and will 

 be based on fungus groupings. 



