DISTRIBUTION OF SPORES 161 



Washington that over 5 million smut spores lodge on each square 

 foot of soil. 



Of much more interest than the ability of fungi to produce 

 spores in abundance is the development of mechanisms or devices 

 that serve to provide maximum distribution of these spores. 

 Survival of given species mav in large part be conditioned by dis- 

 persal into habitats where food is available. In most species of 

 fungi special mechanisms are lacking, and hence distribution, 

 among both aquatic and terrestrial forms, appears to be largely 

 fortuitous. 



DISTRIBUTION OF SPORES 



Aquatic fungi. The environment in which aquatic species ex- 

 ist is more constant than the habitat of terrestrial species, and cor- 

 related with this fact is the possibility that a larger proportion of 

 their spores may germinate and develop into new individuals. For 

 these reasons problems of dissemination of aquatic fungi might not 

 be expected to stimulate as much interest as similar problems in- 

 volving terrestrial fungi. Nearly all aquatic fungi are among the 

 Phycomycetes, the spores of many of which are motile (planetic). 

 The most primitive of these are holocarpic. Each such plant pro- 

 duces 20 to 30 spores, each of which possesses a single flagellum. 

 After a brief period of motility, which rather closely restricts the 

 distance that the spore may migrate from the parent plant, the 

 spore initiates the assimilatory phase of the cycle of development. 

 After a few days the sporangium is again mature, and conditions 

 for dispersal have once more been prepared. 



Other more highly specialized species possess differentiated 

 sporangia or other sporiferous cells, from which cells having two 

 flagella are liberated. Evidence is lacking that biflagellate species 

 are significantly better able to disseminate themselves and to com- 

 pete to greater advantage than monoflagellate ones. Undoubtedly 

 diplanetism, that is, two morphologically distinct motile stages 

 that always occur sequentially, so highly developed among the 

 Saprolegniales, must be regarded as an evolutionary advance over 

 monoplanetism. In the Saprolegniales diplanetism is accompanied 

 by certain morphological differences in spores whose significances 

 are wholly unknown. Typically, on first escaping from the spo- 

 rangium the swarm spores are pear-shaped and terminally biflagel- 

 late. After swarming for a brief period, they encyst and then 



