168 SPORE DISSEMINATION 



escape from the cvst as reniform, laterally biflagellate swarmers. 

 Encvstment follows; after this stage they give rise to germ tubes. 

 This pattern of behavior varies in the different genera. Sometimes 

 polvplanetism occurs as reported bv Weston (1919) in Dic- 

 tvuchus, bv Hohnk (1933) in Saprolegnia torulosa and Achlya 

 racemosa, and bv Salvin ( 1940) in Achlva, the number of swarm- 

 ings beimj controlled by reserve food in the swarm spore and by 

 unknown environmental factors. 



Terrestrial fungi. Various adaptations occur among terres- 

 trial fungi to aid in their geographic distribution. The spores of 

 many are pulverulent, so that dissemination bv air currents is 

 favored. Others accumulate in a mucous matrix that is water sol- 

 uble, the occurrence of dews and rains being required for spore 

 dissemination. Some become wet with ease, others with difficulty; 

 some have thin walls, others very thick, resistant walls; some are 

 smooth, others are armed with spines, tubercles, or echinulations. 

 The fructifications of some species are malodorous, encouraging 

 visitation by flies, bees, ants, and other insects, whereas others 

 are attractive to mvcophagous animals, such as nematodes, beetles, 

 snails, slugs, and rodents. 



For convenience of discussion the dissemination or dispersal of 

 fungi may be considered to be accomplished by (a) agencies re- 

 lated to the environment of the species and (b) the fungus itself 

 through structural adaptations. 



The environmental agencies include movement of air as con- 

 vection currents and winds, movement of water, occurring as 

 dew, rains, and streams, and transport by insects and other animals, 

 including man. Many species are dispersed on seed, fruits, cut- 

 tings, seedlings, and transplants. 



Air currents as a factor in dispersal. For nearly 150 years 

 it has been taken for granted from observational evidence that the 

 spores of fungi are wind-borne. Proof that wind is an important 

 agency in the spread of pathogenic fungi has been forthcoming 

 only in recent years. It arose from attempts to explain the occur- 

 rence of epidemics, especially of rusts. According to Arthur 

 (1929), Marshall reported the following observations made in 

 1782 upon the results of planting a barberry bush in a field of 

 wheat: "About the barberry bush there appeared a long but 

 somewhat oval-shaped stripe of a dark livid color, obvious to a 

 person riding on the road at a considerable distance. The part 



