REPRODUCTION AND DISPERSAL 



8ll 



amoeboid movements; much more characteristic are non-motile car- 

 pospores and tetraspores (figs. 150, 151), which, like zoospores, are 

 devoid of protective walls. Non-motile spores may occur also in the 

 green algae (as in the aplanospores of Botrydium, fig. 93). 



Asexual spores in the fungi. — • Perhaps the culmination of asexual 

 spore development occurs in the fungi. A few forms that grow in water 

 or in wet places have ciliated zoospores (as in Saprolegnia, fig. 156); 

 in certain myxomycetes there are zoospores which swim for a time, and 

 then lose their cilia and creep with an amoeboid movement. In general, 

 however, fungus spores are not self-motile, and are invested with con- 

 spicuous walls. They may be borne within a 

 special spore-bearing organ, for example, a spo- 

 rangium (as in Mucor, fig. 11 22), or an ascocarp 

 (as in Peziza, figs. 175, 176), or they may be 

 developed externally, as in the conidia of Peni- 

 cillium (fig. 179) and in basidiospores '■ (fig. 

 201). 



Fungus spores commonly are dispersed by 

 wind, and their minute size and their resistance 

 to wetting make possible the remarkable effi- 

 ciency of this agent ; even the very slightest 

 movements of the air are suflacient to initiate 

 dispersal. Many species of fungi are common 

 to widely separated regions, and it is thought 

 that this cosmopolitanism is due in large part to 

 the effectiveness with which their spores are dis- 

 persed by wind. The abundance of spores and the ease with which they 

 are carried is shown by the readiness with which cultures of various 

 fungi may be made anywhere by exposing to the air, bread or cheese, 

 properly moistened, so as to insure good conditions for germination. 

 Fungi surpass all other plants in the number of new individuals that 

 may be produced from a single plant by asexual spores. A single large 

 puffball (as Lycoperdon giganteum) may produce several trillion spores, 

 and in other large fungi their number may run well into the billions. 

 The production of spores in such great numbers is advantageous, since 

 generally only a single spore in many millions falls in a place where it 

 can develop into a plant. 



Fig. 1 122. — The spo- 

 rangiumof a mold (Mucor)^ 

 showing the columella (c) 

 and numerous spores (s); 

 highly magnified. — From 

 Coulter (Part I). 



' Basidiospores, however, though actually external, in the mushrooms are considerably 

 protected by the fruit body on which they develop. 



