PHYCOMYCETES 



243 



has adapted itself in some respects to aerial conditions, yet 

 its only reproductive bodies known consist of the aquatic 

 type of motile spores or zoospores. So long as fungi 

 remained aquatic, motile spores were a decided advantage ; 

 their power of spontaneous movement enabled them to 

 come in contact with new hosts. On dry land the advan- 

 tage of spontaneous movement is not obvious, in fact it 

 retards the diffusion of the spores, and infection can only 

 be secured when the host-plant is covered with moisture. 

 To compensate for this shortcoming, a vegetative method 

 of continuing the species has been evolved. When a 

 potato plant is infected, some of the mycelium of the 

 fungus passes into the tubers \ there it remains in a resting 

 condition, its presence being indicated by rusty patches 

 in the flesh. When a tuber thus infected is planted, 

 the mycelium grows up along with the stems of the 

 potato, and finally passes into the leaves, where, if 

 climatic conditions are favourable, it produces fruit. Some 

 of the mycelium again passes into the young tubers. By 

 means of this vegetative method of reproduction, the dis- 

 ease can pass from one generation of potatoes to another 

 without the intervention of spores. Many other kinds of 

 fungi also depend much on the presence of persistent 

 mycelium in the tissues of the host-plant for their 

 periodical appearance ; in fact some species, as the fungus 

 present in the grain of Lolium temulentum^ have so per- 

 fected this vegetative mode of reproduction, ' that the 

 production of spores has been entirely arrested. 



Future investigation will probably show that the presence 

 of hybernating mycelium has much more to do with the 

 occurrence of fungus epidemics than is at present suspected. 



Distribution cosmopolitan. 



