118 STUDY OF COMMOISI PLANTS. 



In general, parasitic fungi follow their hosts in their 



various habitats and changes of station, and accordingly 



many species have a wide geographical distribu- 

 Distribution, , . , , n , n , 



tion, which may be extended and otherwise 



altered by the natural or artificial extension or restriction 

 of the area occupied by their host. To cite a well-known 

 case, the grape-vine mildew has gone with American grape- 

 vines to Europe, where it has been even more destructive 

 than at home. On the other hand, the extirpation of any 

 species that serves as the only host of a fungus naturally 

 involves that of the parasite. This principle is of course 

 taken into account in the use of preventive measures aim- 

 ing at the restriction of parasitic diseases. 



While so different in appearance and habits from all 



other groups of plants, the simpler forms of fungi exhibit 



in their reproductive processes a close resem- 



Rolationsliipi ,, , , • ^ ^ 'jj^j^ii. 



blance to certain algse, so close, in tact, that a 

 classification based upon this correspondence long held 

 sway in botanical treatises. Thus the production of zygo- 

 spores by essentially the same process in the brown moulds 

 and in Spirogyra and its allies was held to be sufficient 

 reason for bringing both of these groups together into one 

 class (Zygosporeae), and other great classes were formed 

 in the same way. At present the general relationship of 

 the fungi and algae is duly recognized, but the attention of 

 special students is directed more to the difficult problem 

 of their relationships among themselves, — a problem as 

 yet only partially cleared up. 



Physiologically, certain fungi and algae exhibit a curious 

 and interesting relation known as symbiosis. The fungi 

 Symbiosis, are attracted by particular algse to which they 

 Lichens, attach themselves as parasites. After their 

 union both parasite and host continue their growth, and 



