EVOLUTION OF PARASITIC FUNGI. 113 



medium, the third crop of spores was developed in a little more 

 than one week. Kow the fungus grows abundantly and quickly 

 as a saprophyte in artificial cultures. 



The nitrifying organism (a bacterium) studied by Frankland 

 presented similar peculiarities in the attempts to grow it in arti- 

 ficial culture media, and there was also a change of form in the 

 adaptation to new culture media. 



Other cases might be cited of like adaptations, and they lend 

 some support to the statements that certain fungus parasites of 

 one region when carried to another sometimes acquire a rapidity 

 of development and adaptation to the new conditions on new 

 varieties of hosts, which makes them more virulent than in tlieir 

 native land and condition. It also suggests that certain species 

 of fungi may pass slowly from feral plants to cultivated ones, 

 and when they become adapted to the new conditions may, from 

 the numbers of the host, and perhaps the lack of certain hin- 

 drances which existed in the feral state, as well as the more 

 artificial physique of the host, become serious pests. 



Fluctuatiox IX Seasonal or Polymorphic Forms. — 

 There is also a fluctuation in seasonal forms, if they may be so 

 called. De Bary notes that Peronospora aJsinearum, one of the 

 downy mildews, in the neighborhood of Strasburg, in the spring 

 develops couidia which are accompanied or immediately followed 

 by the oospores on the same host, Avhile in the autumn only the 

 conidia are developed. A fluctuation of this kind, or in the 

 appearance or omission for a greater or lesser period of certain 

 phases of many of the polymorphic species, may account for the 

 varying notions as to what phases or stages appertain to certain 

 species of fungi. Yet it would not do to admit, without careful 

 investigation, the inclusion of forms which have been seen to 

 accompany well-determined stages in the life history. Perhaps 

 it is not yet definitely established just what forms belong to the 

 black knot fungus of the cherry and plum. 



Periodicity ix Developmext. — Among other tendencies we 

 may inquire whether there is not a tendency in some species or 

 groups of fungi to a periodicity in the development — a period- 

 icity I mean which would require two or more years in the com- 

 pletion of its round. I do not mean a periodicity which requires 

 several years for the development of the vegetative phase of the 

 parasite before the fruiting occurs, but a periodicity similar to 



