PHYCOMYCETES 87 



of infection is possessed both by the conidial form of 

 Erysiphe graminis, and the spores of its ascigerous con- 

 dition. 



Up to the present, biologic forms have been mostly noted 

 in fungi belonging to the Erysiphaceae or powdery mildews, 

 but judging from the numerous known cases, where the 

 same species of fungus belonging to other orders is known to 

 be parasitic upon only one kind of host-plant, it is highly pro- 

 bable that in the near future the presence of biologic forms 

 of species will be shown to exist in all orders of fungi includ- 

 ing parasitic species. 



From an economic standpoint, it is perhaps difficult to 

 decide as to whether the evolution of biologic species is a bless- 

 ing or a curse. If half a dozen kinds of wild grasses growing 

 round the borders of a wheat field are infected with Erysiphe 

 graminis, it is reassuring to know that the spores diffused 

 from one or all of these grasses cannot infect a wheat plant. 

 On the other hand, if a single stray wheat plant, infected 

 with the fungus, happens to be growing near the wheat field, 

 infection occurs, and spreads like wild-fire over the field, the 

 rate of infection assuming the proportion of an epidemic, 

 on account of the special infective power of the biologic 

 form over one kind of host-plant only. 



Marchal, Comp. Rend., 135, p. 210 (1902); 136, p. 1280 



(i93)- 



Neger, Flora, 90, p. 221 (1902). 



Salmon, in Massee's Text-Book of Fungi, p. 146 (1906). 



Salmon, Phil. Trans., 197, p. 107 (1904). 



PHYCOMYCETES 



The present order includes the most primitive types of 

 fungi, many of which retain the aquatic habit of the algae 

 from which they evolved, as the Saprolegniaceae. Other 

 groups, as the Peronosporaceae, which have in part become 

 modified and adapted to an aerial mode of life, still have 

 their reproductive organs produced in the form of zoo- 

 spores or motile bodies, which require the presence of 

 water to enable them to move from one place to another, 

 hence the infection of new hosts depends on the presence 

 of water. In other members of the group the conidia are 



