34 



During the summer and early fall the asexual reproduction takes 

 place by means of conidia. They are cylindrical or ovate colorless cells, 

 filled with protoplasm, and ijorne on the ends of special branches, — the 

 conidiophores — wldch are erect, colorless or white and possess one to sev- 

 eral septa or cross walls. The conidia are formed from the conidiophores 

 by constriction from the ends, and are freqiuently found in chains, where 

 several from the end of the same hypha have fallen off together. During 

 the grov^ing season large numbers of conidia are produced. These are 

 carried about very easily by the wind and germinate rapidly on the host 

 plant by sending out a tube. Under proper conditions this soon develops 

 intt a new mycelium. Salmon says "Even in a dry atmosphere, but more 

 rapidly in a damp one, or in water, oni^. or more delicate germinal tubes 

 are produced near one end of the conidium." By means of the conidia, 

 then, the mildews are rapidly spread. 



In autumn u^hen the vitality of the plant is low, the perithecia or 

 true fruiting bodies are produced. According to DeBary these represent a 

 sexual stage. They are usually globose in shape and contain resting 

 spores v^^hich carry the fungus over winter. The spores are colorless, gran- 

 ular and oval, and are borne in colorless, sessile or stalked sacs kno^vn 

 as asci. The number of asci varies in the d|ifferent genera. Salmon states 

 that they may be from 1-66 and the number of ascospores m.ay vary 

 from 2-8. These ascospores require to pass a winter before they will 

 germinate. 



When tlie perithecia are about half-grown, appendages begin to 

 grow out from the outer Avail. These appendages vary in form and Avith 

 the number of asci are the characters on which the genera are based. In 

 the spring the -wall of the perithecium opens by an irregular rupture and 

 the asci are liberated. 



This family is parasitized to a great extent in Europe, but it is not 

 common in America. 



RELATIONSHIP TO HOST— SPECIAIvIZED FORMS OR RACES. 



It has for a long time been an unsettled question in the minds of bot- 

 anists whether the powder}^ mildews have the capacity for infect- 

 ing host plants other than the one from which they came. In spite 

 of the fact that some of the species are found on miany host plants of 

 different families, e. g., E. polygoni, E. cichoracearum, yet many of 

 them are limited in their choice of host plants to certain families, 

 e. g., A. salicis on salicaceae, S. pannosa on rosaceae, E. graminis on 

 gramineae. Instances are also known where the hosts of the 



fungus are limited to one genus. 



From the work that has been done by Reed and Salmon there would 

 appear to be several specialized f orins or races of ,; E. graminis. This 



species is known to attack the family Gramineae, including barley, oats, 



