50 James G. Horsfall 



of the substomatal chambers pass but little C0 2 and 2 ; water for the 

 production of carbohydrates is scarce; and the process of changing sugars 

 to starch tends to reverse itself. The ultimate effect can be nothing but 

 an undersized, scrawny plant. 



The results of this experiment support this transpiration theory, because 

 it was noted several times during the course of the experiment, which ran 

 during hot weather, that the mildewed plants wilted very soon if the 

 gardener was a trifle lax with the watering hose. It appears that they 

 lost water more rapidly than the unmildewed plants. 



Conidia produced in the primary cycle constitute the inoculum for the 

 secondary cycles which follow each other at less than two-weeks intervals, 

 because the time from inocu'ation to spore production and further 

 inoculation usually is not longer than about ten days under summer 

 conditions. 



Foex (1924) describes the formation of conidia. He says that the 

 vegetative hypha swells just below a nucleus; the nucleus divides and a 

 septum is laid down. The upper nucleus divides again, and a septum 

 separating these two daughter nuclei is laid down, the upper cell becoming 

 a conidium. The lower cell becomes a spore mother cell from which 

 conidia are cut off in succession. The conidiophores of this species are 

 commonly said to be single-spored, but Salmon (1903:265) points out that 

 chains of from 4 to 8 spores may occur. These chains usually escape 

 detection because the spores separate immediately upon mounting in 

 water. 



The factors governing the formation of perithecia would be of interest 

 because of a possible explanation of their absence in the East. The. 

 opinion is commonly held as a result of Kleb's work (1900) that starvation, 

 or a lack of nutrients, stimulates reproduction in fungi. Klika (1922:75) 

 maintains that, " Die Entwicklung der Perithezien wird entweder durch 

 das Sinken oder ein Schwanken des Substratnahrwertes verursacht." 

 Possibly the report from the State of Washington (Haskell, 1926 : 369) that 

 perithecia of clover powdery mildew were collected on alsike and white 

 clovers is merely another indication of the same tendency, especially if 

 Blumer's (1925) contention is true that these clovers are resistant to 

 the trifolii-pratense form and do not harbor separate forms themselves. 

 Fromme and Wingard (1921:392) found that the uredinial generation was 

 frequently elided in varieties of beans resistant to rust. The telial sori 

 appeared instead. 



Epiphytology 



As has been indicated under inoculation in the primary cycle, the cool 

 nights and warm days of middle and late summer seem to have a marked 

 effect upon the spread of mildew. Wind plays a significant role in the 



