io8 Journal of Agricultural Research voi. xix, N0.3 



of the tassel. Most commonly, however, the conidia appear on the leaves 

 and leaf sheaths, where they occupy principally the conspicuous mottled 

 and discolored areas which have been described. 



On whatever part of the plant they may be found, the conidiophores 

 emerge at night, provided there is present a thin layer of dew, rain, or 

 mist. Damp air alone does not seem to permit their formation. Under 

 favorable conditions the process of conidiophore emergence and conidia 

 production begins about midnight and may continue a few hours after 

 dawn, provided the weather is favorably rainy. When seen at night in 

 the luxuriance of their growth, the innumerable conidiophores projecting 

 slightly from the thin film of moisture on the leaves form a very distinct 

 grayish white down, which is by no means even suggested by the dry, 

 matted fragments which remain when the hot morning sun has dried the 

 surface of the leaves (Pi. 21, A). 



This process of conidiophore development and conidia production has 

 never been described, and, since it shows several points of interest, it will 

 be presented in detail in a subsequent paper. In general, however, it 

 occurs as follows : 



From the stomata of the infected portion one or more club-shaped 

 hyphae grow out. These elongate, and under favorable conditions the 

 paired protrusions finally bud out from their tips and become the stout 

 primary branches. From the tips of these in turn bud out the begin- 

 nings of the secondary, and from these, at length, the tertiary branches, 

 each of which usually terminates in one or two tapering sterigmata. 

 Since the initial protrusions which develop into the branches arise almost 

 invariably in pairs, the structure of the mature conidiophore is character- 

 istically dichotomous, instances of the suppression or delayed formation 

 of a branch being, on the whole, rather rare (PI. 24, D). Finally, from 

 the tip of each sterigma there buds one conidium as a spherical protrusion 

 which enlarges and lengthens until it attains the elongate oval or rounded 

 oblong shape of the mature spore and is separated from the sterigma tip 

 by a cross wall. 



When fully formed the conidiophore appears as in Plate 24, C, and con- 

 sists essentially of a main axis which begins with an elongate basal cell 

 and broadens gradually until it divides into the two to four stout main 

 branches. From each of these extend two to four smaller secondary 

 branches, each of which in turn bears two to four tertiary branches that 

 terminate severally in one or two tapering sterigmata, each bearing at 

 its tip a conidium. Although under favorable conditions the conidio- 

 phores are of the large, well-developed type just described, they fre- 

 quently show such variations in structure as the omission of the second 

 and third series of branches and a general reduction in branches, sterig- 

 mata, and consequently number of conidia (PI. 24, E). On vigorous 

 conidiophores 32 to 96 conidia may be borne, while on poorly developed 

 ones there may be as few as 8 or even 3. In size also there is great varia- 



