MYCOLOGICAL SECTIOX. 97 



orless, granular, and somewhat oily substance. At frequent inter- 

 vals on these threads, as they push their way between the cells.' minute 

 lateral projections are formed, that penetrate the walls of the adja- 

 cent cells of the host, from which they absorb the nourishment for 

 the support of the parasite. These projections have received the 

 name of '"suckers," or haustoria. The contents of the perforated 

 cells quickly turns brown, ultimately effecting the outward changes 

 in the coloration of the leaf or other affected part. 



Upon this mycelium, at different periods and in very unlike ways, 

 two sorts of reproductive bodies, or spores, are formed; one kind pro- 

 duced externally on short filaments, and named conidia, the other 

 developed by a special sexual process on the raycelium within the 

 tissue of the host plant, and termed oospores. The first are produced 

 in great numbers throughout the summer, and serve for the imme- 

 diate propagation of the fungus, effecting its rapid distribution; the 

 second are formed later, and do not germinate until the following sea- 

 son. The former are often called "summer spores," in distinction 

 from the latter, which have been named the ''winter spores." 



Summer spores. — The summer spores, or, as they are technically 

 called, conidia, are borne upon the ultimate branches of slender fila- 

 ments of the fungus, which issue through the natural openings, the 

 breathing pores of the leaves. Four to five, or even more, of these 

 filaments, called conidiophores, or conidia-bearers, issue from each 

 pore, and through their abundance the fungus becomes visible to the 

 naked eye; the downy white patches of mold, so conspicuous on the 

 under surface of affected leaves, being wholly of this growth. 



A few hours of a single night is all the time required for the de- 

 velopment of the conidiophores and conidia, but the mycelium may 

 exist within the tissues of the leaves or other affected parts a long 

 time before this outward development takes place. The conidiophores 

 only appear under certain favorable atmospheric conditions, and, as 

 these conditions may only occur at intervals of considerable length, 

 "we are in the habit of assuming that a new infection takes place each 

 time. That vines previously free from the mildew may become 

 affected at any time during the summer there is no doubt, but the 

 appearance of the mildew on the leaves may come from mycelium 

 that arose from a much earlier infection. 



The number of conidia that may be produced upon a single invaded 

 vine has been estimated at from two to ten millions. Their great 

 number, coupled with the fact that each one may produce a half a 

 dozen or more new individuals, explains how it is that an entire 

 vineyard may be "struck" with the mildew like the sudden falling 

 of a cloud upon it. 



In shape the conidia are generally ovoid, the smaller end being at 

 the point of attachment, their longest diameter being from tfoo o fo 

 lohso of an inch. They are very thin-walled, and are filled with a 

 colorless, nearly transparent, granular fluid. Their formation takes 

 place with great rapidity, and when mature they are most easily 

 detached from their supports. 



One of these bodies, happening to fall upon a grape-leaf where there 

 is moisture condensed in the form of drops of rain or dew, will germ- 

 inate within a couple of hours, the germination taking place in the 

 following manner: The contents of the spore undergoes a process of 

 division, separating into a number of distinct particles, which very 

 soon escape through an opening made in the spore-wall; they are 

 then free, but exceedingly minute masses of naked protoplasm of 

 7 AG— '86 



