MYCOLOGICAL SECTION. 107 



ness, or a moderate amount of humidity only, is sufficient for t-heir 

 germination. Tliey do not require water condensed in the form of 

 drops of rain or dew, as does tlie Downy Mildew, and they germinate 

 hy the immediate production of germ-tubes and not by zoospores. 

 The conidia are tliin-walled, oblong cells, filled with a transparent 

 granuhir matter. One of these bodies falling upon a grape-leaf will, 

 under favorable conditions of temperature and humidity, push forth 

 one or more germ-tubes, which first send haustoria, or suckers, into 

 the epidermal cells and then grow into the thread-like branched my- 

 celial formation (thallus) diffused over the surface. By their multi- 

 tude these threads now become visible to the unaided eye, and we 

 have what has been familiarly termed "Mildew," "■Erysijjhe," "O'ld- 

 ium," &c. 



When the mycelial growth has attained its full development a spore 

 formation of an entirely different character from that above described 

 takes place. Perithecia, or what we may be allowed to term ' ' fruits," 

 are formed, within which spores are produced in a number of little 

 sacs called asci. These ' ' fruits " are especially abundant on the in- 

 vaded organs of the vine during the months of September and Octo- 

 ber. To the naked eye they appear as minute dark-brown or black 

 points, thickly dotting the mildewed surface. (One of these bodies 

 is figured in Plate 11. The appendages which surround the perithe- 

 cium usually stand up at quite an angle to the plane of their attach- 

 ment; they do not lay out flat, as represented in the figure.) 



In their growth the perithecia are at first quite colorless, then pale 

 yellow, and finally very dark brown or black. The appendages are 

 clear and transparent at their extremities, but have a brownish color 

 towards their bases. They are divided into several cells by transverse 

 walls, and are sometimes, though rarely, branched or divided above. 



The asci, developed within the perithecium, are delicately walled, 

 transparent sacs that contain the ascospores or sporidia. These are 

 oblong bodies, rather more rounded in outline than the conidia and 

 somewhat smaller. They are the true Avinter spores of the fungus. 

 Closely incased within the hard, compact walls of the perithecium, 

 they are well protected from injury and the severe weather of winter. . 

 In the spring the walls of the perithecium decay or crack open, allow- 

 ing the sporidia tp escape, and bring about a new infection of the 

 vines. Doubtless a sufficient number of these fungus fruits remain 

 adhering to the vines through the winter to bring about a recurrence 

 of the disease as soon as the conditions favorable to the germination 

 of the sporidia prevail. 



ACTION OF THE POWDERY MILDEW ON THE VINE. 



The Powdery Mildew makes its appearance usually during the 

 early days of June and continues its development late into the autumn. 

 It appears in dull, grayish-white patches, most conspicuous on the 

 upper surface of the leaves, and when growing thickly on the young 

 shoots or berries its mycelium imparts to these organs a similar hue. 

 It never has the bright, lustrous, or frosty apjDearance that charac- 

 terizes the Downy Mildew, and the livid brown or seemingly scorched 

 blotches on the leaves that the latter fungus occasions are wanting, 

 although in thin-leaved varieties of foreign vines a discoloration, 

 takes place through the whole thickness of the leaf, visible at the 

 points below the patches of fungus growth on the surface above. In 

 a few instances I have seen the mycelial growth so dense upon the 



