THE AMETIICAX GRAPE MILDEW. 139" 



They urc called sexual spores because it is necessary that the contents of two 

 (litTerent threads should mingle in order that one of these spores may arise. 



In essence this union of tlie contents of two cells is all that there is in the 

 sexual reproduction in flowcrino,- plants, though in another and perliaps mora 

 evident manner. There is involved in the production of a seed, the male ele- 

 ment represented by the pollen grain, and the female part called the embryonal 

 vesicle. Here we have tlic male re])rcsented by the tip of one filament and the 

 female by the enlarged extremity of another, and after union has taken place 

 the latter develops into an oospore, — one of which is represented highly magni- 

 fied at/. 



These spores are larger than the aerial ones, and are well provided with a 

 thick covering of cellulose. As they are embedded in the tissue of the grape 

 leaf they can serve no purpose in the immediate spreading of the disease. In 

 fact they are not usually formed until late in the season, and their special 

 office undoubtedly is to carry the mildew over the severe months of winter. In 

 the spring they germinate "by zoospores similar to what has been said on this 

 point with the asexual spores. These little motile bodies finding their way to 

 the fresh leaves of the grape vine cause a repetition of the trouble of the pre- 

 vious years and of the circumstances which we have just described. 



There is another grape mildew which is sometimes confounded with the one 

 in question, though they have little in common except being both fungi and 

 growing on the grape. It is closely related to the Oidium Tuckeri, which has 

 proved so disastrous at different times to the vineyards of Europe and Madeira, 

 Botanically it is Uncimila spiralis, and we Avill call it Oidium wlien occasion de- 

 mands its further mention in the treatment of this paper. The Oidium, unlike 

 the Pcronospora, is a surface grower entirely, never sending its threads down 

 through the tissue of the leaf, and besides is not confined to the under side. 

 Eunning here and there over the surface of the leaves, young stems, and berries, 

 it gives, with the aid of a hand lens, a tine cobwebby appearance to the surface, 

 and after it has formed its multitude of spores the parts affected look to the 

 naked eye as if they had been dusted with flour. It never has those well de- 

 fined frosty spores characteristic of the Peronospora, and usually develops itself 

 earlier in the season, and is more deliberate in its progress. Its sexual spores 

 are superficial, and when ripe are enclosed in a thick brown covering of suffi- 

 cient size to be seen with the naked eye. 



The fact that the Oidium often attacks the young grapes makes this mildew 

 quite destructive. As is often the case, the two kinds grow well together, wiiich 

 fact makes it very difficult to decide how much injury is to be attributed to each 

 one. 



The Uncinula is treated by Dr. Farlow in Part II. of Vol. II. of the Bulletin 

 of the Bussey Institution, wliere a full page plate of this mildew may be found. 



As regards the time of the appearance of the Peronospora, the literature, 

 letters and personal experience on the subject lead to the conclusion that it de- 

 pends very largely upon the nature of the weather, in general appearing at any 

 time from thefirst of June to tlie last of September. A series of warm showers 

 alternating with bright sunshine is especially favorable for the development of 

 this fungus, as of most of the other species of the vast group. This year I 

 found it in abundance after the warm rainy weather of the middle of June. It 

 is evident that the variety of grape may determine to some extent the time of its 

 advent. To this conclusion the following observation points : when a number 

 of seedling vines of F. cestivalis in the nurseries of the Arnold Arboretum were^ 



