THE AMERICAN GRAPE MILDEW. 137 



The rei:)lies which have been received have in many cases been prompt and 

 full, and in the treatment of tiie subject the language of the writers will be 

 frc(|ucnt]y used. 



Wliile tlie answers to tlio questions were coming in, some time was si)ent iu 

 looking u]) the literature of the grape disease. Frequent mention of it was 

 found far back in the old volumes of the New England Farmer, ]Iorticulturist, 

 and other agricultural and horticultural journals, but the descriptions were usu- 

 ally so incomplete and unsatisfactory that it is not worth our while to quote 

 passages to prove the fact, — suffice it to say that our grape vines have been the 

 seat of various and often destructive attacks by fungi from early time, and as 

 is perfectly natural, the subject has received considerable attention at the hands 

 of agricultural and horticultural writers, who have mingled much theory in 

 their discussions, and have assigned all imaginable primary causes for their de- 

 velopment, from certain kinds of electricity to bad air. 



Let us first get a clear conception of the Peronospora as it appears to us to the 

 naked eye, after which we will observe it more minutely under t!ie compound 

 microscope. As one passes through a vineyard iu midsummer he may fre- 

 quently notice that tlie grape leaves are more or less covered with yellowish 

 spots : should we take one of these leaves, thus seemingly taking on its au- 

 tumnal tints, and turn it over, we will find that the under surface of just those 

 yellowish portions are covered with a fine white forest growth of a shiny, 

 frosty appearance. Here on the underside of the leaf we then find the exter- 

 nal evidences of the cause of the discoloration which we saw upon the upper 

 side. Figure a in the plate represents a leaf of VUis cestivalis, showing the un- 

 der side with affected portions darkened. 



Those species of grapes, as for example V. cordifolia and the one figured, 

 which have a smooth surface to the leaves, exhibit the mildew much more favor- 

 ably than those like V. lahrusca with a downy covering. Our mildew, then, 

 grows in frosty spots upon the underside of the leaves, causing a discoloration 

 noticeable upon the upper side. Farther than this we cannot go without the 

 microscope. 



When one makes a thin cross section through a leaf at one of these diseased 

 places and puts it under a moderately high power of the microscope, something 

 like what is figured in h and c is observed. The section of the grape leaf is 

 shown in 1). (It must be kept in mind that the underside of the section is for 

 sake of convenience placed uppermost in the drawing.) 



Below them are two rows of cells placed close together, forming the dense upper 

 portion of the leaf, followed by a mass of loosely arranged cells which contain 

 the green coloring matter, and do the Avork of assimilation for the plant. 



Kunning irregularly between these cells are shown the threads (mycelium) of 

 the fungus much enlarged that they may be the better seen. Instead of grow- 

 ing directly through the cells they press their way between them, and at inter- 

 vals put out little projections called liaustoria or "suckers," which penetrate 

 the wall of the cells and rob them of their contents. At e is shown a highly 

 magnified view of a few cells with a portion of a thread of the peronospora 

 between them, and having the liaustoria passing into the interior of the cells. 



When the leaf is first attacked it is only these threads in the interior of the 

 cells that can be found, but soon they turn towards the under side anxi finding 

 the breathing pores (stomata), they pass out sometimes to the number of a 

 dozen from each stoma. After this they begin a series of branchings, and finally 

 each assumes the form of a miniature tree, one of which is represented in c. It 



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