128 INTERNATIONAL CONGRESS OF VITICULTURE 



above or below the cell, as being intracellular, is obviated. In the writer's 

 opinion the mycelium is always intercellular. 



The diameter of the hyphae varies considerably with the conditions of 

 temperature and moisture and with the character of the tissue in which it 

 is located. Where the germ-tube and the conidiophore pass through the 

 stomate it is about I/JL in diameter (PI. I, fig. 1). In certain cases the 

 diameter of the mycelium may be as much as 40 to 60/* (PI. I, fig. 5). In 

 the canes where the tissues are firm and the cell walls relatively thick, the 

 hyphae are smaller and of more uniform diameter. In the leaves the hyphae 

 tend to fill the large, irregular, intercellular spaces and hence are of greater 

 diameter. In general it may be said that in young tissue the mycelium is 

 of greater diameter and more abundant than in the more mature parts. 

 Indeed, in the very old, diseased areas, particularly after the period of 

 oospore formation, the mycelium seems to disappear almost entirely. 



The younger parts of the mycelium are filled with a dense, finely 

 granular protoplasm which, with age, becomes coarser and profusely vacuo- 

 lated. When carefully stained the protoplasm exhibits a netted structure 

 seemingly made up of numerous fine strands. At irregular intervals upon 

 these strands occur deeper staining granules whose exact nature is not 

 evident. 



The mycelial wall is very thin and according to Mangin (1890), is com- 

 posed of a mixture of cellulose and callose. The action of various stains may 

 serve as an indication of its nature. When untreated mycelium is stained 

 with chloriodide of zinc there appears after a short time a brown color. If 

 previously boiled in potassium hydroxide the mycelium becomes a deep 

 reddish purple with this stain. This is in accordance with Mangin's state- 

 ment to the effect that boiling potassium hydroxide will dissolve the mask- 

 ing callose leaving the pure cellulose. 



The nuclei are commonly globose but in certain places they may be 

 oval or at times almost linear. They are especially abundant in the younger 

 portions of the mycelium. Istvanffi states that they divide karyokinetically. 



The haustoria are produced very profusely and occur in all parts of the 

 plant in which the mycelium is to be found. In the leaves of the resistant 

 varieties the haustoria are similar in every respect to those found in the 

 more susceptible vines except that they are usually smaller and are never 

 so abundant. 



The method of haustorial formation is similar to that described by 

 Grant Smith (1900) for the Erysiphae. At first a very small tube is produced 

 laterally from the mycelium into the host cell-wall which becomes greatly 

 thickened at this point. The exact cause of this thickening is not apparent. 

 It would seem that, if it is caused by some enzyme producing gelatinization 

 of the wall, as is suggested by Cuboni (1889), the adjacent portions of the 

 wall would also swell but this is not the case. The other possible view is 

 that it is an indication of an attempt of the host to exclude the haustorium 

 hence is the result of additional depositions at this point by the. protoplasm 

 of the irritated cell. Be this as it may, the haustorium continues to grow 

 inward still enclosed in its sheath of host wall. After reaching a variable 

 length the end of the tube gradually swells into a globose sac accompanied 

 to a certain point by the swelling of its sheath (PI. II, fig. 1). After attain- 

 ing about one-half of its mature size the terminal portion bursts or dis- 



