628 Journal of Agricultural Research voi. xv, No. 12 



are multinucleate ; in C. ribicola they are apparently always uninucleate 

 (PI, 53). In the latter fungus no case of more than one cell in a haus- 

 torium has been observed by the writer, although as many as three 

 tips are not uncommonly seen. It is difficult to determine whether the 

 haustorium is always cut off from the hypha from which it is originated 

 and this point will have to be left undecided. 



When properly stained with any of the combinations given on page 623, 

 a sheath can be made out at the base of each haustorium (PI. 58, C, D), 

 enveloping it for a distance of several microns, as if the haustorium were 

 set in a cuplike holder. The sheath' really extends all over the hausto- 

 rium, but is generally very thin in the middle region. At the tip it is 

 as thick or thicker than at the base (PI. 58, E). In its staining reaction 

 the sheath resembles the v/all of the host cell. There is at least a possi- 

 bility that it is formed by the host cytoplasm in response to the irritation 

 or stimulus caused by the presence of the haustorium. Olive {42) states 

 that the plasma membrane of the host protoplasm is pushed in by the 

 haustorium of Botryorhiza hippocratea^as it invades the host cell. Ap- 

 parently the cytoplasm shrinks away; from the advancing haustorium in 

 some cases, leaving an appreciable space. This phenomenon has not 

 been observed in Cronartium ribicola, but it is certain that the plasma 

 membrane of the host cell is not broken nor pierced by the haustorium ; 

 It must be pushed in as the tip of the haustorium grows. The greatest 

 points of irritation produced by the haustorium in the host cell would be 

 the point in the cell wall through which the haustorium entered and the 

 point of contact of the advancing haustorial tip with the host cytoplasm. 

 Consequently it might be assumed that the greatest results from the 

 irritation would be observed at these two points — namely, at the base 

 of the haustorium and at its tip. At these two points the sheath is 

 thicker than at others. Possibly the narrow space which must occur 

 between the haustorium and the host cytoplasm membrane is the dump- 

 ing ground for precipitation products resulting from the irritation. 

 These products might constitute the beginning of the sheath which 

 would gradually increase in thickness as the age of the haustorium in- 

 creased. 



The sheath certainly does not come into being at the time the hausto- 

 rium penetrates the wall, as Smith {53) has reported for the haustorial 

 sheaths of the Erysipheae. It has not been found in connection with 

 young haustoria in any case and seems to be an accompaniment of ma- 

 turity or old age. The hole in the host cell wall, through which the 

 young haustorium passes, is comparatively small (PI. 58, C, D) and at 

 first the wall is no thicker at this point than at any other. Whatever 

 thickening takes place at the point of penetration occurs after the hausto- 

 rium has entered the cell. 



The young haustorium is full of cytoplasm, with a typical round 

 nucleus (PI. 58, B). As the wall of the haustorium, which is at first 



