CYLINDROSPORIUM ON STONE FRUITS 
149 
serotina, and P. mahaleb, has never been observed to turn black and 
has usually a creamy white and more floccose appearance. 
Relation between Fungus and Host 
Interest in the physiological relation between the fungus and the 
host tissue was aroused by the observation, before mentioned, that 
the spots containing Cylindrosporium are deciduous in some species 
while in others they are persistent. Even in the same species there 
is marked variation in this respect. This variation is very striking 
in P. virginiana where ragged remnants of leaves, from which dozens 
of spots have dropped out, may later be abundantly infected with 
apparently little injury to the leaf tissue except the killing of a few 
cells in immediate contact with the acervulus. Because of this marked 
variation as host, P. virginiana was used chiefly in this part of the 
investigation which was undertaken in the hope of finding some 
explanation for this phenomenon. 
The first problem was to find, by a histological study, what occurs 
just before and at the time of the dropping of the spots. For this 
purpose a series of spots were cut out so as to include some of the sur- 
rounding healthy tissue. These were killed, embedded in parafiin, 
and sectioned. The series began with the first sign of infection (a 
slight yellowing of the leaves) and included all visible changes until 
after the spots had fallen. After sectioning, several stains were tried. 
Durand's (9) method for differentiating mycelium of parasitic fungi 
worked very well, and was used where it was desired to see the position 
and extent of the mycelium. 
The mycelium is intercellular, with haustoria which penetrate the 
host cells (figs. 17, 18). The haustorium enters through a very small 
hole in the cell wall and is very much attenuated as it enters, but the 
end enlarges into an oval or elliptical body which contains a nucleus 
and a comparatively large vacuole. After the haustorium has entered, 
the protoplasm of the invaded cell often deposits a cellulose sheath 
around the haustorium, apparently similar to that formed around the 
haustoria of the Erysiphaceae as described by Smith (25) . This sheath 
often extends along the wall of the host cell for some distance also. 
The host cells are not killed at first except those in contact with 
the stroma; and their death is probably brought about by drying 
rather than as a result of any toxic secretion from the fungus proto- 
