EXPERIMENT STATIOX BULLETINS. 275 



are the chief sources of injury to the host plant. The mycelium consists 

 of slender, branched, colorless filaments, the parts of which, whether for 

 vegetative purposes or not, are often called the hypJuv. These filaments 

 are narrower than the conidial spores which they bear, varying from .001 

 to .003 of a millimeter in breadth. They push their way between the cells of 

 the interior of the leaf, by contact disorganize the contents of such cells, 

 and absorb their fluids. During this process a red fluid appears in the 

 cells in place of disorganized chlorophyll and protoplasm, — a sure sign of 

 lowered vitality, — and gives the reddish appearance seen from without. 

 The mycelium continues to ramify from the center of the spot, where it has 

 begun its work, attacking and destroying other cells, all those within reach 

 of the denser growth of filaments, shrivelling, drying, and finally filling 

 with air. It is this last change which gives the dead-white appearance to 

 the center of the spot. It will be seen from the figures that the white area 

 is only about one fifth the thickness of the healthy portion of the old leaf, 

 and of course its cells have completely lost their vitality. If these centers 

 of the disease are numerous and coalesce, it will be seen that the whole leaf 

 must succumb and die, an event of frequent occu ranee in seasons favoring 

 the fungus. If a second crop of leaves is put out in August, these may in 

 turn be infected from the diseased ones, and either much injured or 

 destroyed, in which case the plants should be plowed up, as no crop can be 

 expected from them the following year. 



Especial attention was directed to the hibernation of the ordinary myce- 

 lium. Leaves were brought in repeatedly during periods when they were 

 not covered with snow, and placed in a warm, moist chamber. It was found 

 that basidia invaribly developed from the borders of the white spots in less 

 than twenty-four hours and produced conidia exactly like the summer con- 

 idia. This demonstrated the presence of active mycelium about the disease 

 centers. In the early spring it was found that wholly new spots would 

 appear on diseased leaves, in so short a time after they were brought into 

 the laboratory, that no infection could have taken place from newly formed 

 "Conidia. Probably these diseased areas arose from a fresh growth of 

 mycelium that had hibernated in the leaf. The mycelium in the leaf does 

 not descend in the winter through the petiole to the stem and roots, to 

 reascend through the new petioles in the spring. At least in the consider- 

 able number of sections made in the sjiring, no trace of it was discovered 

 in the petioles. 



THE PRODUCTIVE OEGANS. 



The mycelium is densest in growth just beneath the epidermis of the 

 white area or at its borders, and from this region certain branches break 

 through the upper epidermis of the leaf. The ends thus exposed to the 

 air undergo a simple segmentation, the lower cells forming the so-called 

 basidia, the upper being the conidia, the reproductive organs of this stage. 

 As conidia fall new growths and segmentation of the basidia form new 

 conidia, and this continued production may go on throughout the life of 

 the host. The chief development of the liyphae and basidia is from the 

 upper surface rather than through the stomates or breathing pores of the 

 lower epidermis. 



The conidia are oblong or cylindrical, and are .026 to .042 millimeters long 

 by about .0035 mm. broad.* If they fall on or are blown to a fresh leaf- 

 surface and lodge in a little dew or moisture, they send out in a few hours 



