ONION SMUT FUNGUS. 125 



tents or ■ — depending on the stage at which one finds them — may con- 

 tain protoplasm and bright red nuclei scattered singly or in pairs. The 

 hyphal tangle may be confined to the lower (inner) part of the cell, and 

 is always more dense there (Fig. 4, C). Its windings are difficult to follow. 

 These intracellular windings stain red with the triple stain. There is a 

 marked contrast between the large, swollen winding intracellular hyphaj 

 and the trim, slender, straight intercellular hyphse between the cells be- 

 low, which stain violet and are of only about one-half the diameter of the 

 former. CFsually the tangle is confined to one epidermal cell, but some- 

 times the adjacent cells may be invaded (Fig. 4, B). The attacked epi- 

 dermal cells do not collapse, and, in fact, appear practically normal. 

 Hyphse pass dowai from the tangle through the inner wall of the epidermis 

 into the intercellular spaces immediately beneath. 



Multiple Infection . 

 The same plant maj^ suffer from a number of infections. In one plant 

 fixed eight days after planting, the mycelium was found passing in thraugh 

 the epidermis at six points on a piece of the cotyledon less than a centi- 

 meter in length. In young stages it is not difficult to trace each mycelium 

 to its limits between the cells, and in this case no one of the six had come 

 into contact with another. It is not unusual to find seedlings which show 

 five or six sori on the same cotyledon. Microscopic examination indicates 

 that these are not the results of a single infection, but that for each sorus 

 there is at least one infection thread which penetrated the epidermis from 

 the outside. This statement, however, does not apply to the sori which 

 appear later on the true leaves. 



V. Incubation Period. 



The incubation period is the time which elapses between infection and 

 the first externally visible symptom of disease. Since the fii'st external 

 symptoms appear at approximately the same time that the spores are 

 forming, we may say that the incubation period is that segment of the life 

 cycle between infection and sporo genesis. In the greenhouse the first 

 symptom, a slight curving and thickening of the cotyledon, has been ob- 

 served here on the tenth day. Since, as previously stated, infection may 

 take place as early as the fifth day, we may consider that this period occu- 

 pies a space of about five days under favorable conditions in the green- 

 house. It may be longer outside, but, at most, is a comparatively short 

 period. During this period the parasite grows rapidly, spreads inside the 

 host and prepares to form spores. 



Young Hypluv in the Intercellular Spaces. 

 After passing through the epidermis the hyphaj are intercellular during 

 the remainder of their development. Just below the inner epidermal wall 

 they^spread in all directions. They are long, slender, and, as they pass 



