3C-Q) and by elongation, branching and septation form a dense mycelium without any 

 indication of sporidia. Subsequent to the publication of these results, smutted leaves 

 were placed in damp soil in test tubes where they were kept for three years. At the ex- 

 piration of that time the spores were again tested for germination by the same method 

 as was used in the previously reported tests. The process of germination was now found 

 to be quite different from that which took place during the first three months after matur- 

 ity. Most of the sterile cells had now colJapsed and no longer presented the plump 

 appearance which they have on fresh spores. Germination began within 2-5 days after 

 the spores were sowed on the cold agar plates. The first striking difference was that 

 there was no large globose body developed on the face of the fertile cell, but a single 

 slender tube grew out directly from the central cell (Fig. 3R-U). Occasionally two tubes 

 seemed to come from the same place, but the second one may have been merely a branch 

 of the first arising very close to the surface of the spore. Usually the germ tube remained 

 simple and unbranched until it had reached a length of 20-30 times the diameter of the 

 spore. Then it became septate and branched sparingly. The protoplasm disappeared 

 from the older cells but remained dense in the advancing tip cells. No sporidia were 

 observed at any time even when the further development was examined daily until 

 masses of mycelium a half -inch or more in diameter were formed. All the further devel- 

 opment and behavior of the mycelium was the same as previously described after the 

 germination of the spores according to the first process. The process of germination of 

 these "after-ripened" spores is very similar to that described by Thaxter (44:142) and 

 Whitehead (53) except that both of them found sporidia. The difference in germination 

 between fresh spores and after-ripened spores is susceptible of various interpretations, 

 but none will be atteznpted until after further investigations have been made. 



It is commonly stated in the literature on onion smut that the clilamydospores 

 live for many years — any number up to 25 may be found — in the soil and then germ- 

 inate to infect the seedlings when onions are again planted. Such statements are 

 erroneously based on the observation that smut recurs on a certain piece of land 

 which has not been planted to onions for that particular number of years. It seems 

 more probable that the smut is carried through those years in the form of mj^celium 

 living on the organic matter in the soil. Another possibility is that the spores or 

 other organs of the fungus may ha\ e been washed or carried in some other way from 

 fields where onions had been grown in the interim. During the season of 1923 smut 

 occurred in destructive form on a plot of land on the experiment station farm which, 

 as the records showed, had not been planted to onions in at least 40 years, but this 

 plot was adjacent to another field where onions had been grown during many years. 

 A similar case is reported by Whitehead (52:444). There are numerous ways in 

 which it could be spread from one field to another, e.g., by surface washing, wind, 

 tools, animals, direct spread of the mycelium, workmen, etc. 



From the soil the mycelium enters the very young seedlings by boring directly 

 through the epidermis. Infection may occur only during a very limited period 

 while the epidermis is tender. The exact length of this period was determined by 

 Walker and Jones (50:238) and by Anderson (1:120). Walker and Jones found 

 that "the plants became iimnune . . . between the nineteenth and twentj^-fourth 

 days after sowing, when the cotyledon had about attained its full growth and as the 

 first leaf was emerging." Anderson found that they were no longer susceptible 

 after the 17th day under greenliouse conditions, but that "the period of suscepti- 

 bility is not limited bj^ the number of days diu-ing which the seeds have been in the 

 soil, but by the length of time required for the seedling to pass tlirough certain 

 stages of development" and that "susceptibility begins to diminish from the time 

 that the knees emerge from the ground and little if any infection occurs after the 

 first leaf has emerged from the side of the cotyledon." Thus the conclusions from 

 these two investigations — carried out at the same time, but entirely independently, 

 were identical — that all infection occurs before the first leaf is developed — a period 

 wliich may be rouglily stated as 3 weeks after planting. Walker and Jones very 

 ingeniously demonstrated that immunity after this period depends on the maturity 

 of the tissues of the cotyledon or the successively developed leaves each of which in 

 turn forms a protective sheath about the base of the next developing leaf. 



As soon as the infecting mycelium has passed tlirough the epidermis of the 

 cotyledon, it spreads rapidly in all directions, deri\'ing its food from the cells of the 

 young onion. After growing for a period of 5-10 days the mycelium forms dense 

 knarls between the cells of the host plant and then the knarls are transformed into 

 masses of black spores which have been previously described. It must not be 



9 



