118 TECHNICAL BULLETIN 4. 



3. It enters the soil either as spores or as mycelium from the buried 

 parts of diseased onions. 



4. No typical conidia (sporidia) are produced but it can be widely dis- 

 seminated by the detached mycelial cells which may be carried about by 

 water, wind, rain, tools, animals, workmen, etc. 



5. It probably lives in the soil in this state for years without the pres- 

 ence of onions. 



6. As will be shown later, infection may take place directly from this 

 mycelium, and the presence of spores is not necessary. 



7. The number of years which must elapse before onions can be grown 

 safely on an infested piece of land is not necessarily decided by the lon- 

 gevity of the chlamydospores, but in all probability by the length of time 

 during which the mycelium can continue to live and develop saprophyti- 

 cally without having to pass again through a parasitic stage. 



IV. Infection. 



Very little has been published concerning infection except the bare fact 

 that it occurs at an earh' period in the growth of the plant. Concerning 

 the method and point of entrance, character of inoculum, etc., nothing 

 has been previously ascertained. 



Development of the Onion Seedling. 

 In order to understand the description of infection given below it is 

 necessary that the reader should know something of the stages through 

 which an onion seedling passes during the process of germination. The 

 resting seed consists of a hard, black outer seed coat, a nutritive endosperm, 

 and an embryo. The embryo is coiled like a snail within the endosperm 

 (Fig. 3, A). The larger part of the coil represents the cotyledon; only a 

 short portion of the free end is the radicle. In the lower part of the coty- 

 ledon, just above where it joins the radicle, there is, even at this early 

 stage, a small cavity. A minute bud, lb, arises from the base of and pro- 

 jects into the cavity. This bud is the primordium of the first leaf, and the 

 cavity in this and later stages is called the cotyledonary cavity, cc. Sev- 

 eral layers of elongated cells throughout the length of the center ef the 

 embryo indicate the position which the fibrovascular bundle of the seedling 

 will occupy. Germination begins ^\^t]l rapid elongation of the embrj'o, the 

 radicle and lower part of the cotyledon being thus pushed through the 

 micropyle, a small opening in the seed coat. This elongation is effected 

 both by longitudinal stretching of the cells of the embrj^o and by cell 

 division. Food and water for this acti\ity are absorbed by the upper end 

 of the cotyledon which remains attached in the endosperm. On the third 

 day after planting, the projecting radicle is about 3 to 4 mm. long. The 

 root usually points upward as it emerges, but geotropism soon causes it 

 to turn downward and the cotyledon describes a sharp curve as indicated 

 in Fig. 3, B and C. It will be noticed that the tip of the leaf bud now 



