702 



Journal of Agricultural Research 



Vol. XX, No. 9 



attacked growing scales which were being parasitized by the smut 

 fungus, Urocystis cepulae, but apparently a weakening of the plant 

 is necessary before actual invasion of the growing parts occurs. Fol- 

 lowing harvest there is a gradual invasion of the dormant cells of the 

 fleshy scales of the bulb as previously described. The progress here 

 is usually slow, but in a moist, warm environment there may be a more 

 rapid invasion, resulting in decay of the resting central bud of the onion 

 set. 



RELATION OF THE CAUSAL ORGANISM TO THE HOST TISSUE 



METHODS 



Onion bulbs from which the thin outer scales had been removed 

 were placed in moist chambers. Inoculum consisting of a suspension 

 of spores from pure culture in sterile distilled water was applied to the 

 uninjured surface of the exposed scales, either in drops by means of a 

 platinum loop or as a spray from an atomizer. 



For the stud)' of penetration a razor section was cut tangentially 

 from the surface of the scale directly beneath the infection drop so as 



Fig. 6. — Colletotrichum circinans: Stage of penetration of epidermal cell of onion scale at 66 hours after 

 inoculation. Camera-lucida sketch. Approximately X 430. 



to contain the epidermis with a few layers of the immediately underlying 

 cells. This was examined directly in toto in a water mount, the absence 

 of chlorophyll in the host cells making clearing and staining unneces- 

 sary. For the study of the relation of the fungus to the host tissue 

 following penetration, pieces of inoculated scale as well as of naturally 

 infected fleshy scales were fixed in Fleming's medium fixative, washed, 

 dehydrated, embedded in paraffin, and sectioned according to standard 

 methods of procedure. In some material a satisfactory differentiation 

 of fungus and host was secured by omitting the bleaching of the micro- 

 tome sections (commonly done after using a fixative containing osmic 

 acid), which left the mycelium black, and then counterstaining the 

 host cell walls with orange G. In other cases the iron haematoxylin 

 and Delafield's haematoxylin stains gave satisfactory results. 



PENETRATION 



Under optimum conditions germination occurs within 10 hours and 

 appressoria are formed, either sessile or at the end of short germ tubes. 

 Usually the appressorium is flattened to some extent on the side adja- 



