Sydney G. Paine and Emily M. Berridge 25 



are evidently restricted by cork formation and production of wound 

 gum, and the ultimate death of the organism is the result of desiccation 

 within the area enclosed by the cork. 



Microscopical examination of the tissues. 



The fact that the gummy material present in the spots is softened 

 and slowly dissolved by chromic acid has rendered it possible to get fair 

 microtome sections varying from 4-G/x in thickness after the use of 

 chromic acid fixatives. The most satisfactory stain for the bacteria in 

 the tissues was found to be carbol fuchsin followed by light-green in 

 clove oil. 



Sections through an artificially produced spot seven days after in- 

 fection show masses of a small diplo-bacillus with thick capsule lining 

 the cavity produced by the infecting needle and extending into the 

 neighbouring cells and intercellular spaces. The shrivelled cells sur- 

 rounding the cavity contain clear yellow gum, those beyond a paler 

 vacuolate gum, mostly crammed with bacteria, but in certain cases 

 quite clear and structureless. The view that the granular appearance of 

 the gum in many cells is due to the inclusion of bacteria is confirmed by 

 these sections, and also by the fact that the gum exuded by the dead 

 leaves contains the diplo-bacillus, but in a smaller form than that ob- 

 tained from an agar culture. The strong affinity of this granular gum 

 for bacterial stains such as carbol fuchsin is thus accounted for. 



The infected spots are sometimes, but not always, cut off by a definite 

 layer of cork; multiplication of the host cells has usually occurred and 

 their walls do not give the cellulose reaction with iodine and sulphuric 

 acid ; they and the gum in the intercellular spaces stain red with phloro- 

 glucin, possibly owing to the presence of wound gum. 



The clear structureless gum which accumulates in the cells bordering 

 the spot, mainly in the palisade parenchyma, before the plastids and 

 cell contents are disorganised, is coloured rose-red in fresh hand sections, 

 and on treatment with normal caustic soda turns a bright chlorophyll 

 green. In microtome sections where it is pale and colourless it stains 

 green with chlorophyll extract and yellow with potash, which seems to 

 indicate that it contains something allied to suberin. 



These observations show that the host plant has considerable powers 

 of resisting the invasion of the parasite by producing cork, the strict 

 locahsation of the disease being thereby accounted for. 



