370 



BACTERIAL DISEASES OF PLANTS 



Fig. 285.— Blight- 

 ing pear petiole. A 

 detail from Fig. 280 

 at A', showing more 

 distinctly the stomatal 

 bacterial exudate (Ba- 

 cillus amylovor u .s ) 

 from the blackened 

 petiole. On the right 

 side at X the tissue is 

 still green and turgid. 



cultures of 1905 — not those of 1915; although 

 both were from apple). On thin-sown +15 

 peptone-beef agar plates, held at 20°C. the 

 homogeneous wet-shining surface colonies 

 may reach a diameter of 5 or 6 mm. by the 

 end of the sixth day (Fig. 292). Their ap- 

 pearance when viewed by reflected and ob- 

 lique transmitted light is shown, on Fig. 

 293. In tubes of nutrient agar there is a 

 good growth the whole length of the stab. 

 On gelatin plates the colonies are circular 

 and inclined to pile up rather than to spread 

 widely. In some gelatins a rather prompt 

 pit of liquefaction appears around the colony 

 (Fig. 294). The color of inoculated milk 

 remains unchanged but after some days a 

 soft curd settles, and later this is more or 

 less completely digested (Fig. 295.4). If a 

 lab ferment is produced it must be very 

 sensitive to heat (15 minutes at 57°C.). It 

 does not redden litmus milk, but precipitates 

 the casein usually within a few days (4 or 

 5). There is often a partial reduction of the 

 litmus, the medium being bluer than the 

 check at the top, paler blue at the bottom 

 (in the curd) and wholly reduced in the 

 middle, i.e., changed to a pale brownish 

 white. It grows rapidly in bouillon especi- 

 ally if cane-sugar is added. In beef bouillon 

 or potato broth there is not only a prompt 

 and heavy clouding with presence of pseu- 

 dozoogloeae (turbidity) but if undisturbed 

 a slight granular pellicle forms. Clouds 

 potato juice in closed end of fermentation 

 tubes without gas. Indol reaction scanty 

 or negative. Tolerant of small quantities 

 of malic acid and citric acid but not of lactic 

 acid. I now doubt if malic acid actually 



