Ward. — Thames Bacteria , III. 
245 
Potato. In twenty-four hours, at 20° C., forms a deep golden-chrome 
spot. On the third day the colour is still richer, the surface waxy, but 
rather dull, growing slowly. In seven days a thick waxy mass, red- 
orange, especially in the centre, surface rather dull, margins striated 
(Fig. 60). 
Broth. Slight turbidity in twenty hours, at 20° C., and this increases 
during the second and third days. On the fourth day a yellow deposit 
is formed, but no veil above. In ten days or so the turbidity diminishes 
and a copious yellow deposit has formed below. 
Milk. Slowly peptonized, without previous coagulation. 
Requirements as to air. Aerobic. 
Temperature. Grows slowly at io° C., fairly rapidly at 15 0 C., more 
so at 20 0 C. 
Liquefaction. Sometimes hardly, or even not at all observable. In 
other cases fairly rapid. 
B. arbor escens shows obvious resemblances both to No. 23 
and to No. 17, and the forms allied to them, and there can be 
little doubt as to the propriety of placing it here. But the 
principal interest attaching to B. arborescens is the experi- 
mental proof of the extreme variability of the colonies 
according to conditions. 
When the temperature is very low, or the Bacillus has been 
exposed to unfavourable conditions for some time, e. g. 
exposed to light at 10-12°, or in very old gelatine-tubes, its 
feeble growth is expressed by slowly emerging as transparent, 
contoured, thin plates, like a feeble colony of B. coli-commune , 
only that one can still recognize the dense spider-like or 
star-fish-like zoogloea in the depth (Fig. 62). The fine 
contours are due to the much curved parallel filaments on the 
surface of the gelatine (cf. Fig. 11): these lie closely side by 
side and remain so because no liquefaction occurs, and the 
segments are therefore held in place flat on the surface of the 
gelatine. 
When the plates are exposed to sunlight for three or four 
hours, at ordinary temperatures, taking care that the gelatine 
does not melt ; or if the sowings are made from tubes of water 
exposed to the sun ; the type of colony produced is more like 
