152 DR. A. J. EWART ON THE EVOLUTION OF 
ultra-red ; and according to Engelmann an evolution of oxygen 
is shown in this region, 7. e., when the plants are apparently in 
darkness. 
Of the purple Bacteria the following were found, cultivated, 
and examined: Chromatium  Okenii, Ebr., Chromatium War- 
mingii, Ehr., and Bacterium  photometricum, T. Engelm.* 
These all grow in stagnant water containing a trace of SH, and 
exposed to light. On examining masses of these Bacteria by means 
of Spirilla, a distinct attraction of the latter can be seen to take 
place in light, which ceases in darkness. If the preparations 
have been freshly made, on the contrary, and still contain dis- 
solved oxygen, the red Bacteria may be seen to exercise a 
repulsive effect upon the Spzrilla. By means of Bacterium Termo 
in no case could any evolution of oxygen be detected, though the 
red Bacteria were examined with and without sulphur-grains, 
free, in masses, motile and non-motile, in gelatine preparations, 
and in water. 
For its size, B. photometricum seems to possess a stronger 
power of “evolving oxygen” than Chromatium Okenii; but here 
also no evolution of oxygen could be detected with certainty by 
Bacterium Termo. 
In a culture of Chromatium Okenii it was noticed that in bright 
light the clear surface zone free from colour was deeper than when 
in very weak light. The Bacteria seek out regions where a little 
oxygen is present, and avoid those where it is very abundant. 
Thus, if the cover is removed and the surface exposed freely to 
air, the upper clear zone becomes still broader. These pheno- 
mena were noticed by Engelmann, and are considered by him to 
form a proof of the evolution of oxygen by the purple Bacteria. 
A quite possible explanation of the phenomenon, however, 
that in light the sensibility of the Bacteria to oxygen alters. In 
the presence of oxygen Bacterium photometricum is less sensitive 
to light than in the absence of oxygen. Hence it is also possible 
that exposure to light may increase the sensitivity to oxyge™ 
especially as in the darkness in a few hours the Bacteria come to 
rest, their general sensitivity to oxygen and power of seeking out 
regions where the oxygen percentage is most suitable being lessene 
in weak light and in darkness finally ceasing. Both causes may 
* The B. photometricum showed all the physiological peculiarities with 
regard to light ascribed to it by T. W. Engelmann, in Pílüger's Archiv 
f. d. ges, Physiol. Bd. xxx. p. 95. 
