480 THIRD PART.—BACTERIA OR SCHIZOMYCETES. 
assimilate carbon dioxide; and this is confirmed by Engelmann’s observation * 
that Bacterium chlorinum gives off a small quantity of oxygen in the sunlight. 
Nägeli’s researches show that the Bacteria which contain no chlorophyll generally 
require the same kind of food as the Moulds which have been examined, and that the 
organic compounds employed as food exhibit the same scale of feeding power in 
the one case as in the other (see section XCVIII); but of inorganic nitrogenous com- 
pounds nitric acid serves as food for the Bacteria, though in a less degree than ammonia. 
Bodies which are contained in solution in the substratum but are not nutrient 
substances may greatly influence the vegetation of the Bacteria. It was observed 
above (p. 354) that an acid reaction in the nutrient solution is as a rule unfavour- 
able to the development of the Bacteria. Some species however can put up 
with this state of things to a certain point if they are by themselves in the 
solution. But their vegetation may be entirely stopped by it, and so Moulds or 
sprouting Fungi present with them in the fluid may thrive on thc acid and overpower 
the Bacteria. Other bodies, which are products of the decomposition of the sub- 
stratum caused by the Bacteria and pass into the solution, may also impede the vege- 
tation as soon as they acquire a certain degree of concentration in the solution. 
According to Fitz’s observation in 1882, the vegetation of Bacillus butylicus in 
glycerine solution under otherwise optimum conditions was stopped by 2.7-3.3 per 
cent. of ethyl alcohol, by 0.9-1.05 of butyl alcohol and by 0.05-0.1 of butyric acid, &c. 
It should be observed here that bodies which serve as food, and in the case of 
the aerobiotic forms the oxygen also, act as stimulants on the Bacteria, awakening or 
accelerating their powers of movement and determining its direction. Engelmann? 
has shown that sensitive aerobiotic Bacteria are brought to rest by cutting off the 
supply of oxygen, and are at once set in motion again by renewing the supply, and 
the movement is directed towards the source of the oxygen, for example a cell 
containing chlorophyll which is reached by the sun’s rays. An infinitesimal portion 
of an oxygen-compound, according to Engelmann’s calculation the trillionth part of 
a milligram, is sufficient to set very sensitive aerobionts in motion, and they are there- 
fore the most delicate of reagents for the evolution of oxygen. Such sensitive forms, 
when the oxygen is supplied to them from one direction only, move as close as possible 
to the source of the oxygen, such as a cell containing chlorophyll or the edge of the 
cover-glass when a specimen is grown in drops ofa fluid on a microscopic slide. 
Other forms under the same conditions only approach within a certain distance of the 
source of the oxygen, and this distance increases as the oxygen diminishes. From this it is 
concluded that these forms can do with a pressure of oxygen less than that of the atmo- 
sphere. Anaerobionts behave in this respect in the reverse way to that of the sensitive 
aerobionts. The movement of motile forms in fluids is similarly quickened by a 
supply of proper soluble nutrient material, and it is directed, if the supply comes from 
one side, towards the diffusion-stream of the nutrient solution which flows into the liquid 
substratum from the source of the supply. Hence the Bacteria gather in dense swarms 
round solid bodies containing nutrient substances when placed in the fluid in which 
they happen to be present °. 

1 Bot. Ztg. 1882. ? Bot. Ztg. 1881, p. 441; 1882, pp. 663, 419. 
g P . P 419 
> Further details will be found in Pfeffer in Unters. d. Bot. Inst. z. Tübingen, I, Heft 3. 
