IL. 
CONDITIONS OF GROWTH. 
BACTERIA only grow in presence of moisture, under certain condi- 
tions of temperature, and when supplied with suitable pabulum. As 
they do not contain chlorophyll, they cannot assimilate carbon diox- 
ide, and light is not favorable to their development. 
The aérobic species obtain oxygen from the air and cannot grow 
unless supplied with it. The anaérobic species, on the other hand, 
will not grow in the presence of oxygen, and must obtain this ele- 
ment, as they do carbon and nitrogen, from the organic material 
which serves them as food. 
As aclass the bacteria are supplied with nutriment by the higher 
plants and animals, the dead tissues of which they appropriate, and 
which itis their function to decompose, releasing the organic ele- 
ments as simple compounds which may again be assimilated by the 
chlorophyll-producing plants. 
Water is essential for the development of bacteria, and many spe- 
cies have their normal habitat in the waters of the ocean, of lakes, 
and of running streams ; others thrive upon damp surfaces or in the 
interior of moist masses of organic material. Many species grow in- 
differently either in salt or fresh water, but it is probable that cer- 
tain species will be found peculiar to the waters of the ocean. Some 
of the water bacteria multiply in the presence of an exceedingly 
minute amount of organic pabulum, or even in distilled water. This 
is shown by the experiments of Bolton and others. The author 
named tested two species of water bacteria (Micrococcus aquatilis 
and Bacillus erythrosporus) in the following manner: Ten cubic 
centimetres of distilled water in a test tube were infected with a small 
quantity of a culture of one of these microérganisms. A drop from 
this tube was transferred to the same quantity of distilled water in 
a second tube, and from this to a third. The number of bacteria in 
this tube No. 3 was now ascertained by counting, and it was put 
aside for two or three days, at the end of which time the number was 
again estimated by counting. In every case there was an enormous 
increase in the number of bacteria. In order to be sure that the dis- 
