THALLOPHYTES 13 
anaerobic, the contrasting term for those bacteria that need free oxygen 
being aerobic. These are not names of groups, but of two modes of life 
that may be found in any group. The activities and effects of bacteria 
are remarkable, many of them holding a most important relation to 
human interests. A very brief statement of some of these activities 
must suffice, but it may serve to indicate the economic importance of 
the group. 
Saprophytic bacteria. These forms attack the dead bodies or the 
organic products of plants and animals, and bring about putrefaction 
and fermentation. When they are excluded from such organic ma- 
terial, it does not decay or ferment, and the process of canning, for ex- 
ample, is intended to effect this exclusion. When protein material is 
attacked and broken up, there is an escape of ill-smelling compounds, 
causing the offensive odor associated with putrefaction. In fermen- 
tation, complex carbohydrates are attacked, and simpler substances, 
such as alcohol, carbon dioxid, lactic acid, butyric acid, etc., are pro- 
duced from them, according to the kind of bacteria at work. 
Pathogenic bacteria. These are the disease-producing forms, their 
activities being connected with living organisms. The disease is the 
result either of a direct attack upon the tissues, or of the excretion of a 
poison (toxin), or of both. Modern medicine and surgery are largely 
based upon excluding or destroying or neutralizing these forms. Such 
diseases as erysipelas, tetanus, diphtheria, tuberculosis, typhoid fever, 
pneumonia, cholera, pear blight, cabbage rot, etc., are known to be 
bacterial diseases. Besides the dangerous forms which occasionally 
attack human beings, there are numerous harmless forms constantly 
present throughout the alimentary tract. 
Nitrogen bacteria. These are certain bacteria of the soil that are 
able to utilize the free nitrogen that exists in such abundance in the air. 
Ordinary green plants can use nitrogen only in certain of its compounds, 
so that the power of these bacteria is both remarkable and important. 
They are best known in connection with the tubercles of certain Legu- 
minosae (figs. 1 101, 1 102), as the clovers, which can be used, therefore, in 
the restoration of nitrogen compounds to impoverished soil (see p. 379). 
Nitrifying bacteria. These are also soil forms, and although they 
contain no chlorophyll, they can manufacture their own food. They 
can obtain carbon from carbon dioxid, without the presence of either 
chlorophyll or light; and their "nitrification" consists in taking am- 
monia (and other simple nitrogen compounds) and oxidizing it to nitrous 
