THALLOPHYTA | 309 
and animals, therefore, is the energy of sunlight which is stored 
by plants in photosynthesis. Both saprophytes and parasites 
resemble animals in that they obtain energy by the oxidation of 
organic compounds that had their origin not in themselves but 
in green plants. There are, however, certain kinds of bacteria 
that can live in the absence of both light and organic compounds. 
By the oxidation of inorganic mineral matter they obtain energy 
for all their activities, including the assimilation of carbon from 
carbon dioxide. These bacteria include the sulphur bacteria, which 
obtain energy by the oxidation of sulphur compounds, and nitri- 
fying bacteria, which obtain energy by the oxidation of nitrogen 
compounds. As these bacteria can live without sunlight and in 
the absence of organic compounds, they give us an idea of a way 
in which it is possible that living organisms existed and obtained 
energy before the development of plants with chlorophyll. 
Relationship of bacteria. ‘he bacteria have the simplest struc- 
ture of all known living organisms. Moreover, since there are 
bacteria that can live in the absence of light and of organic 
compounds, and so have a method of obtaining energy which 
could have been used by plants before the evolution of chloro- 
phyll, and since some of the oldest sedimentary rocks show evi- 
dence of bacterial action, there are grounds for the opinion that 
the bacteria are the most primitive of known living organisms. 
While the bacteria are a very primitive group, it does not 
follow that all the individual species are ancient. Indeed, those 
that naturally live only in the human body would appear to be 
of recent origin, as they have physiological properties which 
they could hardly have acquired before the advent of man, or 
of animals rather closely related to him. 
GENERAL SURVEY OF ALG 
Most of the simpler chlorophyll-bearing plants are known by 
the popular name of alge. Algz include four classes: the blue- 
green alow, the green algv, the brown alge, and the red alge, the 
designations referring to the colors of the groups. The blue-green 
