122 CELLS AND TISSUES 
seeds, tubers, and bulbs containing starch and sugars, and in 
many other plant structures, the volume of oxygen consumed 
during active respiration is equal to that of the carbon dioxide 
given off; but in the germination of seeds containing fats and 
fatty oils, the volume of oxygen consumed is greater than that of 
the carbon dioxide given off, in which case some of the oxygen 
is apparently used in changing the fats and fatty oils to other 
forms of food having a larger proportion of oxygen. 
In higher plants the substances oxidized are organic compounds 
including the sugars, fats, proteins, organic acids, and probably 
the protoplasm itself. Some lower forms of organisms oxidize 
inorganic compounds. Some Bacteria obtain energy by oxidizing 
the ammonia of ammonia salts to nitrites, while others obtain 
energy by oxidizing the nitrites to nitrates. Various other sub- 
stances, such as hydrogen sulphide and iron, are oxidized by 
certain Bacteria to secure energy. 
There are some forms of respiration which can continue when 
oxygen is excluded and the one of them best known is fermen- 
tation, which is prominent in the Yeast Plant and other fer- 
menting organisms. When proceeding in the absence of oxygen, 
such forms of respiration are known as anaérobic respiration, that 
is, respiration in the absence of air. In fact, some micro-organ- 
isms can not carry on their processes well except in the absence of 
air. One kind of anaérobic respiration, which is very similar to 
if not identical with fermentation, can be detected in seeds, fruits. 
and all living plant parts when oxygen is excluded, so that the 
process is not obscured by ordinary respiration. This kind of 
respiration is considered by some to be the initial stage of ordinary 
respiration, thus being closely related to it. 
The peculiar feature about fermentation in the absence of air 
is that oxidation of carbon continues with the release of energy 
and the production of carbon dioxide, although no oxygen is ob- 
tainable from without. Furthermore, fermentation, whether in 
the presence or absence of air, differs from combustion and ordi- 
nary respiration in the completeness with which the substances 
involved are broken down. This may be illustrated in the case 
of the fermentation of sugar by Yeast, in which case, as shown by 
the equation CsHi.O. = 2 CO, + 2 CeH,O, the molecule of sugar 
is broken into 2 molecules of carbon dioxide and 2 of alcohol, 
while in case of combustion and often in respiration the molecule 
