474 THE AMERICAN NATURALIST. (Vor. XXXV. 
higher organisms intramolecular respiration is a function very 
limited in importance, taking place only when there is continued 
need of energy in the absence of free oxygen, and capable of being 
maintained for comparatively brief periods only. Like normal 
respiration, it is carried on solely by the living protoplasm, more 
or less actively according to the greater or lesser activity of the 
protoplasm. The substances decomposed are like those oxidized 
in normal respiration and differ in different species of plants. 
The products differ according to the plant, the conditions under 
which it acts, and the substances acted upon. 
Alcohol may be produced in considerable amount. This 
suggeststhat in both fermentation and intramolecular respiration 
(if one may separate the two processes, for the former certainly 
includes the latter as well as nutrition) much of the chemical 
work may be done by enzymes produced by the respiring 
organism. Organic compounds and small quantities of many 
others may also be formed. In germinating peas, the alcohol 
produced may equal as much as 5% the weight of the moist 
seeds, enough to give some support to the idea expressed above, 
that the accumulation of the poisonous products of intra- 
molecular respiration, as in fermentation, may cause the ces- 
sation of respiration and the death of the organism. 
Between those plants for which aérobic respiration is indis- 
pensable to normally active life, and for which anaérobic respi- 
ration is only a means of maintaining life over unfavorable 
periods, and those for which anaérobic respiration is similarly 
and equally indispensable, there are all connecting stages. 
These are found amang the lower plants, especially the fungi ; 
but, as before stated, in all large multicellular organisms, espe- 
cially among animals, there are probably cells, lying deep in 
the tissues, which are forced, by the positions they occupy, to 
supply themselves with needed kinetic energy by the same 
means as the anaérobic organisms, ż.e. by decomposing the 
complex compounds which they contain. There are then cells, 
as well as organisms, which are obligate aérobic, facultative 
aérobic, or obligate anaérobic. Obligate anaérobic cells and 
organisms live where the access of free oxygen is impossible or 
difficult and inadequate; for example, deep in living tissues, 
