1892.] Comparative Physiology of Respiration. 821 
Finally it was proved by Saussure (1804) and others that for 
green plants, and those without green, like the mushrooms, 
oxygen is as necessary for life as for animals. It thus 
became evident that this use of oxygen and excretion of car- 
bon dioxide was a property of living matter, and that the 
very energy that set free the oxygen of the carbon dioxide 
was derived from oxidations in the green plant comparable 
with those giving rise to energy in animals. Further, that the 
purification of the air by green plants in light is a separate 
function—a chlorophyll function, as it has been happily 
termed by Bernard—and resembles somewhat digestion in 
animals, the oxygen being discarded as a waste product. 
Indeed, so powerful is the effort made to obtain oxygen for 
the life processes by some of the lowest plants, the so-called 
organized ferments, that some of the most useful and some of 
the most deleterious products are due to their respiratory 
activity. In alcoholic fermentation, as clearly pointed out by 
Pasteur and Bernard (see 3 of references), the living ferment 
is removed from all sources of free oxygen, and in the efforts 
of the ferment for respiration the molecules of the sugar are 
decomposed or rearranged, and a certain amount of oxygen 
set free; and this oxygen supplies the respiratory needs of the 
ferment. 
It has been found that the motile power of some bacteria, 
like Bacterium termo, depends on the presence of free oxygen 
in the liquid containing them. When this is absent they 
become quiescent. This fact has been utilized by Engelmann 
and others in the study of the evolution of oxygen by green 
and other colored water plants, the bacteria serving as the ` 
most delicate imaginable oxygen test; so that when the min- 
utest green plant is illuminated by sufficient daylight the pre- 
viously quiescent bacteria move with great vigor and surround 
it in swarms. Out of the range of the plant the bacteria are 
still or move very slowly as if to conserve the minute energy- 
developing substance they have in store until it can be used 
to the best advantage. 
May we not now approach the problem directly and answer 
for the whole organic, living world the question, “ What is 
