32 PRACTICAL COURSE IN BOTANY 
invariable product wherever the oxidation of substances 
containing carbon goes on. Heat and moisture are evolved 
at the same time, and if oxidation is very active, as in Exps. 
21 and 22, light also. When the process takes place very 
slowly, no light is evolved, and so little heat as to be imper- 
ceptible without special observation. Hence, oxidation may 
go on around us and even in our own bodies without our 
being conscious of the fact. 
Carbon dioxide is of prime importance to the well-being of 
plants. It furnishes the material from which the greater 
part of their organic food is derived, as will be seen when 
we take up the study of the leaf and its work. To animals, 
on the contrary, its presence is so injurious that if the pro- 
portion of it in the air we breathe ever rises much above 1 
part to 1000, the ill effects become painfully sensible. It 
is not, however, as was formerly supposed, a poison, the 
harm it does being to decrease the proportion of oxygen 
in the atmosphere so that animals cannot get enough of it 
to breathe, and die of suffocation. 
30. Respiration in plants and in animals. — It was shown 
in Exp. 24 that respiration in animals is accompanied by the 
products of oxidation; hence we conclude that respiration 
is a form of oxidation. And since these same products are 
given off by plants (Exp. 25), the inference is clear that the 
same process goes on in them. But in plants the life func- 
tions are so much more sluggish than in animals that it is 
only in their most active state, during germination and 
flowering, that evidence of it is to be looked for. 
31. Respiration and energy. — In plants, as in animals, 
respiration is the expression or measure of energy. Sleeping 
animals breathe more slowly than waking ones, snakes and 
tortoises more slowly than hares and hawks. The more 
we exert ourselves and the more vital force we expend, the 
harder we breathe; hence, respiration is more active in 
children than in older persons and in working people than in 
those at rest. It is the same with plants ; respiration is most, 
