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 i§ the same with plants ; respiration is most 



