254 LEISURE-TIME STUDIES. 



known reactions which take place between animals and 

 plants and the atmosphere, should serve as guides of some 

 value in distinguishing between the two groups. As every 

 school-boy knows, animals, from the lowest to the highest, 

 demand oxygen, as the gas necessary for breathing, that is, 

 for the purification of the blood, whilst they emit carbonic 

 acid gas, as the part-result of their vital wear and tear. 

 Plants, on the contrary, absorb this carbonic acid, and by a 

 subtle chemical process, which really forms one of the mar- 

 vels of plant life, decompose this gas into its constituents, 

 carbon and oxygen, and after retaining the carbon in the 

 light of food, restore the oxygen to the atmosphere for the 

 use of animals. This elementary knowledge undoubtedly 

 founds a distinction of a certain value in separating animals 

 from plants by differences in their breathing processes, but 

 the value is found to be of no very great extent when we re- 

 flect upon the singular alteration in the functions of plants 

 which changes of strictly natural kind induce. Thus, to 

 begin with, plants can only inhale carbonic acid gas when 

 two conditions are supplied them, these conditions being 

 green colouring-matter and sunlight. Of these two condi- 

 tions, the latter appears to be, if anything, the most import- 

 ant, since, when a plant is placed in the dark it not only 

 ceases to absorb carbonic acid and to emit oxygen, but actually 

 reverses its mode of life, and becomes, as far as its breathing 

 is concerned, a true animal, since it inhales oxygen and 

 emits carbonic acid. And, as if to depreciate the value of 

 the distinction in still further measure, we find that plants 

 which want green colour, are by habit and repute animals, 

 not only as regards their breathing, but in respect of their 

 dietary also. A fungus of any kind, not only inhales oxygen 

 and emits carbonic acid like an animal, but feeds on the 

 living matter afforded by the tissues of other plants or of 

 animals. The fungi which cause skin-diseases in man and 

 in his animal-neighbours, or those which infest other plant- 

 tissues, may truly be said to resemble animals in respect of 

 their mode of subsistence ; since they demand living or 



