10 AN INTR0DUC7I0N TO ZOOLOGY. 



from whicli plants take tlie nitrogen required to build 

 up their lifepsubstance or protoplasm. They cannot 

 ■ derive it from any other source, for they are not able 

 to assimilate the nitrogen of the air, as they- do the 

 carbon of its admixed carbonic acid (COj). Thus we 

 see how perfectly interdependent are the two great 

 groups of living things. The substance of the plant 

 is the food ^ of the animal ; the waste excretion of the 

 animal helps to form the food of the plant. The waste 

 gas given off by the animal (COg) is again the air- 

 food of the plant : while the plant, in rejecting the 

 balance of oxygen from the carbonic gas absorbed, 

 supplies the oxygen by the constant inspiration of 

 which the animal lives.^ 



' Primarily, in the phytophagous (vegetable-eating) animals 

 indirectly, in the carnivorous animals which eat them. 



- Guard against the error of saying that plants hreatlie CO2 



'instead of oxygen.. They require the presence of oxygen in 

 the air, indeed their seeds will not germinate if it is not 



.present; and their chemical activity, or metabolism, to some 

 extent results, like that of the animal, in the formation of 

 OOj. These facts represent the breathing of the plant, the 

 respiration of vegetable protoplasm being, in short, the same 

 as that of animal protoplasm. But in green plants, by day, 

 it is entirely masked by. the large inhalation of COo and ex- 

 halation of oxygen which takes place in connection with the 

 feeding of the plant, the green leaves of which are engaged 

 in assimilating carbon. By night the assimilation of carbon 

 stops, oxygen is therefore no longer exhaled in quantities, and 

 the slight expiration of CO2 begins to be perceptible. This is 

 the foundation of the popular superstition that plants in a bed- 

 room are unhealthy : a superstition quite without foundation, 

 for the quantity of COo expired under these circumstances is 

 so small as to have no practical bearing whatever. 



