RESPIRA TION OF PLANTS. 27 



anger, and it is evident at a glance that the war will 

 be waged to the death. 



We cannot too distinctly remember that a stream- 

 less aquarium is a little world, shut off, as it were, 

 from the great world outside. The water, the animals, 

 and the plants have to be so adjusted that no extra- 

 neous addition is required. The marvellous principles 

 of adjustment of animal to vegetable life, and con- 

 trariwise, which holds good all over the surface of the 

 globe, is as much in active operation in a portable 

 aquarium as on a planet. Under the influence of sun- 

 light the aquatic plants obtained from some stream or 

 pond give off oxygen. You may frequently see it, in 

 little bubbles, clinging to the stems and under surfaces 

 of the leaves. We need not say that this gas is vitally 

 necessary for the support of animal life. Plants there- 

 fore provide it. On the other hand, it is equally im- 

 portant that the carbonic acid given out by all animals 

 shall be disposed of, or put out of the way so as not 

 to injure the creatures that have breathed it, after the 

 fashion of the Black Hole at Calcutta. Plants per- 

 form this function ; and not only do they absorb the 

 deleterious gas, but they actually require it for their 

 sustenance and growth, as much as animals do the 

 oxygen ! 



It will be seen, therefore, that in the knowledge of 

 this fact we have the means of adjusting a collection 

 of aquatic animals and plants, in the vessel we call an 

 aquarium, so that there shall be constantly kept up a 



