104 ORGANS AND FUNCTIONS OF PLANTS. 



blue carbon becomes thus tinged, it produces vari- 

 ous shades of orange, and other combinations of 

 red and yellow. Macaire was led by his researches, to 

 attribute the various colours of flowers chiefly to 

 oxygen, accumulated in the petals, producing acids 

 to combine with the other principles. It may be well, 

 however, to caution the young beginner not to take 

 these statements for more than an ingenious and 

 plausible theory. 



It might be supposed, as plants seem to feed 

 chiefly on carbon, that they would thrive well in 

 smoke, or in an atmosphere of carbonic acid gas ; but 

 it is found not to be so, for the particles of carbon 

 in smoke are too large to enter their pores, and too 

 much undiluted carbonic acid gas gorges them, and 

 they will become brown and die. 



Plants, it would appear then, are destined by Pro- 

 vidence to purify the air, which is loaded, from the 

 lungs of animals, with carbonic acid gas ; and to give 

 out a fresh supply of oxygen to replace what is taken 

 up by the lungs. 



During the night, however, the green parts of plants 

 take up oxygen, which is retained, and give out a 

 small portion of carbonic acid gas ; and hence it is not 

 proper to keep plants during the night in a bed- 

 room. When plants indeed are kept in an atmos- 

 phere deprived of oxygen, they soon lose their colours 

 and perish. 



Plants can neither germinate nor live in nitrogen 

 or azote, which kills some species almost instantly, 

 though it is often found in small proportions, upon 



