96 NOT ALTOGETHER ABOUT PLANTS 



atom. The word atom means uncuttable or indivisible. It 

 has turned out to be a poor name, for recent scientists have 

 found that atoms are divisible. But these divisions of 

 atoms need not concern us now. Just how the sub- 

 divisions of atoms affect life we do not know, and in this 

 study we are concerned with these matters only as they 

 affect life. 



From what has just been said you may form the idea 

 that an atom is just the same thing as the molecule of an 

 element. This is not the case, for atoms do not exist in- 

 dependently if they have a chance to unite with other 

 atoms. Thus elements are composed of molecules and 

 their molecules are composed of atoms, just as is the 

 case with compounds. The difference is that the atoms 

 in the molecule of an element are all of the same kind, 

 while the atoms in the molecule of a compound are never 

 all the same. They are always of two kinds or more. 



Atoms may be defined as the units of which molecules are 

 composed. 



Now, going back to our molecule of water, you should 

 have some idea of what is meant when we say that a mole- 

 cule of water is composed of two atoms of hydrogen and 

 one atom of oxygen. Perhaps you have heard of H 2 O. 

 H 2 O is a symbol which stands for the molecule of water. 

 It means a molecule which is composed of two atoms of 

 hydrogen and one atom of oxygen. Hydrogen and oxygen 

 may form molecules of different proportions than this two- 

 to-one proportion. But such molecules are not molecules 

 of water. In molecules of water the two elements can 

 occur in this proportion and in no other. 



Another substance of which we have spoken and which 

 is very important in plant life is carbon dioxide. The 



