4 ELEMENTS OF PLANT ANATOMY. 
From the small size of the average cell, two advantages 
result to the plant. First, strength and solidity; secondly, 
the greatest possible amount of surface for the transfer of cell 
contents. The first insures mechanical support; the second is 
connected with those changes in the chemical nature of the cell 
contents, by which the life processes of the plant as a whole 
are carried on. The discussion of such changes belongs to 
plant physiology rather than to anatomy. 
2. Molecular Structure of Organic Substance. 
The material of which the plant is composed possesses qual- 
ities which are not found in inorganic substances. In order to 
understand some of the theories respecting the origin, growth, 
development, and function of the plant cell, it will be necessary 
to understand some of these qualities, but for a more complete 
treatment of such questions the student is referred to works on 
plant physiology. 
All substances are said to consist of smallest particles, called 
atoms, which are indivisible and separated from each other by a 
small portion of space. The atoms, even of the elements, are 
supposed to be united in groups, and generally in pairs. When 
a certain number of atoms of different elements unite together 
forming a whole, this result is called a chemical compound. 
The union of these atoms is caused by the attraction which 
they possess for each other, — the stronger the attraction, the 
stronger the union. The smallest particle of such a compound, 
or of an elementary substance which can exist by itself un- 
combined, is called a molecule. Its nature depends not only 
upon the atoms composing it but also upon the manner in 
which they are grouped together, or their positions in respect 
to each other. A substance may therefore undergo a change 
of character without change in the number and kind of atoms 
composing it. 
