INTRODUCTORY 5 
in length. Some very pure natural waters contain no plants 
readily visible to the eye, while the foul drainage of a barn- 
yard in warm weather may be of a uniform bright-green color 
from the presence of many thousands of microscopic plants 
in every drop of the water. Plants occur in all the seas as 
well as in all the fresh waters, on every kind of soil from the 
wettest swamps to arid deserts, on rocky cliffs. and on 
branches and leaves of trees. 
4, Structures. and habits of plants. Although the relations 
of plants to human life and the life of the lower animals are 
of the highest importance, it is not this aspect of botany which 
is of largest concern to the beginner. We have no reason to 
suppose that plants exist for the benefit of man. Plant struc- 
tures and processes are of importance primarily in maintaining 
the life of plants themselves, and their use by men is a by- 
product of plant life. In order to gain any scientific knowl- 
edge of elementary botany, it is necessary to try to find out 
what are the forms and internal structures of plants, how they 
carry on their life processes, and what relations exist between 
them and the external conditions of soil and climate. To these 
studies may be added some consideration of the relations be- 
tween the plant and its plant and animal neighbors, and other 
studies about what plants have done for man and what he is 
trying to do with plants. But it must be clearly understood 
that the study of plant structures and functions — what 
plants are and how they live —is fundamental to any other 
study of plants. 
