INTRODUCTION TO PLANTS 195 



the aid of a microscope. But no matter what their size, 

 form, or general structure is, they all require much 

 the same conditions to live. Just as human life, 

 whether in America, Africa, Japan, or Australia, de- 

 pends upon food, air, and water, so plant life, too, 

 depends upon food, air, and water. 



In our study of plants we shall consider principally 

 what they do, and how they do it. The work which 

 any machine does is a direct result of the way in 

 Avhich it is constructed. It is necessary, for example, 

 to know the structure of an automobile before it is pos- 

 sible to understand how it runs. Similarly we must 

 know much about the structure of plants, before we 

 can understand how they work. 



Plants are composed of cells. Much life of which we 

 are ordinarily quite unconscious exists about us. For 

 instance, if we examine under a 

 microscope some scum taken from 

 a pond, we shall find in it many 

 small animals and plants. Among 

 these will be found some of the 

 simplest living things. Upon closer 

 examination each of these small 

 living things will be found to be Fig 74 P7e?/ ,. ococ . 



, . ,,. , -, , -, cus, a one celled plant. 



round in outline and bounded on A. a single ceil; B, 



clusters of cells. 



the outside by a thin membrane, 



r-alled the cell wall. "Within this is a thin, watery 

 substance somewhat granular in appearance, known as 

 protoplasm. In the protoplasm is a denser spot, a more 



