CHAPTER I 

 INTRODUCTION 



1. Occurrence in plants. Plants form the natural cover- 

 ing of the earth's surface. So generally is this true that 

 a land surface without plants seems remarkable. Not only 

 do plants cover the land, but they abound in waters as 

 well, both fresh and salt. One of the most noticeable 

 facts in regard to the occurrence of plants is that they do 

 not form a monotonous covering for the earth's surface, 

 but there are forests in one place, meadows in another, 

 swamp vegetation in another, etc. In this way the general 

 appearance of vegetation is exceedingly varied, and each 

 appearance tells of certain conditions of living. Such plants 

 as appear to the casual observer in a landscape or in a cul- 

 tivated field are by no means the only plants. They are 

 simply the most obvious or the most useful plants, but 

 associated with them are hosts of plants simpler in struc- 

 ture and smaller in size, grading down to forms so small 

 that they are visible only through a microscope. Any 

 general view of the plant kingdom must include all plants. 



2. Plants as living things. It is very important to 

 begin the study of plants with the knowledge that they are 

 alive and at work. It must not be thought that animals 

 are alive and plants are not. There is a common impres- 

 sion that to be alive means to have the power of locomotion, 

 but this is far from true ; and in fact some plants have the 

 power of locomotion while some animals do not. Both plants 

 and animals are living forms, and the laws of living that 

 animals obey must be obeyed also by plants. It is for this 

 reason that the term biology (the science that deals with 



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