CHAPTER FORTY-ONE 



THE CLASSIFICATION OF PLANTS 



Whenever an attempt is made to describe the plants that 

 occur on the earth, it becomes necessary at once to adopt some 

 scheme of classification. About 250,000 plants have been 

 distinguished, and they vary so greatly in size, structure, physio- 

 logical requirements, and life histories that it is obviously im- 

 possible to describe them as a whole. 



Since the earliest times students of plants have proposed 

 schemes of classification which would group together plants hav- 

 ing somewhat similar structures and life histories. At first 

 these attempts were very artificial and unsatisfactory because so 

 little was known about the plants themselves. During the past 

 century and a half, great progress has been made in studying the 

 plants that are now living and the plant forms of past geological 

 ages now found as fossils in the rocks. The large amount of 

 data thus accumulated has made it possible to build classifi- 

 cations that more nearly approach actual or natural relation- 

 ships. Back of all modern classifications is the idea that the 

 plants of the present have been derived through modification 

 from the plants of the past. 



Terminology. Since the time of Linnaeus it has been agreed 

 among botanists that all the individual plants which are essen- 

 tially identical in structure and life history shall be grouped 

 together as a species and given a two- word name. Thus all 

 the millions of corn plants are grouped together as one species, 

 Zea mays. Species have long been recognized and many of them 

 have been given common names, such as Kentucky bluegrass, 

 black mustard, cottonwood, black walnut, and white pine. 



Groups of closely related plants having many characters in 

 common have also been recognized, such as oak, willow, hickory, 

 and pine. These larger groups are called genera (singular, genus) . 

 In each of these groups several or many species are distinguished. 

 For example, the oaks are commonly separated into white oak, 



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