CHAPTER XIII 

 HOW PLANTS ARE CLASSIFIED 



181. Introductory. In preceding discussions we have often 

 had occasion to speak of closely related plants which, while 

 different in some ways, were sufficiently similar to enable us 

 to recognize their likeness. Thus we speak of different kinds 

 of grasses, different kinds of corn, or different kinds of oaks. 

 Also, in actual practice many other bases of grouping are 

 used, as is suggested when we speak of agricultural plants, 

 tropical plants, desert plants, water plants, and poisonous 

 plants. Similarity in structure offers a basis of classification 

 that is commonly used. By means of this basis plants that 

 resemble each other most closely are grouped together ; then 

 the groups having the closest resemblance are combined into 

 a larger group, until finally all the larger groups compose the 

 last and largest group the plant kingdom. 



182. Oaks as illustrations. A good illustration of the 

 smaller groupings of plants may be had by referring to some 

 of the common oaks. In various parts of the United States 

 we find the white oak, bur oak, red oak, black oak ? blackjack 

 oak, live oak, and many others. There are certain special or 

 specific differences between these oaks, as is shown by their 

 acorns and leaves (fig. 164), and each specific kind of oak 

 is called a species and has a species name, as alba (white), 

 rubra (red), macrocarpa 1 (bur), etc. Furthermore, these and 

 all the other species which we call the oaks are grouped to- 

 gether into one genus (or kind), the genus name of the oaks 

 being Quercus. Therefore a genus consists of the different 



1 The original meaning of macrocarpa is "large-fruited," or "large- 

 seeded." 



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