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! (bur), ete. 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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