CHAPTER III 

 THE BASIS OF PLANT CLASSIFICATION 



The Species 



In any scheme of botanical classification the ultimate basis must be the 

 individual organisms. But many individuals resemble each other so closely 

 that we instinctively recognize that they form natural groups or species. 

 It is important to realize that the idea of species arises from our natural 

 intuition and not from scientific definition. 



A child collecting flowers by the wayside can usually tell without difiiculty 

 how many kinds he has found, and these, generally speaking, will correspond 

 to the diff'erent species recognized and defined by the botanist. A species 

 then, may be said to represent the empirical unit of botanical classification. 



Even among the higher and better-known plants, however, many of the 

 so-called species are not sharply distinguishable from one another and we 

 find intermediate forms or connecting links, which can only with difficulty 

 be referred to either of two closely similar species. Among the lower plants 

 this difficulty becomes even greater, for the individuals which comprise the 

 species not only present fewer points for comparison but they vary consider- 

 ably according to the environmental conditions under which they are 

 growing. The more we study plants the more clearly we realize that had we 

 all the past and present individuals before us it might be impossible, except 

 in an arbitrary manner, to arrange them in species at all, for each kind would 

 be found to be connected with others by a series of gradations. 



Our interpretation, then, of a species must be to some extent an arbitrary 

 one, and botanists often disagree as to the extent of the variation in form 

 which may be admitted within any one species. For example, a common 

 weed. Whitlow grass [Erophila verna) (Fig. 19), regarded by Linnaeus as 

 constituting a single species, occurs under something like a hundred * 

 forms, separated by minute but constant diff'erences, which some accept as 

 distinct species. In general, botanists may be said to be divided into two 

 camps, the " lumpers " and the " splitters " ; the former endeavouring to 

 keep the number of species as few as possible, the latter dividing each species 

 up into ever-increasing numbers of sub-species and varieties. 



For purposes of classification it is not sufficient merely to group all the 

 individual plants into species. These species must in turn be collected 

 together in groups of a higher order which are termed genera. The genera 

 are grouped together in families, and the families in turn are assembled in 

 orders, each group having certain characters in common. In this way a 

 system of classification is built up in which the orders are grouped in classes, 

 classes in phyla, and the phyla into kingdoms. Of these kingdoms only 

 * Modern cytological research has greatly reduced this number. 



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