The basis of Plant Classification - The Species

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 difficulty how many kinds he has found, and these, generally speaking, will correspond to the different 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 consider­ably 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 anyone species. For example, a common weed, Whitlow grass (Erophila verna), regarded by Linnaeus as constituting a single species, occurs under something like a hundred * forms, separated by minute but constant differences, 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 two are generally recognized-the plants and the animals-and even they cannot be sharply separated from each other, for the simplest plants and the lowest animals differ but little. 

Every species, genus, family and so on is defined botanically by what is called a diagnosis, generally written, for the sake of precision, in Latin, giving in general terms a description of the characters common to the organisms or groups of organisms which are to be included in the assemblage, and no additional individual or group can be included unless it conforms to the diagnosis of the group into which it is placed. 

The arrangement of individual organisms into species, and the grouping of these into genera, families and so on, constitutes the work of the systematist, whilst the study of the principles in accordance with which the classification should be carried out forms the special branch of biology spoken of as taxonomy. 

Posted in Botany