New Zealand Plants and their Story
Meaning of Terms "Species" and "Genus."
Meaning of Terms "Species" and "Genus."
To write down the word "species" is much easier than to define what a species really is. In fact, when it comes to fixing the limits of a species, scarcely two classifiers can agree. Elementary species, as defined by De Vries (see Chapter I), are the units of the plant kingdom. Such are those groups of plants which differ from all others in certain distinct characteristics, and reproduce themselves "true" from seed. But this experimental method of separating species is not yet in vogue, nor does it seem altogether practicable.
The species, then, of the classifiers are founded by the comparative study of large numbers of individuals, and if a group of such has some distinguishing characteristics which separate it from all other groups of individuals, it is classed as a species. Such a group of individuals may form a true species, which will reproduce its kind, page 155or it may be made up of a number of elementary species. Thus the species of the botanist are by no means equal in value. In practice, however, if a number of plants resemble one another almost exactly, they may at once be concluded as belonging to the same species.
If a number of species agree in certain particulars so that we may conclude they have descended from some common ancestor, they are said to belong to the same genus, and we have the next wider group of plants. Suppose we find a number of plants which, although they differ much in stature, shape of leaves, habit of growth, size and colour of flowers, and in other particulars, yet have all four petals, eight stamens, the calyx-tube attached to the ovary, and produce after flowering a narrow, elongated, 4-angled capsule, which splits open from the apex downwards into four sections, revealing a large number of seeds, each provided with a tuft of hairs at the apex, then all those plants will belong to the genus Epilobium (fig. 63). These plants, again, will vary much amongst themselves; but groups having distinguishing marks for each group can be found, and such groups will each represent a species. There are in New Zealand between thirty and forty species of Epilobium, which are distinguished from one another by distinctive marks, and each bears a name—e.g., Epilobium glabellum, E. Hectori, E. pubens, &c.
Originally the second name had a meaning which was supposed to be appropriate to the plant, but the number of specific names has so increased during the past hundred years that it is no longer possible always to find an appropriate appellation. So modern botany has decreed that a specific name once given must stand for ever, even where the name is quite inappropriate. This means that a name is now considered merely as a name and nothing more, and need have no meaning whatsoever.
Another matter which must be remembered is that generic differences generally depend on the structure of the flowers, and not on the leaves. That a plant has leaves like a willow does not constitute it a willow; similar plant-form, as has been already shown in this book, occurs amongst plants quite unrelated. Leaves, however, amongst other characters, are made use of as marks of specific differences.
Fig. 63.—Epilobium chloraefolium.
[Photo, L. Cockayne.
Classification goes still further. A number of related genera make a family, and so on, until such fundamental divisions of the plant page 157kingdom are reached as—slime-fungi, algae, fungi, liverworts, mosses, ferns, conifers, seed-plants with one seed-leaf in the seedling, and seed-plants with two seed-leaves in the seedling.
The families are now most frequently arranged according to the manner in which they are supposed by some to have originated, the more simple coming first and the more complex last. Thus, amongst seed-plants the pine-tree family begins the list, and the daisy family completes it.
Considering the seed-plants alone, New Zealand has between fourteen and fifteen hundred species, about three-fourths of which are found nowhere else, the number varying according to the computer's conception of a species. Cheeseman gives 1,415 as the number, but the writer's estimate is somewhat higher.
It would be out of place to go at any detail into the families and genera, so only a few of the more interesting are mentioned. Neither can any attempt be made to define the families, &c., in popular language—a task of extreme difficulty, and, when accomplished, harder for the beginner to understand than would be his learning the necessary technical terms, which have a definite meaning and can be used with precision.