The number of different species of flowering plants upon the earth is very great indeed, but a careful examination of all the different forms shows us they can be grouped into a few natural families. A family should consist only of plants showing such a likeness of general features that we consider they are related one to another; that is, that they are probably descended from a common ancestor. A like appearance of some parts is not sufficient, as, for instance, the same shaped leaves or similar fruits. No doubt, even in the present day we have not yet succeeded in perfecting this natural grouping, but we are steadily progressing. This grouping of plants according to general character has only one inconvenience: it makes it difficult to the young student. Classification was much easier when it was arranged according to apparent characters, for instance, upon the number of stamens in the flower. As our object is to ascertain truth, convenience has to a large extent to be left out of consideration. A result of natural grouping is that plants which at first sight appear very different, as Buttercup and Clematis, or a Bean and a Wattle, are brought together, while similar looking plants, as Buttercup and Potentil, are kept far apart. No hard and fast rule can be laid down, though the structure of the flower and seed can be most relied upon.
When we wish to talk about any particular family we generally give it the name of one of its commonest members, as the Ranunculaceae. The peculiar termination of such a word is meant to indicate that we refer to a family and not a genus. Rosaceae means the family of which Rose is the type; Myrtaceae, the Myrtle family; Leguminosae, the legume-bearing family; and so on.
The Ranunculus family, of which Buttercup and Clematis are our commonest forms, bear simple or primitive flowers, which will form a base to enable us to understand the more complex forms. We have only a few species of Buttercup, but they are fairly common, and some of them may be found in flower in most seasons. They are all small herbs with yellow or nearly white flowers, with, in rare instances, tendency to red. Neither the fact that they are herbs, nor that the flowers are generally yellow, is of consequence from a scientific point of view. Size and habit seldom indicate a different relationship; and colour of flowers, however interesting to a gardener, is of little consequence in botany. These factors may change rapidly in response to a change in surrounding condition; they may be classed as characters of secondary importance, or may even be considered accidental.
Field Buttercup. (Ranunculus lappaceus, Sm.) [See p. 6
If we examine a Buttercup flower we find the perianth is very plainly made up of two circles; that is, there is a distinct calyx and corolla, which do not, as in some plants, pass gradually into one another. Though the members of each circle are usually five, they are not as constant in this as we generally find in flowers; six or more may often be found.
The calyx is green and the sepals are all free from each other, and of very much the same consistency as the ordinary leaves. The work of this organ is very evidently to protect the bud from drying, or from the entrance of water while the delicate inner parts are being formed. It is very common in Buttercups for the sepals to fall off soon after the flower opens. The corolla is made up of free petals, which usually agree in number with the sepals. They are of delicate texture, and are commonly larger and broader, but in some species they may be narrow and small The petals arise from the thalamus close above the sepals. This is a point that should be noted, as a departure from this is an important feature in the development of more complex flowers. They are inserted into, or, in other words, arise from, the thalamus by a point or very slender attachment, and it should be observed that they alternate with the sepals; that is, each petal occupies the space between two sepals. This is the common, but not invariable, rule in the placing of the members of each floral circle.
The stamens are numerous, free from one another, and inserted close above the corolla. The anthers are closely blended with the filaments, and consist at maturity of two sacks which split longitudinally to allow the pollen to escape. The stamens of a flower are collectively called the androecium, but we will dispense with the use of this word, as we can do without it. The pistil is made up of numerous carpels, which grow upon a conical enlargement of the thalamus. Each carpel is free from its fellows. and consists of a small chamber called the ovary, the top of which is extended into a short, relatively thick and generally bent, style. Towards the end of the style, on its convex surface, the tissue is softer than the rest, and grows into loose papillae; this is the stigma which receives and retains the pollen. In each ovary there is a single ovule, which grows erect from near the base. This ovule will, after fertilisation by the contents of the pollen grain, become a seed.
Buttercups are cross-fertilised by flies or other small insects which travel from flower to flower in quest of honey. This honey is here developed on the petals. In most Buttercups may be seen rather below the middle on the upper surface of each petal a little pouch, which contains a small but constant supply of sweet fluid.
The fruit is not much altered. The thalamus grows a little larger, and each carpel grows to accommodate the seed, and becomes harder. The style remains as a modified hook which, by sometimes catching on to the fur of a passing animal, assures the fruit being dispersed to a distance. As only one seed is formed in each carpel, nothing would be gained by the carpel bursting to allow it to escape. In ordinary talk we call the mature carpel a seed, but it is in this instance more than that.