CHAPTER XV.

THE COLOURATION OF PLANTS.

The general structure of plants is so simple in comparison with that of animals that our remarks upon this sub-kingdom need only be short.

With regard to leaves, especially such as are brightly coloured, like the Begonias, Caladiums, Coleus, and Anoechtochilus, Plate XI., the colour follows pretty closely the lines of structure. We have border decoration, marking out the vein-pattern of the border; the veins are frequently the seat of vivid colour, and when decolouration takes place, as in variegated plants, we find it running along the interspaces of the veins. These facts are too patent to need much illustration; for our zonale geraniums, ribbon grasses, and beautiful-leaved plants generally, are now so common that everyone knows their character. When decay sets in, and oxidation gives rise to the vivid hues of autumn, we find the tints taking structural lines, as is well shown in dying vine and horse-chestnut leaves, Fig. 1, Plate XI. This shows us that there is a structural possibility of acquiring regional colouration.

We must remember, too, that the negative colouration of these dying leaves is of very much the same character as the positive colouration of flowers, for flowers are modified leaves, and their hues are due to the oxidation of the valuable chlorophyll.

In leaves the tendency of spots to elongate in the direction of the leaf is very marked, as may be well seen in Begonia. Fig. 17, drawn to illustrate another point, shows this partly. When leaves are unsymmetrical, like the begonias, the pattern is unsymmetrical also.

Among parallel veined leaves we find parallel decoration. Thus, in the _Calatheas_ we have dark marks running along the veins. In _Dracæna ferrea_ we have a dark green leaf, with a red border and tip, the red running downwards along the veins. This action may be continued until the leaf is all red except the mid-rib, which remains green. In long net-veined leaves we may cite _Pavetta Borbonica_, whose dark green blade has a crimson mid-rib. Of unsymmetrical leaves those in the plate may suffice.

When we come to flowers, the same general law prevails, and is generally more marked in wild than in cultivated forms, which have been much, and to some extent unnaturally, modified. Broadly speaking, when a flower is regular the decoration is alike on all the parts; the petals are alike in size, the decoration is similar in each, but where they differ in size the decoration changes. Thus, in _Pelargoniums_ we may find all five petals alike, or the two upper petals may be longer or shorter than the lower three. In the first case each is coloured similarly, in the other the colour pattern varies with the size of the petal. The same may be seen in Rhododendron.

Where the petals are united the same law holds good. In regular flowers, like the lilies, the colouration is equal. In irregular flowers, like the snapdragon and foxglove, the decoration is irregular. In Gloxinia the petals may be either regular or irregular, and the decoration changes in concert.

A very instructive case was noticed by one of us in _Lamium galeobdolon_, or yellow Archangel. This plant is normally a labiate with the usual irregular corolla, but we have found it regular, and in this instance the normal irregular decoration was changed to a regular pattern on each petal.

In gamopetalous flowers the line of junction of the petals is frequently marked with colour, and we know of no case in which a pattern runs deliberately across this structure line, though a blotch may spread from it.

When we remember that flowers are absolutely the result of the efforts of plants to secure the fertilizing attention of insects, and that they are supreme efforts, put forth at the expense of a great deal of vegetable energy--that they are sacrifices to the necessity for offspring--it does strike us forcibly when we see that even under these circumstances the great law of structural decoration has to be adhered to.