LEAVES.
[CHAPTER XV.]
The Colouration of Plants.
T
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 Anœchtochilus, [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.