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FIG. 74.-Acacia melanoxylon.

CHAPTER VI.

HITHERTO I have dealt with plants in which one main consideration appears to be the securing as much light and air as possible. Our English trees may be said as a general rule to be glad of as much sun as they can get. But a glance at any shrubbery is sufficient to show that we cannot explain all leaves in this manner, and in tropical countries some plants' at any rate find the sun too much for them. I will presently return to the consideration of other characteristics of tropical vegetation. In illustration, however, of the present point, perhaps the clearest evidence is afforded by some Australian species, especially the Eucalypti and Acacias. Here the adaptations which we meet with are directed, not to the courting, but to the avoidance, of light.

120

AUSTRALIAN ACACIA.

[CHAP.

The typical leaves of Acacias are pinnate, with a number of leaflets. On the other hand, many of the Australian Acacias have leaves (or, to speak more correctly, phyllodes) more or less elongated or willowlike. But if we raise them from seed we find, for instance, in Acacia salicina, so called from its resemblance to a Willow, that the first leaves are pinnate (Fig. 75), and differ in nothing from those

FIG. 75.-Seedling of Acacia salicina..

characteristic of the genus. In the later ones, however, the leaflets are reduced in number, and the leaf-stalk is slightly compressed laterally. The fifth or sixth leaf, perhaps, will have the leaflets reduced to a single pair, and the leaf-stalk still more flattened, while, when the plant is a little older, nothing remains except the flattened petiole. This in shape, as already observed, much resembles a narrow willow-leaf, but flattened laterally, so that it carries its edge upwards.

VI.]

EUCALYPTUS. IVY.

121

and consequently exposes as little surface as possible to the overpowering sun. In some species the long and narrow phyllodes carry this still further by hanging downwards, and in such cases they often assume a scimitar-like form. This I would venture to suggest may be in consequence of one side being turned outwards, and therefore under more favourable conditions.

In one very interesting species (Acacia melanoxylon, Fig. 74), the plant throughout life produces both forms, and on the same bough may be seen phyllodes interspersed among ordinary pinnate leaves, the respective advantages being, it would appear, so equally balanced that sometimes the one, sometimes the other, secures the predominance.

In the case of the Eucalyptus, every one who has been in the South of Europe must have noticed that the young trees have a totally different aspect from that which they acquire when older. The leaves of the young trees (Fig. 76) are tongue-shaped, and horizontal. In older ones, on the contrary (Fig. 77), they hang more or less vertically, with one edge towards the tree, and are scimitar-shaped, with the convex edge outwards, perhaps for the same reason as that suggested in the case of Acacia. There are several other cases in which the same plant bears two kinds of leaves. Thus, in some species of Juniper the leaves are long and pointed, in others rounded and scale-like. Juniperus chinensis has both.

In the common Ivy the leaves on the creeping or climbing stems are more or less triangular, while those of the flowering stems are ovate lanceolate; a

122

FALL OF THE LEAF.

[CHAP.

difference, the cause of which has not, I think, yet been satisfactorily explained, but into which I will

not now enter.

We have hitherto been considering, for the most part, deciduous trees. It is generally supposed that in autumn the leaves drop off because they die. My impression is that most persons would be very much surprised to hear that this is not altogether

FIG. 76. Eucalyptus-Young.

FIG. 77.- Eucalyptus-Old.

the case. In fact, however, the separation is a vital process, and, if a bough is killed, the leaves are not thrown off, but remain attached to it. Indeed, the dead leaves not only remain in situ, but they are still firmly attached. Being dead and withered, they give the impression that the least shock would detach them; on the contrary, however, they will often bear a weight of as much as two pounds without coming off.

VI.] LENGTH AND LONGEVITY OF LEAVES. 123

In evergreen species the conditions are in many respects different. When we have an early fall of snow in autumn the trees which still retain their leaves are often very much broken down. Hence, perhaps, the comparative paucity of evergreens in temperate regions, and the tendency of evergreens to have smooth and glossy leaves, such as those of the Holly, Box, and Evergreen Oak. Hairy leaves especially retain the snow, on which more and more accumulates.

Again, evergreen leaves sometimes remain on the tree for several years; for instance, in the Scotch Pine three or four years, the Spruce and Silver Fir six or even seven, the Yew eight, Abies pinsapo sixteen or seventeen, Araucaria and others even longer. It is true that during the later years they gradually dry and wither; still, being so long-lived, they naturally require special protection. They are, as a general rule, tough, and even leathery. In many species, again, as is the case with our Holly, they are spinose. This serves as a protection from browsing animals; and in this way we can, I think, explain the curious fact that, while young Hollies have spiny leaves, those of older trees, which are out of the reach of browsing animals, tend to become quite unarmed.

In confirmation of this I may also adduce the fact that while in the Evergreen Oak the leaves on well-grown trees are entire and smooth-edged like those of the Laurel, specimens which are cropped and kept low form scrubby bushes with hard prickly leaves.1

1 Bunbury, Botanical Fragments, p. 320.

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