The leaves of our English trees, as I have already said, are so arranged as to secure the maximum of light; in very hot countries the reverse is the case. Hence, in Australia, for instance, the leaves are arranged not horizontally, but vertically, so as to present, not their surfaces, but their edges, to the sun. One English plant, a species of lettuce, has the same habit. This consideration has led also to other changes. In many species the leaves are arranged directly under, so as to shelter, one another. The Australian species of Acacia have lost their true leaves, and the parts which in them we generally call leaves are in reality vertically-flattened leaf stalks.
In other cases the stem itself is green, and to some extent replaces the leaves. In our common Broom we see an approach to this, and the same feature is more marked in Cactus. Or the leaves become fleshy, thus offering, in proportion to their volume, a smaller surface for evaporation. Of this the Stonecrops, Mesembryanthemum, etc., are familiar instances. Other modes of checking transpiration and thus adapting plants to dry situations are by the development of hairs, by the formation of chalky excretions, by the sap becoming saline or viscid, by the leaf becoming more or less rolled up, or protected by a covering of varnish.
Our English trees are for the most part deciduous. Leaves would be comparatively useless in winter when growth is stopped by the cold; moreover, they would hold the snow, and thus cause the boughs to be broken down. Hence perhaps the glossiness of Evergreen leaves, as, for instance, of the Holly, from which the snow slips off. In warmer climates trees tend to retain their leaves, and some species which are deciduous in the north become evergreen, or nearly so, in the south of Europe. Evergreen leaves are as a rule tougher and thicker than those which drop off in autumn; they require more protection from the weather. But some evergreen leaves are much longer lived than others; those of the Evergreen Oak do not survive a second year, those of the Scotch Pine live for three, of the Spruce Fir, Yew, etc., for eight or ten, of the Pinsapo even eighteen. As a general rule the Conifers with short leaves keep them on for several years, those with long ones for fewer, the length of the leaf being somewhat in the inverse ratio to the length of its life; but this is not an invariable criterion, as other circumstances also have to be taken into consideration.
Leaves with strong scent, aromatic taste, or acrid juice, are characteristic of dry regions, where they run especial danger of being eaten, and where they are thus more or less effectively protected.
ON HAIRS
The hairs of plants are useful in various ways. In some cases (1) they keep off superfluous moisture; in others (2) they prevent too rapid evaporation; in some (3) they serve as a protection against too glaring light; in some (4) they protect the plant from browsing quadrupeds; in others (5) from being eaten by insects; or, (6) serve as a quickset hedge to prevent access to the flowers.
In illustration of the first case I may refer to many alpine plants, the well-known Edelweiss, for instance, where the woolly covering of hairs prevents the "stomata," or minute pores leading into the interior of the leaf, from being clogged up by rain, dew, or fog, and thus enable them to fulfil their functions as soon as the sun comes out.
As regards the second case many desert and steppe-plants are covered with felty hairs, which serve to prevent too rapid evaporation and consequent loss of moisture.
The woolly hairy leaves of the Mulleins (Verbascum) doubtless tend to protect them from being eaten, as also do the spines of Thistles, and those of Hollies, which, be it remarked, gradually disappear on the upper leaves which browsing quadrupeds cannot reach.
I have already alluded to the various ways in which flowers are adapted to fertilisation by insects. But Ants and other small creeping insects cannot effectually secure this object. Hence it is important that they should be excluded, and not allowed to carry off the honey, for which they would perform no service in return. In many cases, therefore, the opening of the flower is either contracted to a narrow passage, or is itself protected by a fringe of hairs. In others the peduncle, or the stalk of the plant, is protected by a hedge, or chevaux de frise, of hairs.