The cactus family has a few representatives which grow wild as far north as Nantucket, but most of its members live in the hottest situations in tropic or semi-tropic lands. In such localities there is danger that the plant's juices be scorched or dried out, and Nature guards against this by exposing the least possible proportionate surface to the rays of an ardent sun. The plant substance, instead of being spread out into a great number of thin, flat leaves, is collected into a solid mass, almost globular in some varieties, and this living lump is covered with a skin, which is richly colored with chlorophyll, and acts as one all-enfolding leaf.
The real leaves, superseded in their original work, have become converted into spines or prickles, and act as a deterrent to vegetarian enemies.
A member of the widely-differing family of the spurges, which lives on dry ground under an African sun, has adopted like habits. Its branches are succulent, spiny prongs, whose surfaces contain chlorophyll, and the plant, when not in bloom, might be mistaken for one of the many varieties of cactus, while the exigencies of the South African climate have driven a native milkweed to do as the cactuses do. In all three of these plants the vegetable substance is condensed into a mass, the inner tissues are full of juice, the bark is converted into an all-enfolding leaf, and the plant body is protected from thirsty vegetarians by thorns, hairs, or prickles (Fig. 22).
Only the blossoms show that the plants are representatives of four widely-divergent botanical families.
Of all parts of the plant, the leaf is most subject to change, and the readiest, like Poo Bah, to fill all offices at once.
The same plant may bear two kinds, differing in form and in habits.
Some water-plants have both floating and submerged leaves. The floating foliage breathes atmospheric air, and the submerged foliage lives, as fishes do, by breathing the air which is in the water. The water-crowfoot, for instance, bears some floating leaves, and some which live beneath the surface. The floating leaves are broad, like those of the plant's near relations, the meadow buttercups, but those which live in the water are fringed.
In the common arrow-head, another amphibious vegetable, the submerged leaves are long and narrow, like blades of grass, and the terrestrial ones are arrow-shaped. Every leaf which spends its life under water, whatever its family habits and traditions may be, and whatever its aerial sisters may look like, is either a fringe or a narrow ribbon. Thus submerged foliage is doubly fitted for its habitat. The slender blades and delicate fringes are adapted, like fishes' gills, to bring the greatest possible area of surface into contact with the water, and thus, also, with the air, which is diffused through it.
And the waves and currents, which might tear a broad leaf to ribbons, glide harmlessly through these blades and fringes, just as the ocean gales, which rip the Carina leaves in our summer cottage-gardens into "smithereens," sough harmlessly through the slender needles of the coast pines.
The thick, fat foliage of the house-leek, the aloe, and the century-plant does double duty. These leaves not only prepare nourishment for the plant, but also serve as storehouses to hold it.
Their whole interior is white as that of a potato, and, like that useful vegetable, they are heavily loaded with starch, while their green surfaces fulfil the ordinary use of foliage - transpiration and digestion.
As an Irishman might put the case, there are leaves which are not leaves at all - but are something else.
At the end of a climbing spray of the pea or vetch the topmost leaf - or a part of it - becomes (Fig. 23) a tendril by means of which the vine clings to whatever it can reach. Always on the Irish Gorse, and sometimes on growing tips of our native barberry bushes, leaves are metamorphosed into thorns. In the case of the cactus, and some other succulent dwellers in thirsty lands, they are transformed into pickles.
Fig. 23. - A climbing spray of the pea. (The topmost leaflets are converted into tendrils.) (From the Vegetable World).
And a whole category of plants bear leaves which are traps for the luring and snaring of insects.
So the functions of leaves vary widely, and their forms vary still more. "The leaves of the herbage at our feet," says Ruskin, "take all kinds of strange shapes, as if to invite us to examine them. Star-shaped, heart-shaped, spear-shaped, arrow-shaped, fretted, fringed, cleft, furrowed, serrated, in whorles, in tufts, in wreaths, in spires, endlessly expressive, deceptive, fantastic, never the same, from footstalks to blossom, they seem perpetually to tempt our watchfulness, and take delight in outstripping our wonder".