BOTANICAL CHARACTERS.
LTHOUGH strictly botanical information may be considered as falling outside the limits of a treatise intended only for the cultivator, yet a short account of the principal characters by which Cactuses are grouped and classified may not be without interest.
From the singular form and succulent nature of the whole of the Cactus family, it might be inferred that, in these characters alone, we have reliable marks of relationship, and that it would be safe to call all those plants Cactuses in which such characters are manifest. A glance at some members of other families will, however, soon show how easily one might thus be mistaken. In the Euphorbias we find a number of kinds, especially amongst those which inhabit the dry, sandy plains of South Africa, which bear a striking resemblance to many of the Cactuses, particularly the columnar ones and the Rhipsalis. (The Euphorbias all have milk-like sap, which, on pricking their stems or leaves, at once exudes and thus reveals their true character. The sap of the Cactuses is watery). Amongst Stapelias, too, we meet with plants which mimic the stem characters of some of the smaller kinds of Cactus. Again, in the Cactuses themselves we have curious cases of plant mimicry; as, for instance, the Rhipsalis, which looks like a bunch of Mistletoe, and the Pereskia, the leaves and habit of which are more like what belong to, say, the Gooseberry family than to a form of Cactus. From this it will be seen that although these plants are almost all succulent, and curiously formed, they are by no means singular in this respect.
The characters of the order are thus defined by botanists: Cactuses are either herbs, shrubs, or trees, with soft flesh and copious watery juice. Root woody, branching, with soft bark. Stem branching or simple, round, angular, channelled, winged, flattened, or cylindrical; sometimes clothed with numerous tufts of spines which vary in texture, size, and form very considerably; or, when spineless, the stems bear numerous dot-like scars, termed areoles. Leaves very minute, or entirely absent, falling off very early, except in the Pereskia and several of the Opuntias, in which they are large, fleshy, and persistent. Flowers solitary, except in the Pereskia, and borne on the top or side of the stem; they are composed of numerous parts or segments; the sepals and petals are not easily distinguished from each other; the calyx tube is joined to, or combined, with the ovary, and is often covered with scale-like sepals and hairs or spines; the calyx is sometimes partly united so as to form a tube, and the petals are spread in regular whorls, except in the Epiphyllum. Stamens many, springing from the side of the tube or throat of the calyx, sometimes joined to the petals, generally equal in length; anthers small and oblong. Ovary smooth, or covered with scales and spines, or woolly, one-celled; style simple, filiform or cylindrical, with a stigma of two or more spreading rays, upon which are small papillae. Fruit pulpy, smooth, scaly, or spiny, the pulp soft and juicy, sweet or acid, and full of numerous small, usually black, seeds.
Tribe I.—Calyx tube produced beyond the Ovary. Stem covered with Tubercles, or Ribs, bearing Spines.
1. MELOCACTUS. Stem globose; flowers in a dense cap-like head, composed of layers of bristly wool and slender spines, amongst which the small flowers are developed. The cap is persistent, and increases annually with the stem.
2. MAMILLARIA. Stems short, usually globose, and covered with tubercles or mammae, rarely ridged, the apex bearing spiny cushions; flowers mostly in rings round the stem.
3. PELECYPHORA. Stem small, club-shaped; tubercles in spiral rows, and flattened on the top, where are two rows of short scale-like spines.
4. LEUCHTENBERGIA. Stem naked at the base; tubercles on the upper part large, fleshy, elongated, three-angled, bearing at the apex a tuft of long, thin, gristle-like spines.
5. ECHINOCACTUS. Stem short, ridged, spiny; calyx tube of the flower large, bell-shaped; ovary and fruit scaly.
6. DISCOCACTUS. Stem short; calyx tube thin, the throat filled by the stamens; ovary and fruit smooth.
7. CEREUS. Stem often long and erect, sometimes scandent, branching, ridged or angular; flowers from the sides of the stem; calyx tube elongated and regular; stamens free.
8. PHYLLOCACTUS. Stem flattened, jointed, and notched; flowers from the sides, large, having long, thin tubes and a regular arrangement of the petals.
9. EPIPHYLLUM. Stem flattened, jointed; joints short; flowers from the apices of the joints; calyx tube short; petals irregular, almost bilabiate.
Tribe II.—Calyx-tube not produced beyond the Ovary. Stem branching, jointed.
10. RHIPSALIS. Stem thin and rounded, angular, or flattened, bearing tufts of hair when young; flowers small; petals spreading; ovary smooth; fruit a small pea-like berry.
11. OPUNTIA. Stem jointed, joints broad and fleshy, or rounded; spines barbed; flowers large; fruit spinous, large, pear-like.
12. PERESKIA. Stem woody, spiny, branching freely; leaves fleshy, large, persistent; flowers medium in size, in panicles on the ends of the branches.
The above is a key to the genera on the plan of the most recent botanical arrangement, but for horticultural purposes it is necessary that the two genera Echinopsis and Pilocereus should be kept up. They come next to Cereus, and are distinguished as follows:
ECHINOPSIS. Stem as in Echinocactus, but the flowers are produced low down from the side of the stem, and the flower tube is long and curved.
PILOCEREUS. Stem tall, columnar, bearing long silky hairs as well as spines; flowers in a head on the top of the stem, rarely produced.
With the aid of this key anyone ought to be able to make out to what genus a particular Cactus belongs, and by referring to the descriptions of the species, he may succeed in making out what the plant is.
For the classification of Cactuses, botanists rely mainly on their floral organs and fruit. We may, therefore, take a plant of Phyllocactus, with which most of us are familiar, and, by observing the structure of its flowers, obtain some idea of the botanical characters of the whole order.
Phyllocactus has thin woody stems and branches composed of numerous long leaf-like joints, growing out of one another, and resembling thick leaves joined by their ends. Along the sides of these joints there are numerous notches, springing from which are the large handsome flowers. On looking carefully, we perceive that the long stalk-like expansion is not a stalk, because it is above the seed vessel, which is, of course, a portion of the flower itself. It is a hollow tube, and contains the long style or connection between the seed vessel and the stigma, a (Fig. 2). This tube, then, must be the calyx, and the small scattered scale-like bodies, b (Fig. 2), which clothe the outside, are really calyx lobes.
FIG. 2.—FLOWER OF PHYLLOCACTUS, CUT LENGTHWISE
a, Calyx Tube. b, Calyx Lobes. c, Ditto, assuming the form of Petals. d, Stamens. e, Style. f, Ovary or Seed Vessel.
Nearer the top of the flower, these calyx lobes are better developed, until, surrounding the corolla, we find them assuming the form and appearance of petals, c (Fig. 2). The corolla is composed of a large number of long strap-shaped pointed petals, very thin and delicate, often beautifully coloured, and generally spreading outwards. Springing from the bases of these petals, we find the stamens, d (Fig. 2), a great number of them, forming a bunch of threads unequal in length, and bearing on their tips the hay-seed-like anthers, which are attached to the threads by one of their points. The style is a long cylindrical body, e (Fig. 2), which stretches from the ovary to the top of the flower, where it splits into a head of spreading linear rays, ½ in. length. When the flower withers, the seed vessel, f (Fig. 2), remains on the plant and expands into a large succulent fruit, inside which is a mass of pulpy matter, inclosing the numerous, small, black, bony seeds.
It must not be supposed that all the genera into which Cactuses are divided are characterised by large flowers such as would render their study as easy as the genus taken as an illustration. In some, such for instance as the Rhipsalis, the flowers are small, and therefore less easy to dissect than those of Phyllocactus.
The stems of Cactuses show a very wide range of variation in size, in form, and in structure. In size, we have the colossal Cereus giganteus, whose straight stems when old are as firm as iron, and rise with many ascending arms or rear their tall leafless trunks like ships' masts to a height of 60 ft. or 70 ft. From this we descend through a multitude of various shapes and sizes to the tiny tufted Mamillarias, no larger than a lady's thimble, or the creeping Rhipsalis, which lies along the hard ground on which it grows, and looks like hairy caterpillars. In form, the variety is very remarkable. We have the Mistletoe Cactus, with the appearance of a bunch of Mistletoe, berries and all; the Thimble Cactus; the Dumpling Cactus; the Melon Cactus; the Turk's cap Cactus; the Rat's-tail Cactus; the Hedgehog Cactus; all having a resemblance to the things whose names they bear. Then there is the Indian Fig, with branches like battledores, joined by their ends; the Epiphyllum and Phyllocactus, with flattened leaf-like stems; the columnar spiny Cereus, with deeply channelled stems and the appearance of immense candelabra. Totally devoid of leaves, and often skeleton-like in appearance, these plants have a strange look about them, which is suggestive of some fossilised forms of vegetation belonging to the past ages of the mastodon, the elk, and the dodo, rather than to the living things of to-day.
By far the greater part of the species of Cactuses belong to the group with tall or elongated stems. "It is worthy of remark that as the stems advance in age the angles fill up, or the articulations disappear, in consequence of the slow growth of the woody axis and the gradual development of the cellular substance; so that, at the end of a number of years, all the branches of Cactuses, however angular or compressed they originally may have been, become trunks that are either perfectly cylindrical, or which have scarcely any visible angles."
A second large group is that of which the Melon and Hedgehog Cactuses are good representatives, which have sphere-shaped stems, covered with stout spines. We have hitherto spoken of the Cactuses as being without leaves, but this is only true of them when in an old or fully-developed state. On many of the stems we find upon their surface, or angles, small tubercles, which, when young, bear tiny scale-like leaves. These, however, soon wither and fall off, so that, to all appearance, leaves are never present on these plants. There is one exception, however, in the Barbadoes Gooseberry (Pereskia), which bears true and persistent leaves; but these may be considered anomalous in the order.
The term "succulent" is applied to Cactuses because of the large proportion of cellular tissue, i.e., flesh, of their stems, as compared with the woody portion. In some of them, when young, the woody system appears to be altogether absent, and they have the appearance of a mass of fleshy matter, like a vegetable marrow. This succulent mass is protected by a tough skin, often of leather-like firmness, and almost without the little perforations called breathing and evaporating pores, which in other plants are very numerous. This enables the Cactuses to sustain without suffering the full ardour of the burning sun and parched-up nature of the soil peculiar to the countries where they are native. Nature has endowed Cactuses with a skin similar to what she clothes many succulent fruits with, such as the Apple, Plum, Peach, &c., to which the sun's powerful rays are necessary for their growth and ripening.
The spiny coat of the majority of Cactuses is no doubt intended to serve as a protection from the wild animals inhabiting with them the sterile plains of America, and to whom the cool watery flesh of the Cactus would otherwise fall a prey. Indeed, these spines are not sufficient to prevent some animals from obtaining the watery insides of these plants, for we read that mules and wild horses kick them open and greedily devour their succulent flesh. It has also been suggested that the spines are intended to serve the plants as a sort of shade from the powerful sunshine, as they often spread over and interlace about the stems.