To enable them to endure the excessive drought to which they are naturally exposed they are furnished with an unusually tough skin, the evaporating pores or stomates of which are few in number and very often to all appearance merely rudimentary. This contrivance prevents their losing the scanty moisture which they collect from the burning soil, and enables them to sustain the full ardour of the brightest equinoctial sun without inconvenience ; in this respect resembling the succulent fruits of Europe, such as the plum, the grape, the peach, which by the absence of stomates from their tough skin are equally enabled to bear the powerful action of the bright sun that is necessary for their maturation.
These facts teach us what the points are that it is most necessary to attend to in the cultivation of the numerous species which now abound in our gardens. Their skin is so formed that perspiration takes place very slowly through it, unless under the influence of powerful stimulants and when in a young state. It is therefore obvious that they should be sparingly watered or not watered at all during a considerable period of the year. Dry as the places usually are in which Cacti naturally grow, they are periodically visited by heavy rains, which, combined with a bright light and a high tempera ture, force into activity even the sluggish vital powers of such plants as those under consideration. At such a time the annual growth of a Cactus takes place, secretions which enable the species subsequently to form its flowers are deposited, and a general impulse is given to all the torpid energies of its constitution. But by degrees the rains moderate and finally cease ; the young cuticle which at its first for mation perspired freely becomes thicker and tougher, and impermeable to moisture ; what food has been obtained during the short period of growth is securely inclosed within the recesses of the stem ; and when the air and earth become dry the plant is provided with the means of enduring another long period of fasting and inactivity. With the fall of rain the beat moderates, but the light to which the Cacti are exposed is but little if at all diminished ; so that the assimilation and alteration of the food contained within the stem keeps continually going on, however slowly. It is by following this natural course of events that gardeners have succeeded in bringing their Cacti to that extraordinary state of beauty for which they are now conspicuous ; it is by attending practically to such points in the habits of the species that we obtain the myriads of large, brilliant, red, or blush or snow white blossoms that form the glory of our green-houses in the spring. A Cactus is placed in a clamp stove, exposed to all the light that can be collected without being concentrated, and it begins to grow : it is then watered, at first gently, afterwards copiously with water holding a quantity of organisable matter (manure) in solution ; this practice is continued for three months, when the quantity of moisture is diminished and the temperature is lowered, but exposure to light is still attended to, till at last the plant sinks to rest. In this state it
is kept till the season for again forcing it into growth shall have returned, when it is subjected to a repetition of the same treatment as before.
If the Cactacece are to be propagated, their branches or joints, if they have any, are cut off, a little dried, and then placed in a hot and damp place, when they strike root immediately. Among the practi cal consequences, De Candolle observes, that result from the facility with which they are thus multiplied, is one which deserves to be noticed on account of its importance, namely, the manner in which the Opuntia is employed to fertilise the old lavas at the foot of /Etna. As soon as a fissure is perceived, a branch or joint of an Opuntia is stuck in : the latter pushes out roots, which are nourished by the rain that collects round them, or by whatever dust or remains of organic matter may have collected into a little soil : these roots, once developed, insinuate themselves into the most minute crevices, expand, and finally break up the lava into mere fragments. Opuntias treated in this manner produce a great deal of fruit, which is sold as a refresh ing food throughout all the towns of Sicily.
Where however the species have neither branches nor joints, as is the case with some of the species of Mclocactus and Echinocactus, a different mode of propagation is had recourse to : it is then necessary to compel them to branch by artificial means. Each of the mmmerous tufts of spines that occupy the ridges of their stems is a bud, and is capable of being forced into a branch, if by any means the general tendency to grow at the upper extremity only is checked. This is effected either by burning the apex of the plant with a broad flat iron, or by cutting the plant across below the top, in either of which eases several of the spiny buds will gradually swell and develop themselves as little branches, which being broken off will striko root and become new plants.
It is on species of the Cactacece that the cochineal insect feeds. Of these the most common are the three following :—Opuntia Tuna, which seems the moat employed in Peru ; 0. Ilernandezii, which is the most celebrated in Mexico ; and 0. coehenillifera, the native province of which is somewhat doubtful.