P. senilis (Old-Man).—­Stem attaining a height of 25 ft., with a diameter of about 1 ft.; ridges from twenty-five to thirty on plants 4 ft. high; the furrows mere slits, whilst the tufts of thin, straight spines, 1 in. long, which crown each of the many tubercles into which the ridges are divided, give young stems a brushy appearance.  About the upper portion of the stem, and especially upon the extreme top, are numerous white, wiry hairs, 6 in. or more long, and gathered sometimes into locks.  To this character, the plant owes it name Old-Man Cactus; but, by a curious inversion of what obtains in the human kind, old plants are less conspicuous by their white hairs than the younger ones.  Some years ago, there were three fine stems of this Cactus among the cultivated plants at Kew, the highest of which measured 181/2 ft.  There was also, however, a fine specimen in the Oxford Botanic Gardens, with a stem 16 ft. high; and it is stated that this plant has been in cultivation in England a hundred years at least.  A plant twenty-five years old is very small, and, from its slowness of growth, as well as from the reports of the inhabitants of Mexico, where this species is found wild, there is reason to believe that a stem 20 ft. high would be several hundred years old.  The flowers of P. senilis are not known in English collections, the plant being grown only for its shaggy hairiness.

Other species are:  P. chrysomallus, which has a branching habit, P. Bruennonii (Fig. 57), P. Celsianus, P. columna, P. tilophorus, known only in a young state, and several others, all very remarkable plants, but not known in English collections, unless, perhaps at Kew.

[Illustration:  Fig. 57.  Pilocereus BRUeNNONII.]

CHAPTER XI.

THE GENUS MAMILLARIA.

(From mamilla, a little teat; in allusion to the tubercles.)

Something over 300 different kinds of Mamillaria are known, but only a small proportion of these may be considered as garden plants.  They are characterised generally by short, symmetrically-formed stems, sometimes aggregated together and forming a dense tuft, but, as a rule, each plant has only one stem.  The generic name is descriptive of the chief feature in these stems, namely, the closely-set, spirally-arranged tubercles or mamillae, which vary considerably in the different kinds, but are always present in some form or other.  Some kinds have stems only 1 in. high by 2/3 in. in diameter, and the tubercles hidden from view by the star-shaped cushions of reddish or white spines.  In some, the spines are erect and hair-like, giving the plant the appearance of tiny sea-urchins; another group has the principal spines hooked at the tip, and the points in these so sharp that if the hand comes in contact with them they hook into it and stick like fish-hooks.  The purpose of these hooked spines seems doubtful; certainly, they cannot serve as any protection to the plant itself, as they are so strong that the plant must be torn up by the roots before the hooks will give way.