[Illustration: FIG. 368.--_A Echinocactus_: _a_ position of a leaf-lamina; _b_ a lateral shoot on the displaced axillary bud. _B_ Pereskia: _b_ a foliage-leaf on a small thorny branch which is subtended by a foliage-leaf which has fallen off and left a scar(_a_).] [Illustration: FIG. 369.--_Echinopsis._] Order =Cactaceæ= (=The Cacti=). The flower is epigynous, ☿, regular, and remarkable for its _acyclic_ structure; there are, for instance, a large number of spirally-placed sepals and petals, which gradually pass over into one another, and which in some species, to a certain extent, arise from the walls of the ovary as in _Nymphæa_ (Fig. 383 _A_, _B_). The petals are free; rotate, opening widely in _Opuntia_, _Pereskia_, and _Rhipsalis_; erect and united at their base into a shorter or longer tube in _Cereus_, _Epiphyllum_, _Mammillaria_, _Echinocactus_, _Melocactus_, and others (Fig. 369). _Stamens numerous_, attached to the base of the corolla; gynœceum formed of _many carpels_, with one style, dividing into a number of branches corresponding to the number of carpels; the ovary has _one loculus_ with _many parietal_ placentæ; the ovules are anatropous, on long and curved funicles. Fruit a berry with exendospermous seeds. The fruit-pulp is mainly derived from the funicles.--The external appearance of the Cactaceæ is very peculiar; _Pereskia_, which has thick and fleshy leaves (Fig. 368), deviates the least; foliage-leaves of the usual form are wanting in the other genera, or are usually very small, and quickly fall off and disappear (_Opuntia_), or are modified into thorns; the stem, without normal foliage-leaves,--so characteristic a feature in this order,--makes its appearance after the two normally developed cotyledons. The stems are fleshy, perennial, and may finally become woody. In some they are elongated, globose, pointed, and more or less dichotomously branched, _e.g._ in several of the _Rhipsalis_ species, which live mostly as epiphytes on trees; in others, elongated, branched, globose, or, most frequently, more or less angular (prismatic) or grooved and provided with wings, and either columnar and erect (as much as about 20 metres in height and 1 metre in circumference, as in _C. giganteus_ in New Mexico) or climbing by roots (_Cereus_ and _Rhipsalis_-species); in others again, compressed, more or less leaf-like, often with a ridge in the centre (winged), branched and jointed: _Epiphyllum_, _Phyllocactus_, _Opuntia_, some species of _Rhipsalis_; others are thick, short, spherical or ovoid, unbranched or only slightly branched, and either studded with prominent warts (_mammillæ_) each of which supports a tuft of thorns (Fig. 368 _A_; _Mammillaria_ and others) or with vertical ridges, separated by furrows (rows of mammillæ which have coalesced) in _Melocactus_, _Echinocactus_, _Echinopsis_ (Fig. 369); at the same time the ovary in some is embedded in the stem so