The position of this family is very doubtful; but it seems in many
respects to approach _Mesembrianthemum_. Some botanists place it near
to the Ribesiaceæ; others, again, to the Passifloraceæ. Only 1 order.
[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
that leaves or leaf-scars, with tufts of thorns in their axils, may be
observed on the ovary just as on the stem.--The flattened shoots of
the Cactaceæ are formed in various ways, either by the compression of
cylindrical axes (_Opuntia_) or, as in _Melocactus_, etc., from winged
stems in which all the wings are suppressed except two.
The thorns are produced directly from the growing points of
the axillary buds, and are modified leaves. The axillary bud
is united at its base with its subtending leaf, which as a
rule is extremely rudimentary; and these together form a kind
of leaf-cushion, larger in some genera than in others. This
leaf-cushion attains its highest development in _Mammillaria_,
in which it is a large, conical wart (see Fig. 368 _A_), bearing
on its apex the tuft of thorns and rudimentary lamina.--The
_seedlings_ have normal cotyledons and a fleshy hypocotyl.
All the species (1,000?) are American (one epiphytic species of
_Rhipsalis_ is indigenous in S. Africa, Mauritius and Ceylon),
especially from the tropical table-lands (Mexico, etc.). Some
species, especially those without thorns, as _Rhipsalis_, are
epiphytes. _Opuntia vulgaris_, the fruits of which are edible,
is naturalized in the Mediterranean. The cochineal insect
(_Coccus cacti_) lives on this and some closely allied species
(_O. coccinellifera_, etc.), particularly in Mexico and the
Canary Islands. Several are ornamental plants.