and, and is perfectly at home there. In England
it was cultivated by Gerard nearly 300 years ago. It grows rapidly if
planted in stony soil, in a position exposed to full sunshine, where it
will creep along the ground, and root all along its stems, which rarely
get elevated more than 6 in. from the ground. This species and O.
Ficus-indica are confused by some authors, owing, no doubt, to the name
O. vulgaris having been given by a botanist to the latter, which is a
much larger and very different-looking plant. O. vulgaris is capable of
withstanding our winters out of doors.

O. Whipplei (Captain Whipple's).--Stem usually prostrate, with slender,
elongated branches, which are cylindrical when old, broken up into short
joints when young. Joints varying in length from 2 in. to 1 ft., less than
1 in. in diameter. Cushions small, round. Spines white, variable in
number, and arranged in tufts on the ends of the tubercles, one being
1 in. long, the others shorter. Flowers nearly 2 in. in diameter, red,
borne in a cluster on the ends of the last-ripened joints in June. Fruit
1 in. long, with a cavity in the top. A compact, Mexican species, with
crowded branches, and very free-flowering. It requires stove treatment.
O. Whipplei is related to O. arborescens, from which, however, it is
easily distinguished by the latter having a stout central spine and
numerous radiating ones.

Of the 150 species of Opuntia known, about one-third have been selected
for description here, and amongst these will be found all the
best-marked kinds in the genus, and most of those of which we have any
knowledge. Botanists find good specific characters in the size and
structure of the seeds, in the character of the fruits, &c.; but for
horticultural purposes these are of little or no value.

CHAPTER XV.

THE GENUS PERESKIA.

(Named in honour of Nicholas F. Peresk, a botanist of Provence.)

The thirteen species included in the genus Pereskia differ so markedly
from all other kinds of Cactus, that at first sight one can scarcely
believe they are true Cactuses, closely related to Cereus and
Epiphyllum. They have erect or trailing stems and branches, and usually
form dense, large bushes; the branches are woody and thin, and bear
large, laurel-like leaves, which remain on the plants several years--so
that they may be termed evergreen. They have, however, the
spine-cushions, the tufts of woolly hair and stout spines, and the
floral characters wh