apices of the joints; calyx tube short; petals irregular, almost
bilabiate.

Tribe II.--Calyx-tube not produced beyond the Ovary. Stem branching,
jointed.

10. RHIPSALIS. Stem thin and rounded, angular, or flattened, bearing
tufts of hair when young; flowers small; petals spreading; ovary smooth;
fruit a small pea-like berry.

11. OPUNTIA. Stem jointed, joints broad and fleshy, or rounded; spines
barbed; flowers large; fruit spinous, large, pear-like.

12. PERESKIA. Stem woody, spiny, branching freely; leaves fleshy, large,
persistent; flowers medium in size, in panicles on the ends of the
branches.

The above is a key to the genera on the plan of the most recent
botanical arrangement, but for horticultural purposes it is necessary
that the two genera Echinopsis and Pilocereus should be kept up. They
come next to Cereus, and are distinguished as follows:

ECHINOPSIS. Stem as in Echinocactus, but the flowers are produced low
down from the side of the stem, and the flower tube is long and curved.

PILOCEREUS. Stem tall, columnar, bearing long silky hairs as well as
spines; flowers in a head on the top of the stem, rarely produced.

With the aid of this key anyone ought to be able to make out to what
genus a particular Cactus belongs, and by referring to the descriptions