und the stem.

3. PELECYPHORA. Stem small, club-shaped; tubercles in spiral rows, and
flattened on the top, where are two rows of short scale-like spines.

4. LEUCHTENBERGIA. Stem naked at the base; tubercles on the upper part
large, fleshy, elongated, three-angled, bearing at the apex a tuft of
long, thin, gristle-like spines.

5. ECHINOCACTUS. Stem short, ridged, spiny; calyx tube of the flower
large, bell-shaped; ovary and fruit scaly.

6. DISCOCACTUS. Stem short; calyx tube thin, the throat filled by the
stamens; ovary and fruit smooth.

7. CEREUS. Stem often long and erect, sometimes scandent, branching,
ridged or angular; flowers from the sides of the stem; calyx tube
elongated and regular; stamens free.

8. PHYLLOCACTUS. Stem flattened, jointed, and notched; flowers from the
sides, large, having long, thin tubes and a regular arrangement of the
petals.

9. EPIPHYLLUM. Stem flattened, jointed; joints short; flowers from the
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
of the species, he may succeed in making out what the plant is.

For the classification of Cactuses, botanists rely mainly on their
floral organs and fruit. We may, therefore, take a plant of
Phyllocactus, wit