, are represented by numerous native species.
The latter comprises mostly tropical plants, and is represented in our
gardens by the showy "four-o'clock" (_Mirabilis_). In this plant, as
in most of the order, the corolla is absent, but here the calyx is
large and brightly colored, resembling closely the corolla of a
morning-glory or petunia. The stamens are usually more numerous than
the sepals, and the pistil, though composed of several carpels, has,
as a rule, but a single cavity with the ovules arising from the base,
though sometimes the ovary is several celled.

[Illustration: FIG. 98.--Types of _Centrospermae_. _A_, plant of
spring-beauty, _Claytonia_ (_Portulacaceae_), x 1/2. _B_, a single
flower, x 1. _C_, fruit, with the sepals removed, x 2. _D_, section of
the seed, showing the curved embryo (_em._), x 5. _E_, single flower
of smart-weed, _Polygonum_ (_Polygonaceae_), x 2. _F_, the pistil, x 2.
_G_, section of the ovary, showing the single ovule, x 4. _H_, section
of the seed, x 2. _I_, base of the leaf, showing the sheath, x 1. _J_,
flower of pig-weed, _Chenopodium_ (_Chenopodiaceae_), x 3: i, from
without; ii, in section. _K_, flower of the poke-weed, _Phytolacca_
(_Phytolaccaceae_), x 2. _L_, fire-pink, _Silene_ (_Caryophyllaceae_),
x 1/2. _M_, a flower with half of the calyx and corolla removed, x 1.
_N_, ripe fruit of mouse-ear chick-weed, _Cerastium_ (_Caryophyllaceae_),
opening by ten teeth at the summit, x 2. _O_, diagram of the flower
of _Silene_.]

The first family (_Polygoneae_) is represented by the various species
of _Polygonum_ (knotgrass, smart-weed, etc.), and among cultivated
plants by the buckwheat (_Fagopyrum_). The goose-foot or pig-weed
(_Chenopodium_) among native plants, and the beet and spinach of the
gardens are examples of the family _Chenopodiaceae_. Nearly resembling
the last is the amaranth family (_Amarantaceae_), of which the showy
amaranths and coxcombs of the gardens, and the coarse, green amaranth
or pig-weed are representatives.

The poke-weed (_Phytolacca_) (Fig. 98, _K_), so conspicuous in autumn
on account of its dark-purple clusters of berries and crimson stalks,
is our only representative of the family _Phytolaccaceae_. The two
highest families are the purslane family (_Portulacaceae_) and pink
family (_Caryophylleae_). These are mostly plants with showy flowers in
which the petals are large and conspicuous, though some of the pink
family, _e.g._ some chick-weeds, have