[Illustration: FIG. 112.--_Calyciflorae_, _Opuntieae_ (_Passiflorinae_).
_A_, flower of a cactus, _Mamillaria_ (_Cactaceae_) (from "Gray's
Structural Botany"). _B_, leaf and flower of a passion-flower,
_Passiflora_ (_Passifloraceae_), x 1/2. _t_, a tendril. _C_,
cross-section of the ovary, x 2. _D_, diagram of the flower.]

The fourth order (_Passiflorinae_) are almost without exception
tropical plants, only a very few extending into the southern United
States. The type of the order is the passion-flower (_Passiflora_)
(Fig. 112, _B_), whose numerous species are mostly inhabitants of
tropical America, but a few reach into the United States. The only
other members of the order likely to be met with by the student are
the begonias, of which a great many are commonly cultivated as house
plants on account of their fine foliage and flowers. The leaves are
always one-sided, and the flowers monoecious.[13] Whether the begonias
properly belong with the _Passiflorinae_ has been questioned.

[13] Monoecious: having stamens and carpels in different flowers, but
on the same plant.

[Illustration: FIG. 113.--_Calyciflorae_ (_Myrtiflorae_, _Thymelinae_).
_A_, flowering branch of moosewood, _Dirca_ (_Thymelaeaceae_), x 1. _B_,
a single flower, x 2. _C_, the same, laid open. _D_, a young flower of
willow herb, _Epilobium_ (_Onagraceae_), x 1. The pistil (_gy._) is not
yet ready for pollination. _E_, an older flower, with receptive
pistil. _F_, an unopened bud, x 1. _G_, cross-section of the ovary,
x 4. _H_, a young fruit, x 1. _I_, diagram of the flower. _J_,
flowering branch of water milfoil, _Myriophyllum_ (_Haloragidaceae_),
x 1/2. _K_, a single leaf, x 1. _L_, female flowers of the same, x 2.
_M_, the fruit, x 2.]

The fifth order (_Myrtiflorae_) have regular four-parted flowers with
usually eight stamens, but sometimes, through branching of the
stamens, these appear very numerous. The myrtle family, the members of
which are all tropical or sub-tropical, gives name to the order. The
true myrtle (_Myrtus_) is sometimes cultivated for its pretty glossy
green leaves and white flowers, as is also the pomegranate whose
brilliant, scarlet flowers are extremely ornamental. Cloves are the
dried flower-buds of an East-Indian myrtaceous tree (_Caryophyllus_).
In Australia the order includes the giant gum-trees (_Eucalyptus_),
the largest of all known trees, exceeding in size even the giant trees
of California.

Among the commoner _Myrt