[Illustration: FIG. 111.--_Calycifloræ_ (_Saxifraginæ_): _A_, flowers and leaves of wild gooseberry, _Ribes_ (_Ribesieæ_), × 1. _B_, vertical section of the flower, × 2. _C_, diagram of the flower. _D_, flower of bishop's-cap, _Mitella_ (_Saxifragaceæ_), × 3. _E_, flower of stonecrop, _Sedum_ (_Crassulaceæ_), × 2. _F_, flowers and leaves of hydrangea (_Hydrangeæ_), × ½. _n_, neutral flower. _G_, unopened flower, × 2. _H_, the same, after the petals have fallen away. _I_, flower of syringa, _Philadelphus_ (_Philadelpheæ_), × 1. _J_, diagram of the flower.] The third order (_Opuntieæ_) has but a single family, the cacti (_Cactaceæ_). These are strictly American in their distribution, and inhabit especially the dry plains of the southwest, where they reach an extraordinary development. They are nearly or quite leafless, and the fleshy, cylindrical, or flattened stems are usually beset with stout spines. The flowers (Fig. 112, _A_) are often very showy, so that many species are cultivated for ornament and are familiar to every one. The beautiful night-blooming cereus, of which there are several species, is one of these. A few species of prickly-pear (_Opuntia_) occur as far north as New York, but most are confined to the hot, dry plains of the south and southwest. [Illustration: FIG. 112.--_Calycifloræ_, _Opuntieæ_ (_Passiflorinæ_). _A_, flower of a cactus, _Mamillaria_ (_Cactaceæ_) (from "Gray's Structural Botany"). _B_, leaf and flower of a passion-flower, _Passiflora_ (_Passifloraceæ_), × ½. _t_, a tendril. _C_, cross-section of the ovary, × 2. _D_, diagram of the flower.] The fourth order (_Passiflorinæ_) 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 monœcious.[13] Whether the begonias properly belong with the _Passiflorinæ_ has been questioned. [13] Monœcious: having stamens and carpels in different flowers, but on the same plant. [Illustration: FIG. 113.--_Calycifloræ_ (_Myrtifloræ_, _Thymelinæ_). _A_, flowering branch of moosewood, _Dirca_ (_Thymelæaceæ_), × 1. _B_, a single flower, × 2. _C_, the same, laid open. _D_, a young flower of willow herb, _Epilobium_ (_Onagraceæ_), × 1. The pistil (_gy._) is not yet ready for pollination. _E_, an older flower, with receptive pistil. _F_, an unopened bud, × 1. _G_, cross-section of the ovary, × 4. _H_, a young fruit, × 1. _I_, diagram of the flower. _J_, flowering branch of water milfoil, _Myriophyllum_ (_Haloragidaceæ_), × ½. _K_, a single leaf, × 1. _L_, female flowers of the same, × 2. _M_, the fruit, × 2.]