| Saxifrage family | Saxifragaceae | Perennial herbs; leaves various, simple or compound, stipules mostly absent; flowers bisexual, mostly regular, in various clusters; fruit a lobed capsule or nearly separate follicles. |
| Sedge family | Cyperaceae | Annual or perennial herbs, often with triangular stems (culms), the pith usually present; leaves alternate, the blades simple, grasslike (linear with parallel venation), with a tubular sheath usually closed at the apex; flowers (florets) bisexual or unisexual, very small, in spikelets; fruit an achene or nutlet, often triangular or lenticular. |
| Smartweed family | Polygonaceae | Annual or perennial herbs, or woody vines; leaves alternate, simple, mostly entire, stipules typically united into an ocrea (a tubular, papery sheath); flowers bisexual or unisexual, regular, in short axillary clusters or in various elongate inflorescences; fruit a trigonous or lenticular achene. |
| Sour Gum family | Nyssaceae | Nyssaceae is a family of flowering trees sometimes included in the dogwood family (Cornaceae).[1] Nyssaceae is composed of 37 known species in five genera |
| Spiderwort family | Commelinaceae | Annual or perennial herbs; leaves alternate, simple, tapering to sheathing bases; flowers bisexual, regular or irregular, in various clusters, opening for only several hours; fruit a capsule. |
| Spike-Moss family | Selaginellaceae | Stems slender, leafy and mosslike, often branched; leaves 1-veined; sporangia-bearing. |
| Spleenwort family | Aspleniaceae | Rhizomes lustrous-scaly; leaves simple and entire to 3-pinnate, evergreen in some, the petiole wiry and reddish brown at least toward base, glabrous or sparsely scaly, with 2 vascular bundles; sori dorsal, linear, and not crossing lateral veins, with indusia attached on inner edge; sporangia with annulus complete. |
| Spurge family | Euphorbiaceae | Annual or perennial herbs, often with milky sap; leaves various, simple, stipules sometimes present; flowers unisexual (plants mostly monoecious), regular; fruit a capsule. |
| Stonecrop family | Crassulaceae | Succulent annual or perennial herbs; leaves various; flowers usually bisexual, regular, hypogynous, in cymes; fruit of several follicles. |
| Sumac family | Anacardiaceae | Deciduous woody vines, shrubs, or trees, often resinous; leaves alternate, compound or simple; flowers regular, in terminal or axillary panicles; fruit a drupe. |
| Sweet-Flag family | Acoraceae | Perennial aromatic herbs from rhizomes; leaves basal and equitant (overlapping), linear and swordlike, parallel-veined. |
| Sweetgum | Altingiaceae | Altingiaceae is a small family of flowering plants in the order Saxifragales,[1] consisting of wind-pollinated trees that produce hard, woody fruits containing numerous seeds. |
| Sycamore family | Platanaceae | Trees with outer bark exfoliating to expose whiter inner bark; leaves alternate, simple, palmately veined and lobed, stipules leafy and encircling the twigs; flowers unisexual (plants monoecious), regular, in globose heads; fruit a multiple of achenes in a globose head. |
| Teasel family | Dipsacaceae | Biennial herbs with prickly stems and leaves; leaves opposite, simple, the opposite pairs often fused at the bases; flowers bisexual, irregular, in bracteate heads; fruit a nutlet. |
| Touch-Me-Not family | Balsaminaceae | Annual herbs with translucent, watery sap; leaves alternate, toothed; flowers bisexual, irregular, with one sepal enlarged and spurred; fruit a capsule with explosive dehiscence. |
| Valerian family | Valerianaceae | Annual, biennial, or perennial herbs; leaves opposite; flowers bisexual or unisexual, regular or regular, in terminal cymes; fruit a dry nutlet. |
| Vervain family | Verbenaceae | Annual or perennial herbs, or shrubs, usually with square stems; leaves opposite, simple or compound; flowers bisexual, slightly irregular, in spikes or heads; fruit types vary. |
| Violet family | Violaceae | Annual or perennial herbs; leaves alternate, simple, entire to lobed, stipules conspicuous and often lobed or fringed; flowers bisexual, irregular, solitary; fruit a capsule. |
| Walnut family | Juglandaceae | Deciduous trees, often resinous and aromatic; leaves alternate, odd-pinnately compound; flowers unisexual (plants monoecious), bracteate, the male in catkins, the female solitary or in spikes; fruit a drupelike nut, enclosed in an involucre. |
| Water Starwort family | Callitrichaceae | Tiny glabrous annuals in water or on wet soil; leaves opposite or sometimes appearing whorled, simple, entire; flowers unisexual and axillary, perianth absent, and flowers reduced to a single stamen or single 2-locular ovary; fruit dry and splitting into four 1-seeded nutlets. |
| Water-Lily family | Nymphaeaceae | Aquatic, perennial herbs; leaves floating or emersed, attached to the rhizome by long petioles, simple and ovate to orbicular with a basal notch at the point of petiole attachment; flowers bisexual, regular, solitary on long peduncles from the rhizome; ovaries superior; fruit berrylike, with many seeds. |
| Water-Plantain family | Alismataceae | Aquatic or wetland perennial or annual herbs; leaves basal, simple, often with a well-developed blade and petiole. |
| Waterleaf family | Hydrophyllaceae | Annual, biennial, and perennial herbs; leaves alternate, simple or compound; flowers bisexual, regular, blue or white, mostly in 1-sided cymes, or solitary; fruit a capsule. |
| Willow family | Salicaceae | Deciduous trees or shrubs; leaves alternate, simple with venation pinnate, stipulate; flowers unisexual (plants dioecious), in bracteate catkins; fruits a 2-4-valved capsule, the seeds tufted with white hairs (comose). |
| Witchhazel family | Hamamelidaceae | Deciduous trees or shrubs; leaves alternate, simple, stipulate; flowers unisexual or bisexual, bracteate, regular; fruit a capsule. |
| Wood Fern family | Dryopteridaceae | Rhizomes creeping, scaly; leaves various, the petioles with 2-many vascular bundles; sori dorsal on veins or at vein tips, an indusium present in most, linear to reniform to hoodlike or lobed but not marginal; sporangia with annulus complete. |
| Wood-Sorrel family | Oxalidaceae | Perennial herbs, containing oxalic acid; leaves alternate or basal, palmately compound and cloverlike, stipules in some; flowers bisexual, regular, solitary or in cymes or umbels; fruit a cylindric, dehiscent capsule. |
| Yam family | Dioscoreaceae | Perennial, twining herbs from a thick rhizome; leaves simple, cordate to hastate with parallel veins converging at apex, on long petioles; flowers unisexual (plants dioecious), regular, in axillary spikes or panicles; fruit a 3-winged capsule; family includes both edible and poisonous species. |
| Yew family | Taxaceae | Shrubs and trees, monoecious or dioecious, with green twigs; leaves linear, evergreen; cones absent, the seeds solitary, partly surrounded by a fleshy red aril; foliage and seeds highly toxic to humans and livestock. |