| Heath family | Ericaceae | Small shrubs to trees, deciduous or evergreen, often in acidic soils; leaves simple, usually alternate; flowers bisexual, regular, solitary or in racemes; fruit a capsule or berry. |
| Hemp family | Cannabaceae | Members of the family are erect or climbing plants, including trees, lianas (woody vines), and herbaceous plants. The leaves are borne oppositely or in spirals and are often palmately lobed or compound |
| Holly family | Aquifoliaceae | Shrubs or trees, deciduous or evergreen; flowers mostly unisexual (plants usually dioecious), regular, in axillary clusters; fruit a drupe, attracting songbirds, especially robins, catbirds, bluejays, bluebirds, thrushes, and mockingbirds. |
| Honeysuckle family | Caprifoliaceae | Vines, shrubs, or small trees; leaves opposite, simple or compound, stipulate in Viburnum; flowers bisexual, regular or irregular, in cymes; fruit a berry or drupe. |
| Hornwort family | Ceratophyllaceae | Aquatic perennial herbs, rootless and suspended below the water surface; leaves whorled, sessile, divided; flowers unisexual, solitary in leaf axils, plants monoecious; fruit an achene with an apical spinelike beak; family now thought to represent an ancient angiosperm lineage possibly related to some of the earliest flowering plants. |
| Horsetail family | Equisetaceae | Aerial stems erect, hollow, with vertical grooves embedded with silica; leaves scalelike and whorled; sporangia-bearing. |
| Hydrangea family | Hydrangeaceae | Vines or shrubs; leaves opposite, simple; flowers bisexual, regular, in cymes; fruit a capsule. |
| Indian-Pipe family | Monotropaceae | Non-green (lack chlorophyll), myco-heterotrophic (absorbing nutrients from mycorrhizal fungi that are attached to tree roots), herbaceous perennials, often whitish, pinkish, or yellow-brown; leaves alternate, scale-like; flowers bisexual, regular, solitary or in racemes; fruit a capsule. |
| Iris family | Iridaceae | Perennial herbs; leaves basal or alternate, usually distichous, linear to swordlike, parallel-veined and often equitant; flowers bisexual, regular or irregular, bracteate, in terminal clusters; fruit a capsule. |
| Laurel family | Lauraceae | Aromatic, deciduous shrubs and trees; buds scaly; leaves alternate, simple, entire or lobed; flowers unisexual (plants dioecious), regular, yellowish, in panicles or umbels; fruit a drupe. |
| Legume family | Fabaceae | Herbaceous or woody plants; leaves alternate or basal, simple or compound, stipulate, the petiole and petiolule base typically swollen; flowers bisexual, mostly irregular, in various clusters; fruit a legume. |
| Lily family | Liliaceae | Perennial herbs from bulbs; leaves various, simple and parallel-veined, with sheathing bases; flowers bisexual, regular, solitary or in various terminal clusters; superior ovary; fruit a berry or capsule; often toxic if eaten. |
| Lizard's-Tail family | Saururaceae | Perennial herb of wetlands, often in shallow water; leaves alternate, simple, entire, and cordate, stipulate; flowers bisexual, densely clustered in erect spikes, the tip drooping; fruit a schizocarp; family now considered as primitive angiosperms (paleoherbs) closely allied with the monocots. |
| Loosestrife family | Lythraceae | Annual and perennial herbs, or shrubs, mostly in wetland or aquatic habitats; leaves mostly opposite or whorled, simple and entire; flowers bisexual, regular or irregular, in various clusters; fruit a capsule. |
| Lotus family | Nelumbonaceae | Aquatic, perennial herbs; leaves floating, attached to the rhizome by long petioles, orbicular and peltate, entire with radiate venation; flowers bisexual, regular, solitary, on long peduncles; ovaries superior; fruits nutlike, embedded in leathery, inverted-conical receptacle. |
| Madder family | Rubiaceae | Annual or perennial herbs, woody vines, and shrubs; leaves opposite or whorled, simple, stipulate; flowers bisexual, regular, in various clusters; fruit a capsule, berry, or nutlet. |
| Magnolia family | Magnoliaceae | Deciduous or evergreen trees; leaves alternate, simple, entire or lobed, stipulate; flowers bisexual, regular, solitary; fruit an aggregate. |
| Maidernhair Fern family | Pteridaceae | Rhizome scaly; leaves 2-4 pinnate, the petiole with 1-several vascular bundles; rachis and costae dark brown and wiry; sporangia marginal in continuous or interrupted lines around edges of pinnules; covered by a false indusium; sporangia with annulus complete. |
| Mallow family | Malvaceae | Annual and perennial herbs, or shrubs; leaves alternate, simple, often palmately veined, stipulate; flowers bisexual, regular, mostly solitary or clustered in leaf axils; fruit a capsule, sometimes splitting into mericarps. |
| Mangosteen family | Clusiaceae | Annual to perennial herbs or shrubs; shrubby species with papery bark; leaves opposite, simple, entire, punctate or black-dotted; flowers bisexual, regular, solitary or in cymes; fruit a capsule. |
| Maple family | Aceraceae | Small to large deciduous trees; leaves opposite; flowers mostly unisexual, regular; fruit a samara. |
| Marsh Fern family | Thelypteridaceae | Rhizomes creeping, scaly; leaves various, often pubescent with ciliate margins and with needlelike trichomes on the rachis; petiole yellowish with 2 vascular bundles; sori dorsal, rounded to reniform; sporangia with annulus complete. |
| Meadow-Foam family | Limnanthaceae | Annual herbs; leaves alternate, pinnately divided; flowers bisexual, regular, solitary on long axillary peduncles; fruit separating into 1-3 mericarps. |
| Mezereum family | Thymelaeaceae | Shrubs with leathery bark; twigs with jointed nodes, the buds silky-hairy and end buds lacking; leaves alternate, simple, entire; flowers bisexual, regular, in axillary clusters; fruit a drupe; fruit and bark considered toxic. |
| Milkweed family | Asclepiadaceae | Perennial herbs, usually with milky sap; leaves various; flowers bisexual, regular, often in umbel-like clusters; fruit a pair of follicles, one often aborting; food source for Monarch Butterfly larvae; toxic to many animals. |
| Milkwort family | Polygalaceae | Annual, biennial, or perennial herbs; leaves various, simple and entire; flowers bisexual, irregular, in spikelike or headlike racemes; fruit a capsule. |
| Mint family | Lamiaceae | Annual or perennial herbs or shrubs, the stems square, and foliage often aromatic; leaves opposite, simple; flowers bisexual, mostly irregular, the corolla frequently 2-lipped; fruit splitting into 4 nutlets. |
| Moonseed family | Menispermaceae | Twining woody vines; leaves alternate, simple, venation palmate; flowers unisexual (plants dioecious), regular, greenish, in racemes or panicles; fruit of several drupes. |
| Morning-Glory family | Convolvulaceae | Annual or perennial herbs, twining or trailing in most; leaves alternate, simple; flowers bisexual, regular, in cymes or solitary; fruit a capsule. |
| Moschatel | Adoxaceae | Characterised by opposite toothed leaves, small five- or, more rarely, four-petalled flowers in cymose inflorescences, and the fruit being a drupe. A small family of flowering plants consisting of five genera and about 150–200 species. |
| Mulberry family | Moraceae | Annual herbs or trees, usually with milky sap; leaves alternate or opposite, simple, stipulate; flowers unisexual (plants monoecious or dioecious), regular, greenish, in axillary clusters; fruit a multiple of drupes or achenes. |
| Mustard family | Brassicaceae | Annual, biennial, and perennial herbs; leaves usually alternate; flowers bisexual, regular (4-petaled, with 6 stamens--- 4 long and 2 short), in racemes; seed capsules two-parted. |