The Linden family with forty-four genera is chiefly tropical, with more representatives in the southern than in the northern hemisphere. Of the three North American genera only Tilia is arborescent.
1. TILIA L. Bass Wood. Linden.
Trees, with terete moderately stout branchlets, without a terminal bud, large compressed acute axillary buds, with numerous imbricated scales, those of the inner rank accrescent, mucilaginous juice, and tough fibrous inner bark. Leaves conduplicate in the bud, long-petiolate, 2-ranked, cordate or truncate at the oblique base, acute or acuminate, serrate, deciduous, their petioles in falling leaving large elevated horizontal leaf-scars displaying the ends of numerous fibro-vascular bundles; stipules ligulate, membranaceous, caducous. Flowers nectariferous, fragrant, on slender clavate pedicels, in axillary or terminal cymes, with minute caducous bracts at the base of the branches, their peduncle more or less connate with the axis of a large membranaceous light green ligulate often obovate persistent conspicuously reticulate-veined bract; sepals 5, distinct; petals 5, imbricated in the bud, alternate with the sepals, sometimes thickened and glandular at the narrow base, creamy white or yellow, deciduous; stamens inserted on a short hypogynous receptacle; filaments filiform, forked near the apex, collected into 5 clusters and united at base with each other and (in the American species) with a spatulate petaloid scale (staminodium) placed opposite each petal, the branches of the filament bearing oblong extrorse half anthers; ovary sessile, tomentose, 5-celled, the cells opposite the sepals; style erect, dilated at apex into 5 spreading stigmatic lobes; ovules 2 in each cell, ascending from the middle of its inner angle, semianatropous, the micropyle centripetal-inferior. Fruit nut-like, woody, subglobose to short-oblong or ovoid, sometimes ribbed, tomentose, 1-celled by the obliteration of the partitions, 1 or 2-seeded. Seeds obovoid, amphitropous, ascending; seed-coat cartilaginous, light reddish brown; embryo large, often curved, in fleshy albumen; cotyledons reniform or cordate, palmately 5-lobed, the margins irregularly involute or crumpled; radicle inferior.
Tilia with some thirty species is widely distributed in the temperate regions of the northern hemisphere with the exception of western America, central Asia, and the Himalayas. Tilia produces soft straight-grained pale-colored light wood, largely used for the interior finish of buildings, in cabinet-making, for the sounding-boards of pianos, wood-carving and wooden ware, and in the manufacture of paper. The tough inner bark is largely manufactured into mats, cords, fish-nets, coarse cloths, and shoes. Lime-flower oil, obtained by distilling the flowers of the European species, is used in perfumery. The flowers yield large quantities of nectar, and honey made near forests of Tilia is unsurpassed in flavor and delicacy. Many of the species are planted as shade and ornamental trees, and some of the European species are now common in the gardens and parks of the eastern United States.
CONSPECTUS OF THE SPECIES OF THE UNITED STATES.
Surface of the leaves glabrous at maturity. Leaves glabrous or almost glabrous when they unfold, coarsely serrate. Leaves furnished with conspicuous tufts of axillary hairs, their lower surface light green and lustrous; pedicels glabrous or nearly glabrous.1. [T. glabra] (A). Leaves usually without tufts of axillary hairs, their lower surface not lustrous; pedicels densely hoary-tomentose.2. [T. nuda] (C). Leaves hoary-tomentose when they unfold. Leaves soon glabrous. Leaves coarsely serrate with stout teeth, their veinlets conspicuous; branchlets stout, bright red.3. [T. venulosa] (A). Leaves finely serrate with straight or incurved teeth, their veinlets less conspicuous; branchlets slender, pale reddish brown.4. [T. littoralis] (C). Leaves crenately serrate, glaucescent on the lower surface.5. [T. crenoserrata] (C). Leaves covered below early in the season with articulate hairs, becoming glabrous or nearly glabrous. Leaves thin, coarsely serrate, green or glaucescent on the lower surface, with or without tufts of axillary hairs; summer shoots not pubescent.6. [T. floridana] (C). Leaves subcoriaceous, finely serrate, bluish green and lustrous below early in the season; tufts of axillary hairs minute, usually wanting; summer shoots pubescent.7. [T. Cocksii] (C). Surface of the leaves pubescent below during the season. Lower surface of the leaves covered with short gray firmly attached pubescence; tufts of axillary hairs not conspicuous.8. [T. neglecta] (A, C). Lower surface of the leaves covered with articulate easily detached hairs. Branchlets without straight hairs. Leaves ovate, acuminate, usually obliquely truncate at base, glabrous above, their pubescence brownish or white.9. [T. caroliniana] (C). Leaves oblong-ovate, cordate or obliquely cordate at base, pubescent above early in the season.10. [T. texana] (C). Leaves semiorbicular to broad-ovate, abruptly short-pointed, deeply and usually symmetrically cordate at base.11. [T. phanera] (C). Branchlets covered with straight hairs; leaves ovate, abruptly short-pointed, oblique and truncate at base.12. [T. lasioclada] (C). Surface of the leaves tomentose below during the season with close firmly attached tomentum. Tomentum white, gray, or brown; leaves usually glabrous on the upper surface; branchlets and winter-buds glabrous (occasionally pubescent in varieties of 13). Branchlets slender; petioles not more than 1½′ in length; leaves oblong-ovate, acuminate or abruptly pointed, oblique and truncate or cordate at base; tomentum on the leaves of upper branches often brown; flowers ¼′—⅓′ long.13. [T. heterophylla] (A, C). Branchlets stout; petioles up to 3′ in length; leaves oblong-ovate, acuminate, obliquely truncate at base; tomentum always white; flowers 5/12′½′ long.14. [T. monticola] (A). Tomentum pale or brownish; leaves thickly covered above early in the season with fascicled hairs; branchlets tomentose; winter-buds pubescent.15. [T. georgiana] (C).
1. [Tilia glabra] Vent. Linden. Bass Wood.
Tilia americana L.
Leaves broad-ovate, contracted at apex into a slender acuminate entire point, obliquely cordate or sometimes almost truncate at base, coarsely serrate with incurved glandular teeth, often slightly pubescent when they first appear soon glabrous with the exception of tufts of rusty brown hairs in the axils of the principal veins below, thick and firm, dark dull green on the upper surface, lighter, yellow-green and lustrous on the lower surface, 5′—6′ long and 3′—4′ wide; turning pale yellow in the autumn before falling; petioles slender, 1½′—2′ in length. Flowers ½′ long, opening early in July on slender slightly angled pubescent pedicels, in few-flowered slender-branched glabrous cymes; peduncle slender, glabrous, the free portion 3½′—4′ long, its bract rounded or pointed at apex, 4′—5′ long, 1′—1½′ wide, decurrent nearly to the base or to within ½′—1′ of the base of the peduncle; sepals ovate, acuminate, densely hairy on the inner surface and slightly pubescent on the outer surface, a third shorter than the lanceolate petals; staminodia oblong-obovate, bluntly pointed at apex, a third shorter than the petals; ovary villose; style covered with rufous tomentum. Fruit short-oblong to oblong-obovoid, rounded or pointed at apex, ⅓′—½′ long, and covered with short thick rufous tomentum.