Oaks of the World

Botanical characters

Genus : Quercus L.
Family : Fagaceae Dumortier
Order : Fagales


FAGACEAE
Trees or shrubs, deciduous or evergreen; monoecious.
Stipules early deciduous. Leaves alternate, simple, spirally arranged.
Inflorescences : unisexual or androgynous catkins (in genus Quercus, inflorescences are always unisexual). Male and androgynous catkins spicate, consisting of few- to many-flowered clusters; pistillate catkins spicate, with one to several cupules bearing the pistillate flowers.
Male flowers with bracts often deciduous; 4-6(9) sepals ; 6-12 or more stamens; petals absent; pistillode often present.
Female flowers with 4-6 sepals, no petals, 1 pistil, 3 or more carpels, ovary inferior, styles and locules as many as carpels, placentation axile, 2 pendulous ovules in each locule, all but 1 aborting; staminodes sometimes present.
Fruits : nuts 1-seeded, enclosed by multibracteate cupule; no endosperm; embryo as long as seed.
Genera 8-9 : Castanea, Castanopsis, Chrysolepis, Colombobalanus, Fagus, Formanodendron, Lithocarpus, Quercus (divided into 2 sub-genera : Quercus and Cerris), Trigonobalanus. Genus Nothofagus has recently been placed in a new monotypic Family : Nothofagaceae Kuprianova.
The main feature of Fagaceae is the cupule : it is the cup of the acorn in Quercus and Lithocarpus, and the spiny bur surrounding fruits of Castanea and Chrysolepis.
900-1000 species; widespread, forming forest trees in temperate to sub-tropical areas of Northern Hemisphere. Interspecific hybridization is common, particularly in Quercus.


Genus Quercus L. 1753

Trees to 55 m, or shrubs; monoecious, deciduous or evergreen; bark smooth or furrowed or scaly, sometimes corky; buds with imbricate scales; leaves spiraly arranged, stipules mostly deciduous and inconspicuous, sometimes retained until the end of the vegetative period; lamina chartaceous or coriaceous, lobed or not, margin entire, dentate, or dentate with bristle tips; lateral veins pinnate; intercalary veins present or not; inflorescences in axils of leaves or buds, usually clustered at base of new shoot; staminate inflorescences lax, racemose to spicate; female inflorescences usually stiff, a simple spike, with terminal cup and sometimes with one to several lateral sessile cups; male flowers subsessile, solitary or in clusters of 2-7: sepals connate, stamens (2-)6(-12), pistillodes reduced and replaced by tufts of silky hairs; female flowers with lobed, connate sepals; 3-6 carpels and styles, sometimes with staminodes, styles with broad stigmatic surface; ovules pendent, anatropus or semi-anatropus; position of aborted ovules apical, basal or lateral; fruit a one-seeded nut (acorn) with maturation annual or biennial, round in cross-section, not winged, cotyledons free or fused; endocarp glabrous or tomentose; cup covering at least base of acorn, with scales or lamellate rings; scales imbricate, flattened or tuberculate, not or weakly to markedly reflexed; chromosome number x = 12.
Ca. 430 species, mostly in Northern Hemisphere.

1/ Quercus L. sub-genus Quercus
Sporopollenin masking rugulae in mature pollen grain surface

a) Section Protobalanus
8-10 stamens with apiculate apex; styles short to long, elliptic in cross-section; stigmata abruptly dilated; fruit maturation biennial; endocarp tomentose; position of aborted ovules variable, even in a single plant; cup-scales triangular, fused at the base, thickened and compressed into rings, often tuberculate and obscured by glandular trichomes, with sharp angled tips; leaf dentition spinose. Five species in North Americ and notrhwestern Mexico.

b) Section Ponticae
Shrubs or small trees, rhizomatous; number of stamens mostly 6; male catkins up to 10 cm long; styles short, fused or free, elliptic in cross-section; stigmata abruptly or gradually dilated; fruit maturation annual; endocarp glabrous; positiion of abortive ovules basal; cotyledon free; cup-scales tuberculate with sharp angled apices, occasionally with attenuated tips; leaves evergreen or deciduous, chestnut-like, dentate with teeth simple or compound, sharply mucronate or with thread-like, curved upwards extension; stipules long, persistent or early shed; lateral veins 10-15(-25); buds with scales loosely attached. Two species in North America and in Turkey and Georgia.

c) Section Virentes
Trees or rhizomatous shrubs; styles short, free or fused, elliptic in cross-section; stigmata abruptly or gradually dilated; fruit maturation annual; cup scales narrowly triangula, free or fused at the base, thinly keeled and barely tuberculate with sharp angled tips; leaves evergreen or subevergreen; cotyledons fused; ca. 7 species in North America, Mexico, Cuba, Central America.

d) Section Quercus
Stamens at least 7; styles short, fused or free, elliptic in cross-section; stigmata abruptly or gradually dilated; fruit maturation annual; endocarp glabrous or nearly so; cotyledons free or fused; position of aborted ovules basal; placenta and funiculus sessile; cup scales triangular, free or fused at the base, thickened, keeled and often tuberculate with sharp angled apices, sometimes with attenuate tips; leaves with teeth without bristle-like, aristate tips. Ca. 150 species en North America, Mexico, Central America, West Eurasia, East Asia and North Africa.

e) Section Lobatae
Female perianth forming a characteristic flange; stamens 6 or less; styles elongated, linear, outcurved, elliptic in cross-section; stigmata slightly dilated, spatulate to oblong; stylopodium conical, often annulate; fruit maturation biennial, rarely annual; endocarp tomentose; cotyledons free or sometimes basally fused; position of abortive ovules apical or rarely lateral to basal; placenta sessile or elongated, funiculus sessile; cupule fused to the peduncle, forming a connective piece covered with small scales similar to those on the cup; cup scales triangular, free, mostly thin, membranous and smooth with broadly angled tips ; leaf teeth and lobes with bristle-like extensions, teeth reduced to bristles in entire or nearly entire leaves. Ca. 120 species in North America, Mexico, Central America and Colombia.

2/ Quercus L. sub-genus Cerris
Sporopollenin not or weakly masking rugulae in mature pollen grain surface

a) Section Cyclobalanopsis
Male flowers in groups of 1-3(-7) along inflorescence axis; stamens 5-6; styles short to very short (< 3mm to < 1mm), elliptic in cross-section; stigmata dilated, subcapitate; stigmatic surface not forming a prominent stigmatic groove; stylopodium annulate with 3-5 distinct rings; fruit maturation annual or biennial; endocarp tomentose or rarely glabrous; cotyledons free; position of aborted ovules apical (position variable); placenta elongated reaching the apical part of the seed, where vascular bundles enter the seed and the aborted ovules; funiculus sessile or with a short petiole; cup with several rows of concentric lamellae; leaves evergreen; teeth with or without bristle-like extensions. Ca. 90 species in tropical and subtropical Asia (including the southern Himalayas).

b) Section Ilex
Stamens 4-6; styles medium-long, elongated, apically gradually dilated, recurved, v-shaped in cross-section; stigmata slightly subulate; fruit maturation annual or biennial; endocarp tomentose; cotyledons free; position of aborted ovules basal or lateral; placenta and funiculus sessile or elongated; cup scales triangular, free or basally fused, mostly thin, membranous, often keeled and tuberculate with sharp angled apices, occasionally with slightly raised tips or narrowly triangular, thickened with elongated recurved tips (as in Q. alnifolia, Q. baronii, Q. coccifera, Q. dolicholepis); leaves persistent, teeth spinose or with bristle-like extensions. Ca. 35 species in Eurasia and North Africa.

c) Section Cerris
Stamen 4-6; styles elongated, outcurved, pointed, v-shaped in diameter; stigmatic area linear; fruit maturation biennial (variable only in Q. suber); endocarp tomentose; cotyledons free. position of abortive ovules basal, lateral or apical; placenta sessile, funiculus elongated or sessile; cup scales narrowly triangular, thickened and keeled with elongated, well-developed recurved tips; leaf dentition with bristle-like extensions. Ca. 10-12 species in Eurasia and North Africa.

The genus Quercus is a very important (perhaps the most) woody genus in the Northern Hemisphere (especially in Mexico with 160 species, followed by Asia with nearly 120 species and by North America with 90 species). In the past, oaks were an important source of fuel, fodder and materials; leaves and cork also gave tannins; acorns were eaten by humans, but today only cattle and wild animals eat them.