AFABEGO-Association française des amateurs de bégonias

A little botany: Leaves

Leaf characters that determine which section a Begonia belongs to:

Arrangement

Alternate leaves

The most common case in the genus Begonia.

    • B. foliosa Kunth / section Lepsia / Colombia, Venezuela, Ecuador: alternate leaves

Opposite leaves

Rarer in the genus Begonia.

    • B. sibthorpioides Ridley / section Heeringia / Malaysia – Thailand: the upper leaves of this Begonia are opposite

Distichous leaves, close together

Arranged in two vertical rows in the same plane; this character is most often found on creeping plants.

    • B. thelmae L.B. Smith & D.C. Wasshausen / section undetermined / Brazil: distichous leaves

Whorled leaves

Inserted at the same level, in a circle, in the same horizontal plane: found on acaulescent (stemless) plants.

    • B. adenopoda Lemaire / section Lauchea / Myanmar: plant with whorled leaves

Number

1 or 2 leaves

    • B. sambiranensis Humbert / section Quadrilobaria / Madagascar: single-leaved plant

More than 2 leaves

Most begonias.

    • B. minutifolia N. Hallé / section Filicibegonia / Gabon (endemic): numerous leaves

Position in relation to the petiole

Straight leaves

When the main vein is exactly in line with the petiole.

    • B. molleri Warburg / section Tetraphila / São Tomé: straight leaf, the main vein is in line with the petiole

Oblique leaves

When the main vein makes an angle of about 45° with the petiole.

    • B. sinuata Wallich / section Parvibegonia / Malaysia: oblique leaf, the petiole makes a 45° angle with the main vein
    • B. horticola Irmscher / section Tetraphila / Central Africa: oblique leaf, the petiole makes a 45° angle with the main vein

Transverse leaves

When the main vein is at (or nearly at) a right angle to the petiole.

    • B. formosana Masamune / section Platycentrum / Taiwan: transverse leaf, the petiole is perpendicular to the blade

Symmetry

Symmetrical blade

Rare in the genus Begonia.

    • B. horticola Irmscher / section Tetraphila / Congo – Zaire – Uganda: almost symmetrical blade

Asymmetrical blade

This is the most common case.

    • B. silletensis CB Clarke subsp. mengyangensis Tebbitt / section Sphenanthera / China: strongly asymmetrical blade

Peltate or not

Peltate leaf

The petiole is attached within the blade.

Non-peltate leaf

The petiole is attached at the edge of the blade.

    • B. garagarana C de Candolle / section Gireoudia / Panama: non-peltate leaf

Divisions

Simple or entire leaf

The leaf has no divisions.

    • B. mariannensis  Wasshausen & McClellan / section Knesebeckia / West Indies (Trinidad) / entire leaf

Palmately lobed leaf

It is divided radially into lobes (divisions not exceeding half the blade), arranged like the fingers of a hand

    • B. China sp. / section Diploclinium / China: palmately lobed leaves

Palmatifid leaf

The same, but the divisions reach halfway into the blade / a rare character in the genus Begonia.

    • B. antsiranensis Aymonin & Bosser / section Quadrilobaria / Madagascar (endemic): palmatifid blade

Palmatisect leaf

The divisions reach almost to the base of the blade.

    • B. diadema Linden / section Platycentrum / Malaysia – Borneo: palmatisect leaf
    • B. mananjebensis Humbert / section Quadrilobaria / Madagascar: palmatisect leaf
    • B. pedatifida Léveillé / section Platycentrum / China: palmatisect leaf

Pinnatifid leaf

Not palmate but pinnate (leaflets arranged like the barbs of a feather on either side of the rachis); the divisions do not reach the midrib.

    • B. aspleniifolia Hooker f. / section Filicibegonia / endemic to Gabon: pinnatisect blade: pinnatifid, with divisions exceeding half the blade

Bipinnatifid or even more divided leaf

When the pinnatifid divisions are themselves pinnatifid.

    • B. bipinnatifida JJ Smith / section Petermannia / New Guinea: bipinnatifid leaf

Compound leaf

The blade is divided into several leaflets, each with its own petiole; a rare character in the genus Begonia.

    • B. hemsleyana J.D. Hooker / section Platycentrum / China: palmately compound leaf

Filiform leaf

The blade is segmented.

    • B. sp. Madagascar / section Erminea / endemic to Madagascar: filiform leaf in twice-divided segments

Veins

Palmate veins

Arranged like the fingers of a hand.

    • B. sericoneura Liebmann / section Gireoudia / Central America: palmate veins
    • B. beccarii Warburg / section Diploclinium / Malaysia (Borneo: Sarawak): palmate veins

Palmate-pinnate veins

The first veins are palmate and the following ones pinnate along the main vein.

    • B. humilis Dryander / section Doratometra / From the West Indies to Peru and Brazil: palmate-pinnate veins

Pinnate veins

The veins are arranged like the barbs of a feather.

    • B. auriculata JD Hooker / section Filicibegonia / Gabon: pinnate veins
    • B. ulmifolia Willdenow / section Donaldia / Guiana Shield: pinnate veins

Indumentum

(Characters not visible to the naked eye)

  • Presence on the leaves of more or less circular or star-shaped “scales” called trichomes : plants of sections Baccabegonia, Squamibegonia and Tetraphila and a few species of section Loasibegonia.
  • Presence of star-shaped “hairs”: on a few Asian and South American species of sections Knesebeckia and Pritzelia.

Characters that do not determine membership of a section:

Petiole

No petiole

Sessile leaves.

    • B. polygonoides JD Hooker / section Tetraphila / Tropical Africa: almost sessile leaves

Glabrous or hairy

    • B. picturata Y. Liu, S-M Ku & Ch-I Peng / section Coelocentrum / China (Guangxi): densely hairy petiole
    • B. roxburghii A. de Candolle / section Sphenanthera / India – Myanmar: glabrous petiole

Shape of the blade

Cordate

Heart-shaped.

Auriculate

Has an “auricle” (small ear) on the wider side of the blade, at its base.

    • Gabon sp. nov / asymmetrical blade, with a well-formed auricle on one side of the base and cordate on the other

Obtuse at the base or apex

Rounded at the tip or base.

    • B. kisuluana Büttner / section Tetraphila / from Nigeria to Angola: leaves obtuse at the apex

Lanceolate

Lance-shaped (widest part at the base).

    • B. henrilaportei Scherberich & Duruisseau / section Quadrilobaria / Madagascar: linear-lanceolate blade, almost symmetrical, with pinnate veins, incised.
    • B. komoensis Irmscher / section Tetraphila / Equatorial Africa: lanceolate blade

Cuneate base

Wedge- or triangle-shaped.

    • B. maynensis A de Candolle / section Knesebeckia / Ecuador – Peru: lanceolate blade with a cuneate base

Circular or subcircular

    • B. atroglandulosa  Sosef / section Loasibegonia / endemic to Gabon: almost circular blade

Falcate

Sickle-shaped.

    • B. poculifera JD Hooker / section Squamibegonia / Tropical Africa: falcate blade

Surface of the blade

Glabrous

Smooth blade, without hairs.

Pubescent or hirsute

Blade with dense or sparse hairs, short or long, spine-like (spiculiform or spiculate), in various colours.

    • B. sizemoreae R. Kiew / section Platycentrum / Vietnam: upper surface of the blade with large white hairs
    • B. limprichtii Irmscher / section Platycentrum / China: hairy blade

Bullate

Blistered surface.

    • B. susaniae Sosef / section Scutobegonia / Cameroon – Gabon: bullate blade, each “blister” bearing a spiculiform hair
    • B. bullatifolia Kollmann / section Knesebeckia / Brazil (Espírito Santo): bullate blade, without hairs

Margin

Entire

The margin has no incisions.

Dentate

Margin cut into teeth that are more or less large and more or less regular; it is called serrate when finely toothed.

    • B. augustinei Hemsley / section Platycentrum / China: serrate margin, with a longer tooth where the veins reach the margin

Crenate

Margin edged with rounded teeth.

    • B. furfuracea JD Hooker / section Tetraphila / Cameroon – Bioko: leaf with a crenate margin

Ciliate

Margin with more or less long hairs.

    • B. jingxiensis D. Fang & Y.G. Wei var. mashanica / section Diploclinium / China (Guangxi): ciliate margin

 

Bibliography

  • The sections of Begonia / J. Doorenbos – M.S.M. Sosef – J.J.F.E. de Wilde
  • Begonias of Peninsular Malaysia / Ruth Kiew

  • BEGONIACEAE, Edition 2 / Part I : Annotated Species List / Jack Golding