Combretum

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Combretum

Description

Woody lianas to at least 35 m, shrubs, or sometimes small trees to 7 m; ‘combretaceous hairs’ plus peltate scales or stalked glands present. Leaves opposite or more or less so, sometimes in whorls of three, not clustered at branchlet tips, sometimes with weak pocket-shaped domatia or axillary hair-tufts; without petiolar glands. Inflorescences terminal or axillary, lax to congested, simple to branched spikes, leafless or leafy, spikes often forming compound panicles; bracts usually very small and caducous, sometimes foliaceous. Flowers bisexual, actinomorphic or sometimes weakly zygomorphic, sessile but sometimes lower hypanthium extended into a pseudostipe, 4- or 5-merous; lower hypanthium extended into a short 'neck' or not so,upper hypanthium cupuliform, campanulate or tubiform, deciduous before fruiting; calyx lobes 4 or 5; petals 4 or 5; stamens 8 or 10, usually well exserted, anthers versatile; disk glabrous to densely pubescent; style free, usually exserted, glabrous, or sometimes pubescent proximally. Fruits 4-5-winged or -ridged, actinomorphic, dry or spongy.

Distribution

Africa: present Americas: present Asia: present Australia in the extreme northern tip of Queensland: present Guianas: present Pacific islands: present Southern America:, Venezuela (Venezuelapresent) eastern tropical Africa: present subtropical regions in N America: present subtropical regions in S America: present tropical America: present
About 255 species distributed throughout tropical America, Africa and Asia, but not in the Pacific islands, just reaching Australia in the extreme northern tip of Queensland, extending to subtropical regions in S America (to 33° S) and subtropical regions in N America (to 30° 35' N); greatest number of species and greatest variation occurs in eastern tropical Africa; in the Americas only 29 species, 9 occur in the Guianas (plus 1 nearby in Venezuela).

Taxonomy

The genus is divided in 3 subgenera and 52 sections, of which 2 and 11 respectively occur in America, and 2 and 5 respectively in the Guianas:
  • Subgenus 1. Combretum. Scales present, stalked glands usually absent; flowers 4-merous, petals glabrous; fruits 4-winged or -ridged. (Sections 1-3).
    • Section 1. Thiloa. Upper hypanthium deeply cupuliform with scarcely developed calyx lobes; petals absent; stamens 4, scarcely or not exserted; ovules 2; fruits 4-ridged; ‘combretaceous hairs’ present only inside flowers; scales large and conspicuous. Combretum paraguariense.
    • Section 2. Combretum. Upper hypanthium cupuliform to broadly infundibuliform; petals 4, from shorter than to about as long as calyx lobes; stamens 8, much exceeding calyx lobes; ovules (3-)5-7(-8); fruits 4-winged; ‘combretaceous hairs’ very sparse to abundant; scales large and conspicuous. Combretum fruticosum, C. rohrii, C. rotundifolium.
    • Section 3. Combretastrum. Upper hypanthium cupuliform, without distinct upper and lower regions; petals 4, exceeding calyx lobes; stamens 8, well exceeding calyx lobes; ovules 1-2 (-3); fruits 4-winged or -ridged; ‘combretaceous hairs’ abundant to extremely sparse; scales small. Combretum fusiforme, C. laxum, C. pyramidatum.
  • Subgenus 2. Cacoucia. Scales absent, stalked glands present (usually very inconspicuous); flowers mostly 5-merous, petals pubescent; fruits usually 5-winged. (Sections 4-5).
    • Section 4. Spinosae. Upper hypanthium cupuliform to infundibuliform, without distinct upper and lower regions; petals 4-5, exceeding calyx lobes; stamens 8-10, well exceeding calyx lobes; ovules 4-8; fruits 4-5-winged or -ridged; ‘combretaceous hairs’ sparse to abundant; peltate scales absent. Combretum spinosum.
    • Section 5. Cacoucia. Upper hypanthium deeply curved-cupuliform, without distinct upper and lower regions; petals 5, slightly to much longer than calyx lobes; stamens 10, well exceeding calyx lobes; ovules 3-4; fruits 5-ridged; ‘combretaceous hairs’ sparse to abundant; peltate scales absent. Combretum cacoucia.

Wood

, , or ;1 Growth rings faint to distinct.
Vessels diffuse, of 2 distinct sizes: normal vessels solitary, 11-15 per mm² on av., mean tangential diameter 115-126 μm, mixed with narrow vessels, mean tangential diameter 20-30 μm. Perforations simple. Intervessel pits alternate, round to polygonal, 5-7 μm but those of the very narrow vessels infrequently elongate, vestured; vessel-ray pits similar to intervessel pits but half-bordered, sometimes in distinct horizontal rows and coalescent.
Rays uniseriate, ca 10-20 (25) per mm, composed of square and (weakly) procumbent cells, with some rows of upright cells; upright cells more frequently present in C. fruticosum and C. pyramidatum. Dark contents in the rays of C. cacoucia.
Parenchyma apotracheal diffuse, in marginal bands and paratracheal, vasicentric-aliform; mainly aliform-confluent in C. pyramidatum; strands of 4-6(-8) cells.
Ground tissue fibres thin-walled and septate in C. fruticosum, thin- to thick-walled and partly septate in C. pyramidatum, thick to very thick-walled and non-septate in C. rotundifolium; frequently gelatinous. Pits simple to minutely bordered, mainly on radial cell walls.
Crystals in all species studied. Enlarged cells containing one large solitary crystal in the rays and in few axial parenchyma strands of C. fruticosum. Small to large rhombic crystals in ray cells of C. cacoucia and C. laxum, rhombic crystals filling the cells in axial parenchyma strands of C. rotundifolium.
In the Guianas 9 species occur, 5 of which are represented in this study. Van Vliet (1979) published a very detailed wood anatomical description of Combretum, based on material of 14 species, 3 of which are from the Neotropics. The variation he reports is considerable, particularly in mean quantitative characters such as percentage of solitary vessels, vessel frequency and diameter, and the presence of very narrow vessels. However, the variation was mainly due to African representatives. Although included phloem was found in part of the African species, it was absent from the material from the Neotropics. Likewise, radial vessels, were seemingly absent.
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Notes

Taxonomic problems in the Guianas concern the distinctions between C. laxum, C. pyramidatum and C. fusiforme.
Combretum indicum (L.) Jongkind (Quisqualis indica L.), from Asia, is a widely planted tropical ornamental liana or shrub, easily distinguished from all Guianan species of Combretum by its scented flowers with a long (38-78 mm) narrow (1.5-3 mm) upper hypanthium and 5 patent pink to red petals 9-20 x 6-13 mm. It is naturalized in some areas of tropical America but apparently not so in the Guianas. In the Flora of Suriname (Exell 1935: 170-171) this species was included but the 2 cited specimens from Paramaribo were cultivated examples, like others since.