Flora of Tropical East Africa - Xyridaceae (1999)

J. M. Lock

• Publisher: CRC Press
• Year: 2020
• Print ISBN: 9789061913856
• eBook ISBN: 9781000162479
• Edition: 1
• Topic: Biological Sciences
• Subtopic: Botany
• Index: Biological Sciences

The flora is prepared at Royal Botanic Gardens, Kew, in close collaboration with East African Herbarium and in liaison with the University of Dar es Salaam, the University of Nairobi and the Makerere University. Significant contributions are also made by specialists elsewhere. The flora is designed to a high academic standard and should be a useful resource reference for anyone concerned with the identification and utilization of plants in eastern Africa. Each family is published as a separate part.

FLORA OF TROPICAL EAST AFRICA

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XYRIDACEAE

J.M. LOCK

Annual or perennial herbs, often in seasonally or permanently wet sites; stems upright, base sometimes swollen in perennial species. Leaves alternate, simple, linear, distichous, few to numerous, basal sheath open, blade flattened to terete. Inflorescence a condensed pedunculate spike; flowers in the axils of densely crowded often coriaceous bracts, forming a spherical, ovoid or elongate head (the spike). Flowers 3-merous, bisexual; calyx-lobes 3, the adaxial at first forming a hood-like structure over the bud, the laterals smaller; corolla tubular, divided above into 3 broad spreading usually yellow but sometimes white, blue or orange petals. Stamens 3, opposite the petals, often with 3 staminodes alternating with them. Ovary (1—) 3-locular, with numerous ovules and axile or parietal placentation; style 1, sometimes divided into 3 at the apex. Fruit a loculicidal or irregularly dehiscent capsule. Seeds numerous, small, with endosperm.

Pantropical, with 5 genera and ± 260 species of which 240 are in Xyris. Most diverse in South America, and extending into the warm temperate parts of North America; ± 25 species in East Africa. A medium-sized family of mostly small annual or perennial herbs.

In South America the inflorescences of a few species are collected and sold as dried flowers; otherwise the only economic importance of the family is as weeds of wet places, particularly rice fields.

XYRIS

L., Sp. Pl.: 42 (1753) & Gen. Pl., ed. 5: 25 (1754); N.E. Br. in F.T.A. 8: 6-25 (1901); Malme in E.J. 48: 287-308 (1912)

Annual or perennial herbs, often forming clumps; rhizomes, when present, short, horizontal or ascending. Leaves virtually all basal; ligule present or not. Inflorescence a condensed bracteate spike at the end of elongated peduncles which are often variously grooved or ridged; lower bracts of the spike sterile, occasionally elongated or reflexed. Flowers borne singly in the axils of the fertile bracts, short-lived; calyx of 3 unequal segments, the adaxial hooded and enclosing the flower-bud, the laterals asymmetric and often keeled, free from one another (sometimes fused basally in American species); corolla yellow (blue in at least one West African species). Stamens alternating with 3 usually bifurcate staminodes divided apically into many moniliform hairs. Ovary with parietal placentation (in East Africa). Fruit a loculicidal capsule.

Useful descriptions and illustrations of character states are given by Kral (Ann. Missouri Bot. Gard. 75: 522-722 (1988)).

All the African species belong to section Xyris, characterised by parietal placentation in a unilocular ovary. Of the other two sections, Pomatoxyris Endl. occurs only in Australia and Nematopus Seub. only in Central and South America. However, Kral has cast doubt on the validity of the sections as intermediate types of placentation occur.

Although the species of the genus generally look rather different, accurate identification usually depends on the observation of small features for which a x 10 or x 20 lens or binocular microscope is essential. Anatomical characters may be useful, but have not been investigated for most African species. Characters of value, which should be borne in mind when collecting, include:

Life-span — annual or perennial? If perennial, is there a swollen bulb-like base? Collect if present.

Leaf sheaths — these should be complete and clean; surface texture and colour useful.

Leaf — make sure these are present and not grazed or broken off.

Leaf surface ornamentation — spare leaves for sectioning.

Peduncles — material for

Bracts of the — a of is useful; include for dissection.

Lateral sepals — the keel may be entire or variously dentate or ciliate; examine dissected sepals from both sides to get a clear view of the keel.

Flowers — do not dry well; material is useful.

Fruits — usually present in mature spikes, but tend to dehisce and lose their seeds; spare mature spikes.

Seed shape and surface ornamentation — seeds must be ripe; use x 20 or x 40.

The flowers of some species have been noted as opening in the morning. Doust & B.J. Conn (Austr. Syst. Bot. 7: 455-484 (1994)) give fairly precise opening times for the flowers of the species they describe; similar data for African species would be welcome.

The family has probably been undercollected in East Africa and this account should be regarded as more preliminary than many in the Flora. Most major recent collecting expeditions have produced either new species or new records for East Africa and there can be no doubt that new taxa remain to be found and described. Areas worthy of special attention include south-western Uganda, the Kitale area in Kenya, and western and southern Tanzania. Various collections mentioned in the notes, and the three taxa here listed as sp. A, B & C may well represent new species but without really good material I hesitate to describe them.

Purseglove 3634 (Uganda, Kigezi District, Kachwekano Farm, June 1951) has the lowest sterile bract greatly elongated so that it is ± 5 x as long as the spike. While the specimen is poor and sparse, and may well be from an abnormal plant, species with similar bracts occur in South America and collectors in SW. Uganda should look for more material.

X. valida Malme occurs in Rwanda and Zaire (Kivu), and is to be expected in SW. Uganda. It is usually a very robust perennial plant of swamps. It would probably key out as X. capensis but the peduncle is 2-winged, and the wings are pilose below the spike.

Fertile bracts red-brown, papillose, with ciliate-lanate margins; a small annual....	20. X. sp. C
Bracts not as above; annual or perennial:
Outer (sterile) bracts white, pale grey or pale buff, sometimes with a narrow darker median line:
Leaves and peduncles strongly rugulose; outer bracts erect, thin and papery, more than 10 mm. long....	1. X. kornasi ana
Leaves and peduncles not rugulose; outer bracts erect or spreading, firmer in texture, smaller:
Fertile bracts irregularly ciliate-dentate on the margins, with a strong dorsal mark....	2. X. erosa
Fertile bracts with entire margins, with a paler median line but no distinct dorsal mark....	3. X. ednae
Outer (sterile) bracts yellowish, reddish, brown or black:
Keel of lateral sepals entire:
Inflorescence bracts with a distinct subterminal triangular or rhomboid mark; inflorescence broadly ovoid to subspherical at maturity....	9. X. anceps
Inflorescence bracts without such a mark; variously shaped when mature:
Inflorescences 5 mm. or more long, ellipsoid or subspherical:
Mature inflorescences ellipsoid; peduncles two- ridged below the inflorescence; plants usually clearly perennial....	12. X. makuensis
Mature inflorescences subspherical; peduncles ± terete below the inflorescence; plants usually clearly annual (but see notes to text)....	14. X. capensis
Inflorescences less than 5 mm. long (or, if more, then leaf surfaces rugulose at least in the lower parts of the leaf):
Inflorescences dull brown, obtriangular at maturity; small perennial,,,,	13. X. huillensis
Inflorescences brown, straw-yellow or reddish, ellipsoid or subspherical; annuals:
Leaf sheaths rugulose, at least towards the base:
Mature inflorescence ellipsoid; bracts yellowish brown....	21. X. straminea
Mature inflorescence subspherical; bracts yellowish or reddish:
Fertile bracts pale red-brown with black apiculate apices; lowest sterile bracts arist...

Table of contents

• Cover
• Title Page
• Copyright Page
• Xyridaceae
• Index to Xyridaceae