Introduction

As an adjunct to seed storage for genome preservation, orchid pollen storage has much to offer: to the hybridist wishing to overcome flowering asynchrony in species and/or to introduce wild genome contributions into cultivated taxa; to the gene bank manager seeking to preserve a large quantity of genetic material in a small facility; and to the conservationist anxious to preserve species, even if this can only be achieved through the pollination of plants held ex situ in orchid collections with stored pollen. Pollen storage may also be of considerable future importance to the biotechnologists, if the process of haploid plant production via embryo development from pollen grains (i.e. androgenesis) can be extended to orchids.

The literature on orchid pollen germination is surprisingly limited with previous studies concentrating on optimising the composition of the germination medium, particularly the sugar level. Pfundt (1910), working with two European species and a range of sugar levels from 5–20%, observed a preference for 5–10% sugar. Miwa (1937) similarly found orchid pollen generally germinates best when using cane sugar at around 3–10%. Although more exacting studies on the optimal chemical composition of the germination medium have been performed, particularly with reference to plant hormones (Curtis & Duncan 1947; Rao & Chin 1972), sugar level remains the most important single chemical factor in stimulating orchid pollen germination on artificial medium; its action being osmotic, preventing grain bursting whilst avoiding plasmolysis, rather than as a heterotrophic source of carbon (see Stanley & Linskens 1974).

Introduction

The application of in vitro techniques to the propagation of plants at Kew originated from Dr Peter Thompson's work in the 1960s on the formulation of a medium for orchid seed germination. By 1971 the commercial and research potential of plant propagation by aseptic culture was becoming more apparent and the Director of Kew at the time, Professor Heslop-Harrison, determined that it was time to make the techniques available to the living collection. Plans were made for a unit to provide in vitro propagation services to the sections of the Living Collections Division at Kew. The Unit based at Aiton House was set up and opened in 1977, under the present Curator, Mr John Simmons.

The living orchid collection at Kew comprises approximately 3,500 species represented in 375 genera. Each year the resultant seed from a pollination programme is germinated at the Micropropagation Unit. Seed is also given to the Unit by botanists from field collections, from private and commercial orchid growers and from members of the general public. The main function of the Unit is to supply the orchid collection with new species and in many cases provide a back-up of viable seedlings for those species which present special cultivation problems. As much information as possible is recorded about the seed including such details as the site and habitat type where the collection was made. This type of information helps decide the type of culture conditions used.

Summary

The understanding of the host–fungus relationships in the mycorrhizas of orchids is important in relation to the application of symbiotic methods to seed germination and seedling development, and also for re-establishment in natural conditions either from seed or tissue culture as one of the contributions to conservation. Despite extensive progress in knowledge of the role of fungi in the early stages of germination, the inherent difficulty in the use of the symbiotic technique has inhibited its application.

Orchid fungi are extremely variable and relatively few root inhabitants are true mutualistic symbionts. The outcome of the relationship between the partners is a finely balanced one and many fungi isolated from orchid mycorrhizas may, after a period in culture, become incompatible with, or even pathogenic to, orchid protocorms.

Mutualistically symbiotic fungi enhance the nutrition of germinating seeds by the transference of carbon compounds. Photosynthetically active seedlings and mature plants, however, may be quite independent of their fungal partners. Evidence suggests that in conditions of nutrient stress the fungal partner may mediate in the movement of phosphate and/or nitrogen compounds, as in other mycorrhizal systems.

Introduction

Orchids, whether epiphytic or terrestrial, generally grow and thrive in conditions of nutrient impoverishment. As with other higher plants growing in similar environments this may only be possible by the association of fungi with the roots and other subterranean parts – the mycorrhizal association.

A large family well represented in West Africa, at least half of the genera containing species which are widespread throughout West Africa, many of these, in addition, in Gambia.

Introduced garden species give a useful introduction to the general appearance of members of this family. Allamanda spp., oleander (Nerium oleander), Madagascar periwinkle (Catharanthus roseus), frangipani (Plumeria spp.) and herald's trumpet (Beaumontia grandiflora) are all widely available.

Members of the family may be recognised by their opposite, simple, entire, generally exstipulate leaves (though often with interpetiolar ridges), possession of latex, and cymes of fragrant, gamopetalous 5-part flowers with contorted corolla lobes. The fruit is a berry of two carpels, two berries (of one carpel each), or a pair of follicles (or one by abortion) with plumed seeds. Most of the widespread species are climbers (some with branch tendrils) or under-storey trees in forest, a few only being open woodland savanna species.

Four genera diverge sufficiently from the ‘family pattern’ as to be recognisable on sight. The only species with spirally arranged leaves (and prominent leaf scars) is a fleshy stemmed deciduous shrub, Adenium obesum, found (and often planted) in the northern guinea and sudan zones of Nigeria.

Carissa edulis, in the same zone (Ghana–Nigeria) has branch spines, the only species in the family in West Africa to do so.

A tropical and temperate family, represented in West Africa by large woody or fibrous perennial herbs up to 3 m high, less often by shrubs and rarely by trees. Most species are deep rooting, withstand drought, and are prominent in drier savannas.

The decorative introductions include several Hibiscus spp.: H. mutabilis (blushing Hibiscus) and H. rosa-sinensis (rose of China, shoe flower) from China, H. schizopetalus (frilled Hibiscus) from East Africa.

Members of the family may be recognised by their alternate, stipulate simple leaves with digitate venation (pinnate in Malvastrum and Sida) with stellate hairs on young growth (scales in Thespesia). Ptyxis is mostly conduplicate, conduplicate-plicate in Abutilon and Pavonia. On the back of the leaf, glands occur in patches, depressions or pits (concealed in small swellings) (absent in Pavonia). The flowers are axillary, five-part and often large, lasting a day only, their colour often changing during that time. The petals are veined or basally blotched, and the column of anthers in the centre of the flower is prominent.

Flowers ⊕ (rarely dioecious) 5-part. The flower is usually surrounded by 3(–13) united bracteoles, adnate to the calyx and forming an epicalyx in Abelmoschus, Pavonia and Urena etc., free from the calyx and forming an involucel in most genera and often associated with extrafloral nectaries (Cienfuegosia); bracteoles free in Gossypium and Thespesia; in Malachra, there is an involucre round each head of flowers, while bracteoles are lacking in Abutilon etc.

A family of woody plants in all the warmer parts of the world, represented in West Africa by trees, some shrubs and shrubby epiphytes, and a few stranglers.

Members of the family may be recognised by their latex and alternate stipulate leaves. The leaves may be pinnately veined, relatively small, and conduplicate in the bud, when the stipules are also small (but see Ficus carica and Artocarpus heterophyllus) or the stipules may be intrapetiolar, large and hoodlike, leaving circular scars on the stem. The leaves are then rolled (supervolute) in the bud (Ficus etc.) or pleated (plicate) in the bud and later large and digitately divided or compound (Artocarpus communis (now A. altilis), Musanga). The leaves are usually leathery or rough. The flowers are small, ♂ or ♀, apetalous and often four-part, closely packed in cymose inflorescences, the inflorescence axis (the receptacle, cf. Euphorbia) often enlarged. It may be shaped like a spike, a disc or a bag. The fruit is false, sometimes a syncarp or a syconium.

The boundary between the Moraceae and the Urticaceae rests on rather few characters, and Musanga and Myrianthus are now generally considered to be part of the latter family as a consequence of their possession of clear (not milky) sap, which turns black, basal ovules, small achenes and mineral bodies (cystoliths) in the epidermal cells. In conformity with the second edition of the Flora of West tropical Africa, however, the two genera are here retained in the Moraceae.

A large woody tropical family represented in West Africa by c. 60 genera, mainly trees. In this family, and the two that follow, the Mimosaceae and the Papilionaceae, the pod (legume) is regarded as the characteristic fruit. It is formed from the single superior carpel, and becomes dry, opening by splitting down both margins. Both pods and the seeds they contain are commonly referred to as ‘beans’. In some classifications, the three families are united under the name Leguminosae (Fabaceae) (with subfamilies), thus forming the second largest family of flowering plants.

Members of the Caesalpiniaceae may be recognised by their alternate, stipulate, compound pinnate leaves without stipels (except Amphimas and Pellegrinodendron). The conduplicate leaflets may be alternate or paired, and there may (imparipinnate) or may not (paripinnate) be an odd terminal leaflet. A pulvinus occurs at the base of the petiole, a pulvinule at the base of the petiolule. Both have a wrinkled surface. Compound racemose inflorescences of zygomorphic five-part flowers each with a single superior carpel are usual. The fruit is generally a pod.

Unifoliate leaves are uncommon, most often being seen in three whiteflowered savanna species: Bauhinia (now Adenolobus) rufescens (Schmitz, 1973) (with flowers) in riparian woodland, and, with ♂ and ♀ flowers on separate plants, Piliostigma reticulatum in sudan savanna and P. thonningii in forestsavanna mosaic, both also in northern guinea savanna. The leaves are twolobed in all cases. Half a dozen other genera have species with entire unifoliate leaves.

Except for some species of the genus Garcinia, these are upper-storey, evergreen forest trees. Most are glabrous, the slash exuding yellow or orange resinous sap or latex. The family is a pantropical one, of about 600 species. No purely decorative species have been introduced. The Hypericaceae may be included as a subfamily.

Members of the family may be recognised by their pairs of exstipulate, simple entire leaves, which are streaked and spotted with resin glands, and carried on branches which appear to be whorled. The flowers are axillary, solitary or in cymes, red or white and conspicuous, with numerous stamens often in bundles. The fruits are large and berry like, and of economic importance.

Flowers ⊕, ♂, ♀ and, often 5-part, though 4-part in Calophyllum etc., when the sepals are orthogonal and the petals diagonal. Anthers mostly united in bundles each opposite a petal and alternating with the stigmata, but free in Garcinia sp., united only basally in Calophyllum and Mammea, and united into a staminal tube in Symphonia, cf. Chapter 18, Meliaceae; present as staminodes in ♀ flowers. An extrastaminal disc is usually present. G(2)–(5)–(8) with the same number of cells and axile placentation, the terminal style sometimes very short; 1-celled with 1 basal ovule in Calophyllum and 5-celled with parietal placentae in Allanblackia.

Pollination The disc in Allanblackia, Garcinia and Symphonia, thought to be staminodal in origin, and the numerous stamens suggest insect pollination, the shape of the flower providing ‘open access’ to short-tongued insects.

One of the largest families, cosmopolitan, and the next largest West African family, in terms of species, after the grasses. Shrubs and under-storey trees in forest are particularly important.

Members of the family may be recognised by their opposite or whorled, simple, entire leaves accompanied by interpetiolar stipules or ridges. Ptyxis conduplicate as a rule, revolute in Rothmannia sp. The inflorescence is a cyme, sometimes head like. Flowers are ⊕, mostly four- to five-part, always gamopetalous, with an equal number of epipetalous stamens alternating with the corolla lobes, and with a two-celled inferior ovary.

Rather few species have been introduced. Cinchona spp., yielding quinine, and the garden ornamental Gardenia jasminoides, the most usually cultivated Gardenia sp. of temperate climates (originally from China and Japan), are sometimes seen. The red and white Ixoras, I. coccinea and . parviflora, respectively, and, in Sierra Leone and Ghana, Pentas lanceolata (from East Africa), are more common. Many indigenous West African species deserve a place in the garden.

The tribe Naucleae (Adina, Mitragyna, Nauclea and Uncaria) showing similarities to the Combretaceae, is sometimes separated as a family, the Naucleaceae. Verdcourt (1958) proposed an entirely different intrafamilial classification to the one employed in the Flora of West tropical Africa.

Very few West African plants have obviously unique features. One is the genus Uncaria, climbers in forest with sensitive hooks formed from inflorescence branches. Rutidea membranacea, another forest climber, has recurved, but not hooked, axillary spines.

When I wrote Introduction to the flowering plants of West Africa (1965), I was particularly aware of the needs of post-O level GCE students and their teachers, whether in schools or in the first year of some form of higher education or further training. In the intervening twenty years or so, not only has a concentrated research effort in West Africa vastly increased our knowledge of West African plants, but uses for, and attitudes to, this natural resource have changed. There have been two periods of drought and famine south of the Sahara, their severity directly and mainly attributable to economic policies, whether wrong in themselves, wrongly applied or simply non-existent.

A knowledge of species biology is not merely the basis for the academic study of a discipline such as taxonomy, but is the essential foundation for decision-making in rural economics in both its aspects, development and conservation. Famines will recur while population pressure, deforestation and traditional farming starved of investment continue. Yet India is self-supporting in food production and has no foreign debt. The difference between the two subcontinents is striking, and the expertise of a wide range of specialists is going to be needed if the lot of millions of people south of the Sahara is to be ameliorated.

Atropical family of both Old and New Worlds, though only one or two genera (introductions excluded) occur in both floras. In West Africa, the family is a small one (six genera, nine species) and yields no major economic products. However, its species are commonly seen and are of ecological prominence.

Numerous species have been introduced into West Africa. From the Americas, jacaranda (Jacaranda mimosifolia) and yellow tecoma (Tecoma starts) are often seen, as well as a red tecoma (Tecomaria capensis) from South Africa. This last climbs by twining, and a number of other genera in the family either twine or have claw-like hooks or tendrils (replacing the terminal leaflet), or clasping roots. Bignonia capreolata, introduced from America, has tendrils. Only one West African species, Dinklageodoxa scandens, reported only from Liberian coastal savanna, is a climber (by twining). Anomalous secondary thickening occurs in these species.

In West Africa, members of the family may be recognised by their large, opposite, compound imparipinnate leaves and conspicuous panicle-like cymes of large, brightly coloured zygomorphic flowers, with darker nectar guides, followed by long, slim two-valved capsules with winged seeds.

Flowers .1. 5-part. K(5), 5-toothed or spathaceous, in which case the calyx develops in 1 piece and splits at bud-opening, either down the adaxial side (Markamia, Spathodea) or down the abaxial side (Newbouldia). C(5), 2- lipped, but in Spathodea with well-developed lobes, splitting adaxially. Stamens 4, epipetalous, with apically attached anthers and of 2 different heights. The staminode is adaxial.

Trees and shrubs with latex and fleshy fruits associated with a persistent calyx. In West Africa, most species are large, evergreen forest trees with simple, entire, alternate, leathery leaves with prominent pinnate venation. Flowers are small and white, in axillary clusters, each one three- to eight-part; flower structure in some species is complex. The flowers seldom open widely, never before senescence, and the corolla and staminodes absciss circularly round the base and fall as a piece. The flowers are followed by fleshy or woody berries, sometimes very large, with seeds having relatively large scars. The family is not yet reported growing indigenously in Gambia.

The introduced sapodilla (Manilkara zapotilla) from Central America has grey or brown fruits with sweet red flesh, and the star apple (Chrysophyllum cainito), from the same area, has berries with delicious white or purple flesh. Mimusops elengi (tropical Asia) is sometimes planted for shade.

Flowers ⊕. Simple 5-part flowers occur in Aningeria etc. (K5 [C(5) A5] G(5)), with 5 staminodes alternating with the stamens only in Vincentella and Aningeria. The more complex flowers of Butyrospermum, Tieghemella and Manilkara have 2 whorls each of sepals and petals, appendaged petals (each petal represented by 3 lobes) and petaloid staminodes. Butyrospermum and Tieghemella have 4-part flowers (K4+4 (the whorls dissimilar) [C(8) A8] G(8)), Manilkara has 3-part flowers (K3+3 [C(6) A6] G(6)), all have petaloid staminodes. In Omphalocarpum the flowers are of the simple 5-part type, but there is a group of 2–6 stamens opposite each petal lobe.

A tropical and temperate family of herbs and soft-wooded shrubs, a few members being climbers or small trees. Prickles and/or stellate hairs are frequently present. The family is centred in central and south America, and is best known for its economic products, which include tobacco, tomato, Irish potato and egg plant, all introduced, and the indigenous Solatium nigrum (ogumo, bologi).

Ornamentals introduced from tropical South America include the shrub Brunfelsia americana, which has blue .1. flowers. A number of Solanum and Datura spp. with ⊕ flowers are weeds. The flowers of Solatium wrightii are large and easily examined.

Members of the family have sympodial growth, bearing alternate, simple exstipulate leaves, which are sometimes lobed and usually adnate to the axis above their node of origin. Latex is lacking. The flowers are in cymes, and are generally white or purple and delicate, often with very short free corolla lobes (limbs), the whole folded or pleated. The pistil is of two joined carpels with axile placentation and numerous ovules. Bracts often occur in pairs in the inflorescences, one larger than the other, the larger one remaining adnate to the shoot it subtends.

Flowers ⊕ (but see Solatium) 5-part (occasionally more, e.g. tomato), except for the pistil. K(5) persistent. C(5), .1. in only 1 small tribe; ribbed and at least conduplicate (also contorted as in Convolvulaceae in Datura) though valvate in Capsicum.

Shrubs and twiners, occasionally stem succulents resembling cacti superficially, e.g. Caralluma dalzielii, sometimes seen in gardens. Milky juice is always present, though not always copiously so (Leptadenia), while leaves are simple, exstipulate and whorled or opposite, with flat or curved ptyxis. Some genera are, however, leafless, and in a few others the leaves are very short lived. There are cymes of complex five-part flowers, in which each anther contains one or two pollen masses (pollinia) in each loculus, and the paired carpels are joined only distally by the styles. The fruit is a pair of follicles (or a single one by abortion), containing plumed seeds.

In West Africa, the family is well represented in savanna and sahel, but only a few species are common in forest. Many of our species occur also in similar habitats elsewhere in tropical Africa, but within West Africa most of the species have very local distribution.

Only sodom apple (Calotropis procera) and blood flower (Asclepias curassavica) seem to have been introduced successfully into West Africa, blood flower, generally seen as a small shrub, being common in gardens and very suitable for the investigation of the flowers.

Leptadenia and Sarcostemma are ecologically important genera on the sahel zone and driest savannas, possessing also distinctive habits. Leptadenia consists of two species of twining shrubs and an erect one. This last is L. pyrotechnica, which produces tiny leaves, a few at a time, on its new, wiry stems.

As defined in the Flora of West tropical Africa (1968), this is a family of sympodial herbs with strap-shaped, mainly supervolute leaves ± pseudopetiole, having parallel veins converging at the leaf tip, perennating by rhizomes, stem tubers or bulbs. Flowers are three-part in terminal inflorescences lacking bracteoles and with superior ovaries. There are genera with leafy aerial stems and rosette herbs in which the only aerial axis is a leafless peduncle (scape).

On the basis of a great amount and variety of information (Dahlgren & Clifford, 1982), a new classification is presented (Dahlgren, Clifford & Yeo, 1985)(Table 35.1) in which the Liliaceae as so defined is replaced by six new taxonomic units at the family level. These new families have the advantage of now being as distinct but internally cohesive as for example, the Amaryllidaceae and Iridaceae in Hutchinson & Dalziel's (1927-36) treatment. A synoptic key to the six families is included for convenience (Table 35.2). Each family displays a complex of characters, including easily observed morphological ones.

The largest, most common and widespread genera in the former Liliaceae are the bulbous herbs, now Hyacinthaceae, and two genera with erect ‘rootstocks’ which otherwise resemble the bulbous genera in many features (now Anthericaceae). These two groups of genera are described in Field recognition, below. The remaining genera are all of distinctive habit, possessing welldeveloped, persistent and leafy aerial stems.

Almost all species of this family are confined to the frost-free zone between latitudes 30° N and 30° S. The flora of West Africa is small, with only 12 genera of indigenous palms; of these only Elaeis and Raphia also have New World species. The palms are, however, important ecologically in West Africa, second only to grasses in economic importance, as well as being prominent by reason of habit.

West African palms are trees or climbers (rattans), all woody and of relatively large dimensions. Species of Calamus and Raphia provide the largest known climber and leaf, respectively, other genera providing the largest inflorescence and fruit.

Palms lack secondary thickening and depend on the extensive deposition of lignin in existing primary tissue for their rigidity, and on the production of a mass of adventitious roots from the lower nodes to provide increased support and water supply as the stem grows.

Growth is monopodial, infloresences being axillary, and most West African genera show pleonanthic growth, i.e. the stems grow indefinitely and flower repeatedly. There is a small group of genera (Ancistrophyllum, Oncocalamus and Raphia) in which the stems flower only once (hapaxanthic growth), the equivalent (in effect) of the sympodial growth of most other monocotyledons. These short-lived aerial shoots are replaced by suckers growing from the base of the plant (also seen in Calamus and Phoenix), but in Nypa, the infloresence is terminal on one of the dichotomies of the rhizome, the leafy shoot being produced separately. Hyphaene shows dichotomous branching.