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Description

Trees, shrubs, scramblers, lianas, or herbs with ± woody base; evergreen or deciduous; sometimes armed with thorns or spines (Caesalpinieae); buttresses sometimes present. Leaves usually spirally arranged, bipinnate, pinnate, or unifoliolate, petiolate. The leaves are usually spirally arranged, but in the relict genus Ceratonia opposite or subopposite. They are bipinnate in Caesalpinia and some other genera, but most groups have pinnately compound leaves, often showing within the genus a gradual reduction in number of leaf-pairs, from species with numerous pairs to a single pair or a single leaflet. The petiolules are usually unspecialized but in Afzelia and Crudia they are twisted. Stipules are nearly always present, often interpetiolar. Intrastipular spines and trichomes are present in some groups of Bauhinia, particularly in America, but in Malesia found in, e.g., B. viridescens. Extrafloral nectaries (glands) among the leaflets or on the petiole are found in Senna and Chamaecrista where they offer excellent taxo- nomic characters. Swollen and myrmecophilous petioles are found in extra-Malesian groups. Ant domatia have been described in two S American and six African genera; in Asia only in the Indian genus Humboldtia. Glandular dots caused by translucent resin ducts are found in the tribe Detarieae.
Stipules nearly always present, paired, often caducous; stipels usually lacking, minute when present. Inflorescence types are various: most commonly paniculate, in other cases reduced to racemose, spicate, or even 1-flowered inflorescences. Usually they are terminal or axillary on the young branches, but cauliflory does occur in few genera. Bracts and bracteoles are variously developed; in the tribes Detarieae and Amherstieae the bracteoles are enlarged and often enclosing the flowers before anthesis; they are in these groups often persistent while in other tribes they are most often caducous. When the bracteoles are strongly developed a reduction of the calyx is often observed.
Inflorescences axillary and/or terminal, or cauli- florous, solitary or fascicled, commonly paniculate, through reduction and condensation leading to simple racemes or spikes and, rarely, to solitary flowers. The hypanthium is a characteristic feature. It is developed in various ways, from almost absent in very few genera to a low, cup-shaped, sometimes zygomorphic structure, to long and tubular as in some Bauhinia species. The hypanthium probably is of a complex nature, consisting of fused torus and perianth bases. Often, but not always, it is nectariferous. As the stipitate ovary is often inserted on the side or at the mouth of the hypanthium there is a close resemblance between the structure of the Bauhinia flower and the flowers of the Chrysobalanaceae, a family in which the style, however, is gynobasic, as is never the case in the Caesalpiniaceae.

Floral morphology shows a great deal of variation from small, almost regular flowers, similar to the mimosoid flowers, to large zygomorphic ones with a papilionoid similarity. The simplest floral structures with regular pentamerous flowers and often with 10 stamens are found in the tribe Caesalpinieae. In Gymnocladus and Gleditsia the flowers are effectively unisexual or polygamous, collected in racemes or small panicles. The sepals are not covering the petals in bud. Acrocarpus has more robust flowers with sepals almost covering the small, greenish petals spreading during anthesis and exposing a red cupular hypanthium. Similar primitive structures are found in Erythro- phloeum and here the presence of root nodules also suggests a close link to the Mimosa- ceae. In the Peltophorum group, mainly found in the Neotropics, the flowers develop from rather small, simple types over larger ones as in Peltophorum to very large, showy flowers in Delonix. In Caesalpinia and related genera as Pterolobium large zygomorphic flowers occur with winged dehiscent or indehiscent fruits often with a reduced number of ovules and seeds; further a variety of defence systems in the vegetative parts and fruits in the form of thorns and bristles are found.

In the tribe Cassieae there is a rich variety of floral structures from the small, regular, apetalous, polygamo-dioecious flowers of the relict genus Ceratonia over other small- flowered genera as Dialium with cymose, paniculate inflorescences to the often large, complexly structured flowers of Cassia, Senna and Chamaecrista. Particularly the development of the androecium in these last three closely related genera shows a unique diversity; basically they are 10-merous, often strongly accrescent towards the abaxial side except in Chamaecrista.

In the small tribe Cercideae the hypanthium is usually well developed and the flowers vary from small, almost regular pentamerous to large and irregular, with reduction mainly in the androecium (see further under the genus Bauhinia).

The floral morphology in the tribe Detarieae is the most diverse in the whole of the Caesalpiniaceae, as Cowan & Polhill (1981) express it: "in no other part of the family has one basic type of flower been modified repeatedly in so many different ways." As a basic floral type the genus Cynometra has small, regular, pentamerous flowers with a cupular hypanthium; from this, zygomorphic flowers develop and reductions as well as multiplications in the androecium occur. There is also a great variation in the nectarial structures.

The small tribe Amherstieae is in its floral structures closely related to the Detarieae and particularly to the Cynometra type.

The caesalpinioid pistil comprises a single unilocular ovary. In an old specimen of Bauhinia aurea from China flowers with several carpels per flower have been observed (unpubl.) in the Jardin Botanique at Paris. In a collection of B. variegata from Myanmar (Sittang C137613, CAL) two free carpels of normal size were observed in one flower, in two flowers one normal and one abortive carpel were found. However, it seems that only one carpel develops into a mature pod (personal comm. by S. Bandyopadhyay, Calcutta).
Flowers large to small, often showy, usually bilaterally symmetrical or ± zygomorphic, less often actinomorphic, bisexual and/or unisexual (then the plants polygamous or dioecious), usually 5-merous except for gynoecium, mostly perigynous, rarely hypogynous. Sepals 5 or 4, free or (infrequently) partly united, usually imbricate. Petals 5, sometimes reduced to 4(-1), or absent, often unequal, the upper one being the largest, sometimes vexillum-like; usually imbricate-ascending. Stamens basically 10, rarely through reduction 5, 3, 2, 1 or in female flowers absent, rarely many; filaments free or variously connected at basal parts; anthers variously developed, basi- or dorsi- fixed, often versatile, longitudinally dehiscent, rarely by (apical or basal) pores. Ovary with (l-)few to many anatropous, often superposed ovules in 2 rows on either side of the adaxial suture; style short or long, often curving inward; stigma large or small, capitate or peltate, sometimes obscure. Fruits variable, usually oblong to linear or strap-shaped, laterally compressed, straight or curved, dehiscent and 2-valved, valves chartaceous, fleshy, coriaceous, thick, or woody; inde- hiscent and drupaceous or samaroid; with or without pulp; glabrous, hairy, or spiny. Seeds varying in shape, often flattened; testa membranous, coriaceous, or crustose; sometimes arillate; endosperm present or lacking; cotyledons fleshy or foliaceous; radicle straight or slightly oblique, never folded.

Distribution

Asia-Tropical: Tropical Africa: present tropical America: present
The Caesalpiniaceae are a predominantly tropical group of c. 160 genera with c. 2000 species. The greatest number of genera is found in tropical Africa and tropical America. In Malesia there are 25 indigenous genera, containing 200 native species and more than 30 cultivated, introduced, or naturalized ones. In addition there are also at least eight commonly introduced or widely cultivated genera with twelve species.

Very few genera occur in the subtropical zones and only three reach the temperate regions, i.e. Cercis, Ceratonia and Gleditsia; the last one is also represented in Malesia.

The largest genus is Bauhirtia with more than 300 species throughout the tropics; it has c. 70 species in Malesia, c. 60 in Indochina and c. 40 in S China. Several species are endemics of very restricted distribution. Cassia is mainly neotropic and so are the segregate genera Senna and Chamaecrista. Caesalpinia is pantropic with 19 indigenous species in Malesia. Most other genera are relatively small and several restricted to tropical Asia. Few genera have a disjunct distribution as e.g. Copaifera occurring in tropical America, tropical Africa, and Borneo, and Gymnocladus in eastern N America and eastern Asia.

Pollination

Most species have maintained self-incompatibility but observations are scarce. Primitive Caesalpiniaceae appear to rely on colour contrast between anthers and petals; in advanced genera as, e.g., Parkinsonian Caesalpinia and some Bauhinia species nectar guides are evident. In Caesalpinia the upper petal and anthers are UV-absorbant. Xylo- copa bees have been recorded as visitors in, e.g., Cassia, Bauhinia and Caesalpinia. Butterfly pollination is known from Africa and the Americas; K. & S.S. Larsen have observed butterfly pollination of Bauhinia nervosa and B. glauca in N Thailand (unpublished). Bird pollination is of some importance in South America and bat pollination has been observed in Bauhinia and Caesalpinia in Costa Rica and Mexico.

A number of characters important for the pollination system of Bauhinia has not yet been studied; e.g. the nectariferous disc present in several species of the Lasiobema group and the peculiar red outgrowth on the mouth of the hypanthium in B. bracteata. The opening of the anthers by pores, usually indicating buzz pollination, is found in Senna, Chamaecrista and the Palmatifolia group of Bauhinia, common all over Malesia, but it has yet to be studied in nature. Within the same group the nectar-like fluid produced by the pollensacs and excreted through the pores with the pollen may also be of importance to pollinators. Many Caesalpiniaceae are also showing a colour change from the young to the mature flower; in several Bauhinia species the colour changes from yellow or orange to red. Extrafloral nectaries are present in species of Amherstia, Caesalpinia, Chamaecrista, Delonix, Intsia, Kingiodendron, Maniltoa, and Senna.

Dispersal

The fruits and seeds vary considerably, suggesting a variety of dispersal methods. Here as with the pollination biology, observations in nature are scarce. The samaras of Ptero- lobium clearly have wind dispersal; the heavy fruits, however, do not fly over long distances. In Koompassia dispersal by wind has been observed by Ridley (1930). The trees reach a height of over 50 m, overtopping the highest jungles, and the trees are often left untouched in clearings because of the very hard wood. The samaras have a slow flight, lying horizontally in the air and rapidly rotating; they have been observed to land c. 20 m from the mother tree, but most certainly they are able to fly over much longer distances.

The arillate seeds of genera as, e.g., Afzelia and Sindora are, after they have reached the ground, attacked by rats and other rodents; it is also suggested by Ridley that the spiny pods of some Sindora species are spread epizoic. Dialium pods are dispersed by monkeys and birds. The peculiar indehiscent pods of Cassia fistula will also attract animals. Dispersal by water currents is, however, also demonstrated for this species and for, e.g., Senna siamea on the Malay Peninsula. Senna alata with winged pods could be spread by'water over shorter distances; it is in any case often found along water courses. Dispersal by water currents was observed for several species of Caesalpinia in the Malesian area. Afzelia bijuga, which is a common trees in the tidal zone, is also spread by sea currents. There are reports of monkeys feeding on several species of Senna. Bird dispersal is shown for other Senna species. In several Bauhinia species the pods open explosively spreading the seeds up to 12m according to Kerner cited in Ridley (1930).

Anatomy

One of the characteristics of the Leguminosae is the root nodule caused by fungal symbiosis. With few exceptions nodulation is the rule in the Mimosaceae and Papilionaceae, it is rare within the Caesalpiniaceae. Our knowledge is still limited and some results are contradictory. In two genera, however, nodules have been induced experimentally, i.e. Chamaecrista and Erythrophleum; it has also been induced in Swartzia, a genus now placed in the Papilionaceae, but formerly in the Caesalpiniaceae. Erythrophleum is also a 'marginal' genus or, as Corby (1981) expressed it "lies near the vague border between Mimosoideae and Caesalpinioideae." This leaves us with Chamaecrista, and the occurrence of root nodules here is also one of the characters that Irwin & Barneby (1981) have used to support the separation of this group from Cassia.

The epidermis is often papillose and sometimes mucilaginous. Stomata structure is diverse, two main types are found, paracytic and anomocytic. Tanniniferous cells are common in the cortex and phloem. Secretory cavities and canals sometimes occur. The vascular bundles of the leaf veins are often accompanied by sclerenchyma. Solitary and clustered crystals can be found in all parts of the plants; cluster crystals are particularly characteristic of the mesophyl and thus differentiate the Caesalpiniaceae from the Papilionaceae and Mimosaceae. The pericycle of the young stem nearly always contains a continuous ring of sclerenchyma, or a continuous ring of fibres. Grooved stems occur in some species of Bauhinia, Cassia and Senna. In Bauhinia the stems of the lianas are sometimes flattened, forming 'monkey ladders'. Anomalous thickening is another characteristic feature of the lianas in Bauhinia', this includes types with successive growth rings and segmented xylem.

The wood anatomy is best known for the many valuable timber trees (Ilic 1991; Quirk 1983; Sudo 1988, 1991). Generally the wood is characterized by few specialized characters. Baretta-Kuypers (1981), in her survey of the wood anatomy of the family concludes further that the Caesalpiniaceae and the Papilionaceae anatomically merge into each other, while the Mimosaceae, on the other hand, seem to be quite distinct in their wood anatomy.

Below family level the wood structures usually provide good diagnostic characters, which often agree well with the groups based on the morphology. Two groups may be distinguished on the basis of ray features as well as parenchyma characteristics: 1) Caesalpinieae, Cassieae and Cercideae; 2) Detarieae and Amherstieae. In the first group the rays are mostly homocellular, in the second they are predominantly heterocellular.

Taxonomy

The fundamental works on the Leguminosae are those of Bentham (1865) and Taubert (1894). Taubert divided the family into three subfamilies but commented that there are no sharp limits between them. Following Bentham he divided the Caesalpinioideae into eight tribes, again stating that this is in many ways an artificial division and that it is difficult to delimit the tribes as there are several transitional groups. Hutchinson (1964) divided Caesalpiniaceae into two subfamilies: Caesalpinioideae with small and often early caducous bracteoles (or if bracteoles larger then mostly petaloid) and Brachystegioideae with large, usually persistent bracteoles. Also in this system difficulties arise in placing several genera.

Later Cowan (1981) recognized five tribes within the Caesalpinioideae: Cercideae, Caesalpinieae, Cassieae, Detarieae and Amherstieae.

Recently Breteler (1995) studied the demarcation of Amherstieae and Detarieae and concluded that they should be united as one tribe under the name of Detarieae. He also added a new tribe, i.e. Macrolobieae.

The important characters for the recently delimited tribes and their Malesian genera (including introduced or cultivated ones) are listed below. For more details see Cowan (1981) and Breteler (1995).
  1. Cercideae Bronn — Many characters indicate that this probably is the most primitive tribe or, as Cowan (1981) suggests, an early offshoot of the Caesalpinioideae. The leaves are simple, bilobed or consisting of two free leaflets, palmately veined. A group of only 5 genera; in Malesia Bauhinia is the sole genus.
  2. Caesalpinieae Polhill & Vidal — Stipules lateral or absent. Leaves are generally compound or, if consisting of a sole foliole, then pinnately veined. Hypanthium most often tubular or cupular. Stipules lateral or absent; bracteoles small or absent. The following Malesian genera belong here: Acrocarpus, Caesalpinia, Gleditsia, Pelthopho- rum, Pterolobium, Sympetalandra, and Delonix (introduced).
  3. Cassieae Bronn — Stipules lateral. Hypanthium absent or short and filled with secretory tissue. In Malesia the following genera occur: Cassia, Chamaecrista, Dialium, Kalappia, Koompassia, Senna, and Uittienia.
  4. Detarieae DC., following the redelimitation by Breteler (1995) — Bracteoles not always present and never protecting the flower bud till anthesis. The calyx always well developed. The following genera occur in Malesia: Afzelia, Amherstia, Brownea, Copaifera, Crudia, Cynometra, Endertia, Hymenaea, Intsia, Kingiodendron, Leuco- stegane, Maniltoa, Saraca, Sindora and Tamarindus.
  5. Macrolobieae Breteler — Important characters as published in the original description: Bracteoles valvate, often thick, covering the flower bud till anthesis, resembling a calyx, often persistent. Calyx usually reduced. Distributed in African and American tropics. No representative genera cultivated in Malesia.

Cytology

About half of the genera in the Caesalpiniaceae have been studied cytologically with regard to the chromosome numbers. Two major groups emerge, corresponding well with the current overall classification. The first group include the tribes Caesalpinieae, Cassieae and Cercideae with basic number x = 7 or 14; there are exceptions as, e.g., Acrocarpus, Pterolobium and Erythrophleum with x = 12 and in Cassia sensu lato, where the chromosome cytology is quite complex with a high degree of polyploidy and aneuploidy. In Chamaecrista x = 7 and 8 have been found, probably due to derived de- scending aneuploidy from ancestors with x = 14. In Bauhinia, where more than 50 spe- cies have been studied, most are diploid with 2n = 28, some are polyploid and some have deviating numbers, e.g. 2n = 26, found in several species in Asia and Australia. Diploidy is only found in Cercis with 2n = 14.

The second group, comprising the tribes Detarieae and Amherstieae, has mostly the basic number x = 12.

Several old chromosome counts have shown to be erroneous and in most cases where the basic number x = 8 has been suggested previously this has been shown to be incorrect. Much work has yet to be done before a clear pattern of the cytological condi- tions is established.