Bixaceae
H.-H. POPPENDIECK
Evergreen trees or shrubs with yellow or reddish sap. Leaves, fruits and young parts of stems with brown peltate hairs. Leaves alternate, simple and entire, palmatinerved, long-petiolate, stipulate. Flowers in terminal thyrsoids, large and showy, bisexual, hypogynous, actinomorphic. Pedicels with 5 conspicuous glands below the calyx. Sepals 5, free, imbricate, caducous. Petals 5, free, imbricate, alternating with sepals, white or pink. Stamens numerous, free, anthers dithecal, horseshoe-shaped, opening with short apical slits. Ovary superior, syncarpous, 2-carpellate, 1-celled; placentas parietal with numerous anatropous ovules. Capsule short stipitate, loculicidally 2-valvate, unarmed or with long, soft spines. Seeds numerous, obovoid, sarcotesta turning into a fleshy pulp when moistened.
A strictly neotropical family with five species in a single genus. Bixa orellana L., yielding a reddish food colouring commercially known as annatto, is widely cultivated and often naturalised throughout the tropics.
Vegetative Morphology. Germination is epigeal. The hypocotyl may attain up to 7 cm. The primary leaves are very similar to the adult ones. The leaf is ovate with a cordate base and tapers into a drip tip. Venation is brochido-actinodromous (Garwood 1994). The plant has extrafloral nectaries on the stem at the node and on the peduncle of the flower and fruit which are visited by ants guarding the plant. Those on the stem are active only as long as the leaves are young, and those on the peduncle from bud stage to the maturity of the fruit (Bentley 1977).
Vegetative Anatomy. The leaf undersurface bears peltate, disc-like scales which are red-brown in colour and consist of a stalked central area surrounded by rings of cells. In the parenchyma of stems and leaves, secretory canals, surrounded by an epithelium of small cells, contain a dark refractive mass slowly oozing out when hurt, probably containing bixin. Secretory cells are also present, as are cells containing calcium oxalate crystals (Garwood 1994; Dempsey and Garwood 1994).
Similarly to Cochlospermaceae, the phloem of the stem is stratified into soft and hard portions. The vessels are small to medium, about 0.3 mm long (Metcalfe and Chalk 1950). The parenchyma is apotracheal. Fedalto (1983) found radial tracheitis in Bixa arborea.
Floral Morphology. The ontogeny of the flower is rather similar to Cochlospermaceae, except for the gynoecium. The calyx arises as a quincunx. Petal initiation tends to occur in two successive steps, or the petals arise sequentially. Stamens develop centrifugally from a circular pri-mordium (ringwall, Corner 1946; Ronse Decraene 1989). Gynoecium development starts with the formation of a low circular primordium from which the two congenitally fused carpels arise like a chimney with a narrow mouth. Placentation is parietal (Ronse Decraene 1989).
The horseshoe-shaped anthers (Fig. 10D) are unique: the connective bends and fuses congenitally to form a loop. The anthers are bent, too, with their bases and apices close together at the bottom, and open at the top with small (morphologically lateral) slits (Venkatesh 1956).
Embryology. The ovule is anatropous, bitegmic and pseudo-crassinucellate. The embryo sac is of the Polygonum type. Endosperm development is nuclear (Johri et al. 1992).
Pollen Morphology. Pollen is tricolporate, spheroidal, medium-sized, and has a microperfo-rate tectum. Perforations are 0.1-0.6 |Xm wide. The surface is rugulose. The space below the tectum is relatively low and recalls the exine structure of Cochlospermum (Keating 1976).
Karyolog y. The base number is x = 7. B. orellana is diploid with 2n = 14 (Fedorov 1969), other numbers being probably erroneous, according to Morawetz (1986) who considers the karyology of the family to be very similar to that of Cochlospermaceae.
- 10A-J. Bixaceae. Bixa orellana. A Flowering shoot. B Flower bud with extrailoral nectaries at the base of the calyx. C Stamen with horseshoe-shaped anther. D Anther opening with apical (morphologically lateral) slits. E Gynoecium. F Capsule. G Capsule in longitudinal section. H-J Seed. H Lateral view. I Front view with raphe and apical hilum. J Seen from above with hilum. (Takhtajan 1981)
- 10A-J. Bixaceae. Bixa orellana. A Flowering shoot. B Flower bud with extrailoral nectaries at the base of the calyx. C Stamen with horseshoe-shaped anther. D Anther opening with apical (morphologically lateral) slits. E Gynoecium. F Capsule. G Capsule in longitudinal section. H-J Seed. H Lateral view. I Front view with raphe and apical hilum. J Seen from above with hilum. (Takhtajan 1981)
Pollination and Reproduction. Flowers open before dawn and are pollinated by large euglossine and ptiloglossine bees, later on also by smaller bees. The flowers wilt at midday and the corolla falls in the afternoon. Pollen, copiously produced, is the only floral reward (Bentley 1983).
Fruit and Seed. The dry capsules are of variable shape, size and texture and dehisce tardily or remain closed. The conspicuously red or brown exocarp is usually covered with dense stiff spines, rarely glabrous. Placentation is parietal with placentas protruding. The fruit contains usually 6-12 seeds with long funicles.
The seeds of Bixa are well studied (Corner 1976; Nandi 1998). They are anatropous, pyriform, and possess a sarcotesta (eventually drying up) formed by large cells containing the pigment bixin. The aril forms a small whitish rim around the top of the funicle. Endosperm is starchy and encloses the embryo completely, the cotyledons being bent under the chalazal plug. The structure of the chalaza is complicated: The palisade layer of the exotegmen is curved inwards, and a plug of hypostase tissue with an annulus/core structure fits into this dome.
- The sarcotesta of the seed contains the red pigment bixin, a carotenoid of commercial value. The leaf oil contains several sesquiterpenes, ishwaran being the main constituent (Hegnauer 1964,1989). The leaves contain ellagic acid and flavonoids but, in contrast to Cochlospermaceae, kaempferol, quercetin and myricetin are lacking (Harborne 1975).
- Bixa has often been considered the closest relative of Cochlospermaceae but an analysis of rbcL sequences revealed Diegodendron as sister to Bixa, both being sister to Cochlospermaceae. This finding requires maintaining the family status of the three groups, if one does not wish to merge all of them (Fay et al. 1998).
Distribution and Habitats. The family is strictly neotropical. Apart from B. orellana, now widely cultivated and naturalised throughout the tropics, there are four species from the western to central Amazon basin.
Economic Importance. The family is chiefly important for B. orellana which provides a nontoxic dye known as annatto. It is yielded from the sarcotesta and consists mainly of bixin. It has many desirable properties such as resistance to light, heat, etc. and is widely used as a food colouring agent and also as a spice. The plant was probably cultivated already in pre-Columbian times and, among its various applications, the use as a body paint in religious ceremonies or simply as a sun protection has the been most important. Since the leaves are not eaten by cows or goats, B. orellana is also used for fences (Ingram and Francis 1969).
Only one genus:
Characters of the family. Selected Bibliography
Baer, D.F. 1977. Systematics of the genus Bixa and geography of the cultivated annatto tree. Diss. Abstr. Int., B 37(10): 4846.
Bentley, B.L. 1977. The protective function of ants visiting the extrafloral nectaries of Bixa orellana (Bixaceae). J. Ecol. 65: 27-38.
Bentley, B.L. 1983. Bixa orellana (Achiote, Annatto). In: Janzen, D.H. (ed.) Costa Rican natural history. Chicago: Chicago University Press, pp. 193-194.
Corner, E.J.H. 1946. Centrifugal stamens. J. Arnold Arbor. 27: 423-437.
Corner, E.J.H. 1976. See general references.
Dempsey, R.E., Garwood, N.C. 1994. A study of Bixa (Bixaceae), with particular reference to the leaf undersur-face indumentum as a diagnostic character. Bull. Nat. Hist. Mus. Lond., Bot. 24:173-179.
Fay, M.F. et al. 1998. See general references.
Fedalto, L.C. 1983. Estudo anatomico do lenho de Bixa arborea Huber. Acta Amazonica 12: 389-399.
Fedorov, A.A. 1969. See general references.
Garwood, N.C. 1994. Morphology and ecology of seedlings, fruits and seeds of Panama: Bixaceae and Cochlosperma-ceae. Bull Nat. Hist Mus. Lond., Bot 24:161-171.
Harborne, J.B. 1975: Flavonoid bisulphates and their cooccurrences with ellagic acid in the Bixaceae, Frankeniaceae and related families. Phytochemistry 14: 1331-1337.
Hegnauer, R. 1964,1989. See general references.
Ingram, J.S., Francis, B.J. 1969. The annatto tree (Bixa orellana L.) - a guide to its occurrence, cultivation, preparation and uses. Trop. Sei. 9: 97-102.
Johri, B.M. et al. 1992. See general references.
Keating R.C. 1976. Trends in specialization in pollen of Flacourtiaceae with comparative observations of Cochlos-permaceae and Bixaceae. Grana 15: 29-49.
Metcalfe, C.K., Chalk, L. 1950. See general references.
Morawetz, W. 1986. Remarks on karyological differentiation patterns in tropical woody plants. Plant Syst. Evol. 152: 49-100.
Nandi, O.I. 1998. Ovule and seed anatomy of Cistaceae and related Malvanae. Plant Syst. EvoL 209: 239-264.
Ronse Decraene, L.P. 1989. Floral development of Cochlosper-mum tinctorium and Bixa orellana with special emphasis on the androecium. Am. J. Bot 76:1344-1359.
Takhtajan, A.L. 1981. See general references.
Venkatesh, C.S. 1956. The curious anther of Bixa: its structure and dehiscence. Am. Midlands Nat. 55: 473-476.
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