Achatocarpaceae

V. Bittrich

Achatocarpaceae Heimerl in E.-P., Nat. Pflanzenfam. ed. 2,16c:

Dioecious trees or shrubs, sometimes thorny. Leaves alternate, simple, entire, exstipulate. Flowers small, hypogynous, in axillary racemes or panicles, brac-teoles present or absent; sepals 4-5; petals none; stamens 10-20, filaments distinct or connate at base; ovary bicarpellate, unilocular with one basal ovule; styles two; fruit a berry; seeds with an annular embryo surrounding the perisperm.

A poorly known family of two genera and ca. six species occurring from Texas, California and NW Mexico to Paraguay and Argentina.

Vegetative Structures. All species of the family are glabrous or in young parts short-hairy, shrubs or small trees, with the tips of short shoots often transformed into spines. Serial shoots occur in Achato-carpus. The leaves are alternate, sometimes fascicled on short shoots, petiolate, simple, entire, exstipulate, mucronate, rounded or emarginate at the apex. Epicuticular wax crystals form platelets, which are often arranged in clusters (Engel and Barthlott 1988).

Secondary growth is normal, wood rays are present. The pericycle of Achatocarpus contains scleren-chyma and stone cells. Phloem fibres are present. The leaves are mostly bifacial and hypostomatic, sometimes isobilateral; the stomates are anomocytic; a hypodermis is often present. Tanniniferous cells are common in stems and leaves. Various forms of calcium oxalate crystals (druses, sphaerites, prismatic crystals) occur, but raphides or styloids are absent.

Formerly, the Achatocarpaceae were considered to have sieve element plastids with a subperipheral dense ring of proteinaceous filaments and a globular central protein crystalloid (PHIcf-type, Behnke 1976), like most centrospermous families. New investigations, however, revealed that the central crystalloid is polygonal (PHIc'f-type), a type characteristic also for the Caryophyllaceae and Stegnosperma (Behnke 1981).

Inflorescence and Flower Structure. The unisexual flowers are arranged in bracteate determinate panicles or racemes with (Phaulothamnus) or without bracteoles (Achatocarpus). The five tepals of

Achatocarpus are quincuncially imbricate. The four tepals of Phaulothamnus are decussate with two inner and two outer ones, the latter in transversal position. Only the terminal flower of the raceme can rarely have five tepals in Phaulothamnus. There are 10-20 stamens with delicate filaments, which are connate or free at base. The dithecal, basifixed anthers open by longitudinal slits. The ovary is completely lacking in male flowers. In female flowers the syncar-pous, superior ovary is closely enveloped by the tepals. It consists of two carpels, with long, hairy-papillose styles, which are distinct or only very shortly connate at the base. The position of the styles is median in Achatocarpus, and transversal in Phaulothamnus. Septa are absent at least in the mature ovary, which contains only one basal, campylotro-pous ovule, or very rarely two ovules.

Pollen Morphology. Pollen of Achatocarpus and Phaulothamnus was studied by Nowicke and Skvarla (1979) and Skvarla and Nowicke (1982). The grains are spheroidal, with mostly 4-6 large pores or ± irregular apertures, which are often poorly defined. The microperforate tectum is scabrate or coarsely granular; the foot-layer is very thin. Thus the pollen type of this family is different from other centrospermous families and gives no clue concerning the relationships of the family within the order.

Fruits and Seeds. The fruits are one(two)-seeded, translucent white or blackish, sour or bitter berries, in which the black seeds are often visible. The seeds are generally reported to be exarillate, but a very small aril can be seen at the hilum. The seedcoat is tubercu-late, sometimes shining. The peripheral, ± annular embryo is strongly curved around the mealy perisperm. Rudimentary endosperm sometimes forms a thin layer around the embryo (Heimerl 1934).

  1. The presence of berries suggests that the fruits are dispersed by birds, but no observations are reported in literature.
  2. The poor information available is summarized in Hegnauer (1989). In contrast to occasional assertions in literature (e.g. Skvarla and Nowicke 1982), an investigation on the presence of betalains was not yet possible (Mabry pers. commun. 1989). Also another important centrospermous character, the presence of bound ferulic acid in the unligni-fied cell walls, has not yet been checked. Richardson (1981) investigated the flavonoids of Achatocarpus praecox Gris, and found only vitexin and isovitexin.
  3. The family is normally linked to the Phy-tolaccaceae s. lat. and sometimes included in this
Fig.2 A-E. Achatocarpaceae. A-D Achatocarpus balansae. A Branchlet with young fruits. B Young fruit. C Inner (left) and outer (right) tepal. D Young fruit, longitudinal section. EA.bi-cornutus, young fruit. (Schinz and Autran 1893)

family. This is probably due to the presence of racemose inflorescences and of berries in both families. Several other characters, however, contradict a close relationship between them. Most notable are the absence of raphides and styloids, the type of sieve element plastids, the compound unilocular, uni-ovulate ovaries (in the Phytolaccaceae restricted to the subfamily Microteoideae, which is sometimes referred to the Chenopodiaceae), and the absence of an anomalous secondary growth. As the ovaries are built differently in the Achatocarpaceae and the Phytolaccaceae, it seems possible that the berries have been evolved in parallel in both families.

Key to the Genera

  1. Bracteoles present, tepals 5, styles in a median position
  2. Achatocarpus

- Bracteoles absent, tepals 4, rarely 5 in terminal flowers, styles in a transversal position 2. Phaulothamnus

1. Achatocarpus Triana

Fig.2

Achatocarpus Tirana, Ann. Sci. nat. 9: 45 (1858); Walter, Das Pflanzenreich 39:134 (1909).

Shrubs or trees up to 10m high, often thorny, glabrous or hairy; flowers white or greenish, in racemes or panicles; bracteoles two; tepals 5, stamens 10-20; styles 2 in median position; fruit a whitish or blackish berry. About five spp. from S Mexico to Argentina.

2. Phaulothamnus A. Gray

Phaulothamnus A. Gray, Proc. Amer. Acad. 20:293 (1885).

Shrub, 1.5-4 m high, thorny, glabrous; leaves alternate, forming dense fascicles on older stems; flowers inconspicuous in few-flowered racemes; tpracteoles absent; tepals 4, very rarely 5, stamens 12-14, irregularly inserted; styles 2 in transversal position; fruit a white translucent berry. One sp., P. spinescens A. Gray, Mexico, Tres Marias Islands, Texas, and California, in clay washes, plains and lower hillsides.

Selected Bibliography

Behnke, H.-D. 1976. Ultrastructure of sieve-element plastids in Caryophyllales (Centrospermae); evidence for the delimitation and classification of the order. PI. Syst. Evol. 126: 31-54.

Behnke, H.-D. 1981. Sieve-element characters. Nord. J. Bot. 1: 381-400.

Engel, T., Barthlott, W. 1988. Micromorphology of epicuticular waxes in centrosperms. PI. Syst. Evol. 161: 71-85.

Hegnauer, R. 1989. See general references.

Heimerl, A. 1934. Achatocarpaceae. In: Engler, A., Prantl, K. (Eds.) Die Natürlichen Pflanzenfamilien, 2. ed., 16c. Leipzig: Engelmann, pp. 174-178

Nowicke, J.W., Skvarla, J.I. 1979. Pollen morphology: the potential influence in higher order systematics. Ann. Mo. Bot. Gard. 66: 633-700.

Richardson, P M. 1981. Flavonoids of some controversial mem-beis of the Caryophyllales (Centrospermae). PI. Syst. Evol. 138:227-233.

Schinz, H., Autran, E. 1893. Des genres Achatocarpus Triana et Bosia Linné et de leur place dans le système naturel. Bull. Herb. Boiss. 1:1-14.

Skvarla, J.J., Nowicke, J.W. 1982. Pollen fine structure and relationships of Achatocarpus Triana and Phaulothamnus A. Gray. Taxon 31: 244-249.

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