Aizoaceae

H.E.K. Hartmann

Aizoaceae Rudolphi, Syst. Orb. Veg.: 53 (1830); nom. cons.

Predominantly succulent, annual to perennial herbs, subshrubs or shrubs. Leaves mostly opposite, simple and entire, epidermis with bladder idioblasts, or uniform and xeromorphic, often pubescent; base of petioles rarely with stipuliform appendages or leaves sessile with a connate leaf sheath. Cymose inflorescences of principally dichasial pattern complete or in various derived forms, mostly terminal, often seem-

Kg.3A-D. Aizoaceae. Growth forms. A Jordaaniella dubia, creeping on coastal dunes. B Odontophorus nanus, a compact perennial on quartzitic gravel. C Lithops julii, sunken in fine gravel. D Aizoon canariense, short-lived annual, whole plant consisting of inflorescence. (Phot H. Hartmann)

ingly axillary, flowers often solitary; flowers actino-morphic, bisexual, hypogynous to perigynous or epi-gynous; perianth elements (3-)5(-8), free portions often unequal and with dorsal subapical appendages, basally connate and adnate to the filaments, thus forming a perianth-stamen tube, inner surface of upper portion petaloid or green; androecial elements 4 to many; if many, the outer primordia developing often into petaloid organs (= petals), filaments rarely connate at base, anthers dehiscing by longitudinal slits; ovary syncarpous, (l-)5(-°°)-carpellate, placentation axile, basal or parietal, ovules one to many per carpel, anacampylotropous or campylotro-pous, bitegmic, crassinucellate, sometimes pendulous. Fruits mostly hygrochastic loculicidal, rarely septicidal or circumscissile capsules, sometimes hard and indehiscent, rarely drupaceous, occasionally in aggregates. Seeds mostly ± ovoid, rarely arillate, with a curved peripheral embryo, and voluminous starchy perisperm, endosperm reduced to a layer enveloping the radicle.

As understood here, the family consists of five subfamilies, the Molluginaceae are excluded (Bittrich and Hartmann 1988 in contrast to Pax and Hoffmann 1934). One hundred and twenty seven genera, 115 of which are derived from the old genus Mesembryan-themum, comprising ca.2500 spp. distributed in arid parts of the tropical and subtropical zones of both hemispheres, with the main frequency centre in Southern Africa.

Vegetative Morphology. Most Aizoaceae are perennials, and the life span of many annuals can be prolonged by good water supply, so that they may live over two or more seasons. Chamaephytes prevail, very rare are phanerophytes (Stoeberia), hemicrypto-phytes (Delosperma), and geophytes (Phyllobolus); remarkable, though not very common, are subshrubs of the branch-shedding type (Jürgens 1987). Low growing herbs of procumbent, caespitose or compact habit (Fig.3B), subshrubs or shrubs are most frequent growth forms; climbers are rare. Shrubs can be armed with solid thorns, as in Eberlanzia or Ruschia. Occasionally plants consist of a single leaf-pair only, e.g. Pleiospilos, Argyroderma; in some genera the leaf-pair can form a solid body but for a small central slit (e. g. Conophytum) or even grow completely sunken in the soil so that only the tips of the leaves are visible ("living stones", e.g. Lithops, Fig.3C). In contrast to these, creeping forms (e.g. Cephalophyllum, Jordaaniella, Fig. 3 A) can cover several 10 m2. Sym-podial growth is most common, monopodial growth is rare and mostly restricted to the basal parts of the plants (e.g. Mesembryanthemum, Cephalophyllum). Leaf succulence prevails in the family, succulence of roots is rare (e. g. Fenestraria), and succulence of stem (e.g. Psilocaulon, Brownanthus) and/or hypocotyl (e.g. Trichodiadema, Delosperma, Phyllobolus) occurs occasionally; often the leaves of the two last types are short-lived and shed.

Flat leaves are moderately succulent and petiolate. Stipules are reported for Trianthema and Cypselea, but stipuliform appendages occur in all Sesuvioideae, and papery sheaths can be formed; leaf shapes are simple and entire, rarely crenulate, and more rarely pinnatifid (Aethephyllum). Fat, highly succulent leaves occur in various shapes, their margins can be entire or dentate, the bases of the mostly opposite leaves are connate into sheaths of varying length. A "pustule" is often formed at the base of the free portion of the upper side of the leaf; water storage tissue reaches the epidermis in this region. Consequently, high water content inflates the pustule, resulting in an outward movement of the free parts of the leaves thus spreading them. If water storage tissue extends directly to the epidermis at the tip of the leaf, a "window" is formed (e.g. Fenestraria, Lithops, Fig.3C).

Vegetative Anatomy. Many remarkable derived anatomic features of the Aizoaceae represent adaptations to arid and/or saline conditions. Characteristic

Fenestraria Roots

Fig.4A-G. Aizoaceae. Leaf surfaces. 'I\vo different pathways from mesomorphic to xeromorphic surfaces occur within the family: A,C-F reduction in size of bladder cells and subsequent thickening of the epidermis; A, B only the bladder cells become xeromorphic. A Gunniopsis zygophylloides, mesomorphic with prominent epidermal bladder cells, the most widespread type occurring in all subfamilies. C Disphyma cf. crassifolium, mesomorphic with distinct larger cells not elevated above the surface. D Conicosia elongata, mesomorphic, homocellular, with thick wax layer. E Carpobrotus sp., moderately xeromorphic, homocellular, outer epidermal wall encrusted with calcium crystals, hypodermis without chloroplasts. F Ruschianthus falcatus, homocellular, outer epidermal wall thick, stratified, with extensive crystal and wax layers.

G Schwantesia succumbens, xeromorphic, homocellular, with pronounced papillae. B Drosanthemum diversifolium, xero-moiphic, heterocellular, outer walls of bladder cells markedly thickened, but lacking a crystal layer and hypodermis. c cuticle; cl cellulose layer; cr crystal layer; cu cutinised layer. (Orig.)

and unique is the occurrence of bladder idioblasts on leaf and stem surfaces of many Aizoaceae (Fig. 4). These cells serve as outer water reserves, the inner differentiation of tissues is only weakly developed (Jürgens 1987). Kranz anatomy, indicating C4 metabolism has been demonstrated to occur in Trianthema, Zaleya (Carolin et al. 1978), Cypselea, and Sesuvium sesuvioides (Bittrich 1990). A derived much more xeromorphic surface type with a mostly homogeneous pattern of the epidermal cells prevails in most Ruschioideae (Fig.4 D-G). This type includes classical xeromorphic characters including a prominent epicuticular wax layer, which forms a smooth or rough continuous layer with or rarely without various wax crystals (threads, rodlets, vertical irregular platelets, horizontal platelets) or plates (one per cell) in uniform expression over the cell surface or differen tiated into distinct regions (Hartmann 1979, 1991); furthermore, thick outer epidermis walls and sunken stomata, the incrustation of the outer wall with calcium oxalate crystals, and the formation of a thick-walled hypodermis can be mentioned (Fig.4F, Hartmann 1983). In leaves of this surface type a distinct separation of a peripheral assimilation tissue against a central wa'ter storage tissue has taken place. In certain genera (e. g. Lampranthus), homogeneous mesomorphic leaf surfaces with modest outer cell walls lacking crystal incrustations, and with superficial stomata prevail (Fig.4D) in combination with a central water storage tissue. In few groups, the idioblasts possess xeromorphic walls (Jürgens 1987). Hairs can occur on all surface types, they are usually simple and unicellular, but stalked two armed hairs have been reported for several Aizooideae and Tetragonioideae.

Stomata are anisocytic, anomocytic or paracytic; the last type seems to be more frequent in Ruschioi-deae than in any of the other subfamilies. Tanninife-rous idioblasts are formed in the water storage tissue and, particularly in the xeromorphic forms of Ru-schioideae, below the hypodermis. Exceptional are tannin idioblasts in the epidermis of Hymenogyne. Due to differences in reflexion, these areas appear as dots to the naked eye (e. g. Pleiospilos). Smaller idioblasts containing calcium oxalate raphides can occur in the mesophyll; they can be conspicuous in the central water tissue (e.g. Hereroa).

Trilacunar leaf traces have been reported for Trianthema and Zaleya, unilacunar ones for Trian-thema as well (Bhambie et al. 1977), but this complex needs further investigation. Anomalous secondary growth in stems and roots is widespread, several types have been described (Hartmann 1976, 1989), and seemingly the secondary xylem remains rayless. Cortical bundles are known from Mesembryanthe-moideae and few Ruschioideae (Bittrich 1987), they are probably of different origin and therefore not homologous (Dehn 1992).

Sieve element plastids are of subtype PHI, characteristic of many centrosperms (Behnke 1981): a globular protein crystal is surrounded by filaments.

Inflorescence Structure. According to Troll and Weberling (1981), "monotelic inflorescences with thyrsiform mode of ramification" prevail in Aizoaceae. These often appear as pleiochasia or dichasia, the higher orders of branching passing over into monochasia, but reduction of the terminal flower occurs in some members of Tetragonioideae and Plin-thus (Hofmann 1973), resulting in various transitional states of polytelic inflorescences. Uniflory is interpreted as the result of reduction of branching, it is widespread among highly succulent Ruschioideae, where the terminal flower only is retained. Similar processes in lateral dichasia of Tetragonioideae and Sesuvioideae result in the formation of one to two axillary flowers. In the first subfamily, flowers of second order can even develop on the outside of the re-ceptacular cup (Eichler 1878; Hofmann 1973).

Inflorescences can take the shapes of umbels, coils or clusters, flowers being sessile or pedicellate. The predominance of large frondose bracts in Aizooideae, Tetragonioideae, and Mesembryanthemoideae often makes it difficult to discriminate between vegetative shoot and inflorescence. In particular in shortlived herbs the vegetative parts can be reduced to one or few basal internodes with leaf pairs (e.g. Ihlenfeldt and Struck 1987) so that the often large plants consist of a well-developed inflorescence only. Small hypso-phyllous bracts and thus distinct inflorescences characterise Sesuvioideae and many Ruschioideae. In several genera, inflorescences can persist either in a dead sclerotinised state (e.g. Aizoon), or as perennial parts of shrubs (e. g. Ottosonderia) with annual additions of dichasia or single flowers.

Flower Structure. The actinomorphic flowers of Aizoaceae are thought to consist basically of three pentamerous whorls. The outermost of these, the perianth, is often named calyx, and consists of (3-)5 (-8) basally united elements (tepals, sepals, Fig. 6 AC) in quincuncial arrangement. The tepals are + succulent and often possess a subapical dorsal appendage (Fig.6B), their lower (outer) surface tends to be papillate. While all tepals look sepaloid from outside, those of the Aizooideae, Tetragonioideae, Sesuvioideae are coloured white, yellow, pink or purple inside, resulting in a petaloid impression of the open flower.

Lampranthus Pustules
Fig. 5. Aizoaceae. Floral apex of Lampranthus promontorii, showing centrifugal inception of stamen primordia on five primary primordia. (Phot P. Leins and C. Erbar)

As a rule, the androecium develops from (4—)5 al-ternitepalous primordia (Fig. 5; Ihlenfeldt I960). An exception is found in Sesuvioideae, where primorida are found alternate and opposite to the tepals (Bittrich 1990). Higher stamen numbers are interpreted as a result of secondary polyandry that develops cen-trifugally. The stamens are either evenly distributed in one to several circles (as in Mesembryanthemoi-deae and Ruschioideae), or are arranged in several fascicles of one to many stamens (Plinthus, Gun-niopsis, Tetragonia). The number of stamens per fascicle can vary even within one flower. Most remarkable is the development of staminodes in the Mesembryanthemoideae and Ruschioideae, the outer circles of which are brightly coloured in most members and are commonly named petals. The filamentous staminodes (e. g. Ruschia, Vanzijlia) are intermediate elements between petals and stamens. The bases of the filaments fuse completely with the perianth forming a perianth-stamen tube of varying length (absent only in two species of Gunniopsis, Chinnock 1983).

The filaments can fuse above their insertion points into the perianth-stamen tube, forming a separate stamen-tube (in most Mesembryanthemoideae, Fig.6D, rare in Ruschioideae, e.g. Conophytum) or connate fascicles (e.g. Galenia). Nectariferous tissue is said to coat the inside of the perianth-stamen tube. The available data indicate that a smooth nectary ring occurs in Trianthema, Aizoon and Tetragonia (Zandonella 1972). However, a crested ring or single prominent nectaries (Rappa 1912) prevails in Ruschioideae (lophomorphic type, Fig.6E), while shell-shaped, partly tubular nectaries characterize the Mesembryanthemoideae (koilomorphic type, Fig.6D; Bittrich 1987).

The syncarpous gynoecium of (l-)2-5(-°°) carpels is completely septate (only in Acrosanthes incomplete septa are known), the styles are rarely fused and develop a papillate stigmatic tissue along a ventral furrow. This tissue is in contact with the inner transmission tissue (papillose ridges parallel to the placenta).

Perigyny occurs throughout the Sesuvioideae, and partly in Aizooideae; weak epigyny is reported for some Aizooideae, semi-epigyny prevails in Mesembryanthemoideae and some Tetragonioideae, and complete epigyny is found in Ruschioideae and some Tetragonioideae; in epigynous flowers a hy-panthium is very rarely developed (Argyroderma, Erepsia).

A central placenta, mostly in the upper part of the columella, characterizes Aizooideae, Sesuvioideae, Tetragonioideae, and Mesembryanthemoideae. Most genera contain numerous ovules in two rows in their ovaries, uniovulate carpels occur, e. g. in Acrosanthes

Argyroderma Aizoaceae
Fig.6.a Sector of an opened hygroscopic capsule of Ruschioideae, showing different expressions of characters, schematic. A Seen from above; B radial section. CB Closing body; CM1 part of covering membrane falling inwards after removal of seeds; CMR distal recurving of covering membrane. (Orig.)

(ovule basal), Galenia, Plinthus, Tribulocarpus, and Tetragonia (pendulous ovule).

Embryology. About a dozen species of Aizoaceae have been studied embryologically up to now. The tetrasporangiate anthers possess 4 (Ruschioideae, Schmid 1925) or 5 wall layers, depending on the development of 1 or 2 (Sesuvioideae) middle layers, which are obliterated later in any case (Kajale 1940). While the endothecium has fibrous thickenings, the glandular tapetum becomes trinucleate; rarely 7 nuclei occur. Cytokinesis follows the simultaneous type and pollen grains are shed in 3-nucleate or 3-celled state (Prakash 1967). The anacampylotropous or campylotropous ovules are bitegmic with the inner integument forming the micropyle. Both integuments are 2-layered or the outer can be 3-layered; they can be separated by an air space at the chalazal end. The crassinucellate ovule possesses a prominent nucellar cap of radially elongated cells in a

Crassinucellate Type Ovule

Fig. 6b. Aizoaceae. Flowers, longitudinal sections. AAizooi-deae. Gunniopsis zygophylloides, perigynous to slightly epi-gynous. B Sesuvioideae. Sesuvium sesuvioides, perigynous, note dorsal appendages of perianth lobes. C Tetragonioideae. Tetragonia tetragonioides, epigynous, with single pendulous ovules, basal spongy tissue becoming cutinised in fruit, and tissue around locules becoming sclerenchymatous. D Mesem-bryanthemoideae. Sphalmantkus trichotomus, epigynous, with pronounced androecial tube and sunken shell-shaped nectaries. E Ruschioideae. Vanheerdea roodiae, epigynous, with basal to parietal placentation. Nectaries stippled. (Orig.)

single layer in the micropylar region (Dnyansagar and Malkhede 1963).

Embryo sac development follows the Polygonum type in most cases, but occurrence of four other types has been reported (Endymion-, Penaea-, Drusa- and Adoxa type), even in ovules of one ovary (Kapil and Prakash 1966). The ephemeral antipodes disintegrate before, at, or shortly after fertilization; the polar nuclei fuse before fertilization.

Embryogeny follows the Solanad type, a massive, mostly bi- to multiseriate suspensor is developed, which can form buds, but the report of polyembryony (Dnyansagar and Malkhede 1963) needs further confirmation. Starch grains have been found in the embryo sac at various stages. Division of the secondary embryo sac nucleus precedes that of the zygote, free nuclei are formed, wall formation starts at the micropylar end, but is rarely completed. The endosperm protrudes into the nucellus, gaining a curved shape. It has been interpreted as a haustorium, the remains of which are pertained as "endosperm cap" around the radicle.

The central nucellar layers develop into a starchy perisperm in the ripe seed to which the incurved embryo is attached dorsally. A thin, mostly 2-layered true aril is known in Sesuvioideae enclosing the seed almost completely.

Pollen Morphology. Pollen grains are usually tricol-pate, rarely tetracolpate or tricolporoidate, subpro-late to prolate, their diameters range from 15 to 60 |im with maxima around 30|xm. The exine exhibits a spinulose punctate or anulopunctate surface like most Centrospermae (Nowicke 1975; Leuenberger 1976 a,b; Skvarla and Nowicke 1976), it is tectate to semi tectate, rarely reticulate or rugose (Dupont 1977). The sexine is mostly thicker than the nexine, an en-dexine layer seems to be restricted to the aperture regions (Roland 1967; Skvarla and Nowicke 1976).

  1. 6b. Aizoaceae. Flowers, longitudinal sections. AAizooi-deae. Gunniopsis zygophylloides, perigynous to slightly epi-gynous. B Sesuvioideae. Sesuvium sesuvioides, perigynous, note dorsal appendages of perianth lobes. C Tetragonioideae. Tetragonia tetragonioides, epigynous, with single pendulous ovules, basal spongy tissue becoming cutinised in fruit, and tissue around locules becoming sclerenchymatous. D Mesem-bryanthemoideae. Sphalmantkus trichotomus, epigynous, with pronounced androecial tube and sunken shell-shaped nectaries. E Ruschioideae. Vanheerdea roodiae, epigynous, with basal to parietal placentation. Nectaries stippled. (Orig.)
  2. The subfamilies Aizooideae, Sesuvioideae and Tetragonioideae are characterised by the basic chromosome number of x = 8, tetraploid and hexaploid conditions occur rarely and one duodeca-ploid is reported (review in Bittrich 1987), disploidy seems to be very rare (e.g. Trianthema monogyna [= T.portulacastrum] 2n = 28, Sharma and Ghosh 1968). In contrast, Mesembryanthemoideae and Ruschioideae possess a basic chromosome number of x = 9, up to 30% of the species of a genus can be tetraploid, while hexaploids are rare (e. g. Brownan-thus marlothii 2« = 54, Bittrich 1987; Cephalo-phyllum acutum 2 n = 54, Hartmann 1986,1989).
  3. The subfamilies Mesembryanthemoideae and Ruschioideae with showy flowers are obviously entomophilous (Hartmann 1991). Melittophi-lous flowers predominate, pollen and nectar are foraged by the visitors, and for some masarid wasps a dependence on flowers of Ruschioideae and Mesembryanthemoideae has been shown (Gess and Gess 1989). Psychophilous flowers occur in several genera (e.g. Conophytum, Stomatium, Mossia), phal-enophilous ones are rare (Conophytum, Vogel 1954). Bittrich (1987) suggests efficient wind pollination as well for some groups with dry pollen; self-sterility is widespread, but cleistogamy has been reported (e. g.

Dorotheanthinae in Ruschioideae, Ihlenfeldt and Struck 1987). An attractive colouring of the inside of the perianth of Aizooideae, Sesuvioideae and Tetra-gonioideae suggests entomophily for these subfamilies as well, but information is scarce (Gun-niopsis is reported to be visited mainly by bees, Lepidoptera, and Diptera, Chinnock 1983).

Fruit and Seed. Loculicidal hygroscopic capsules predominate in the subfamilies Aizooideae, Me-sembryanthemoideae and Ruschioideae (Bittrich and Hartmann 1988, Hartmann 1991), septicidal opening is rare (Gunniopsis, Chinnock 1983). Indehiscent, hard fruits characterize the subfamily Tetragonioideae, but nuts are rare in Mesem-bryanthemoideae (Bittrich 1987) and Ruschioideae (Caryotophora, Ruschianthemum). Circumscissile capsules occur in Sesuvioideae. Schizocarps are developed in two subfamilies: Sesuvioideae and Ruschioideae, berries are found in Carpobrotus. The most complex fruit morphology is exhibited by Ruschioideae, prominent features for seed retention have been developed (Schwantes 1952; Hartmann 1988). Locules can be open, or covered by covering membranes of different construction: flat covering membranes lack further means for seed retention, curved membranes possess different additional closing devices. Closing bodies block the distal exit of the locules to various degrees.

The number of seeds per locule varies between one (in Acrosanthes, Galenia, Plinthus, and Tetragonia), few (in Trianthema, Zaleya and Stoeberia) to many (iSesuvium, Cypselea, most Mesembryanthemoideae and most Ruschioideae). The seeds are small in most genera (< 1 mm), their shape is determined by the well-developed perisperm (as in most centrosperms). Lateral compression is frequent (Mesembryanthemoideae), but rounded sides are also widespread (most Ruschioideae, Sesuvioideae). In side view, seeds are D-shaped, ± reniform, pear-shaped, or triangular, rarely ovoid. The colour of the seeds varies from black over different shades of brown to a pale cream. Seed coats appear glossy in seeds with smooth testae, rough in seeds with papillate, wrinkled or rugose testae. In Mesembryanthemoideae, Ruschioideae, and some Aizooideae, the occurrence of mostly spinulose epicuticular formations has been recorded (Ehler and Barthlott 1978; Bittrich 1987). In many cases, the exotesta develops a prominent outer epidermal wall; of the inner integument either the inner or the outer layer p ersists as well, the remaining layers are normally crushed.

Dispersal. Hygrochastic opening of the capsule prevails in three subfamilies (Aizooideae, Mesembryanthemoideae and Ruschioideae), two main types of ombrohydrochoric dispersal are correlated with this principle: wash-out and splash mechanisms (Hartmann 1988, 1991). Dispersal of seeds from dry capsules is rare (Sesuvioideae), but retention of seeds in time is widespread (e. g. upper part of capsule in Zaleya, Melville 1952; seed pockets, mericarps in Coni-cosia and relatives, Straka 1955). Enclosure of seeds in carpels or parts thereof occurs in Tetragonioideae, Pseudobrownanthus (Mesembryanthemoideae), Caryotophora, and Ruschianthemum (Ruschioideae).

Long-distance dispersal is rare (Tetragonia tetra-gonioides (Pallas) Kuntze via ocean currents, Bogle 1970, Carpobrotus with berries may be dispersed by birds, Carlquist 1983). Medium-range dispersal by birds in nest building (Dean et al. 1990), by large herbivores in their fleeces (Milton et al. 1990), and by tumble fruits (Hartmann 1988), or washes seems to be more widespread than anticipated, but exclusive anemochory is very rare (mericarps of Hymenogyne; seeds are dispersed by wind from the permanently open capsules of Stoeberia).

The widespread opinion that short-distance dispersal prevails in the family needs re-investigation, taking into account that polychory is dominant, but the sequences of the dispersal steps are known only imperfectly.

Reproductive Systems. Protandrous flowers prevail in the family, during the anthesis the flowers open and close for 3 to 14 days (e. g. Gunniopsis, Chinnock 1983). The movements are caused by basal growth of the androecial elements (Hartmann 1978, 1991), flower size can thus increase considerably (Hartmann and Dehn 1987). Most species flower during the daytime, over noon, some open in the afternoon (Here-roa, Pleiospilos), few in the evening (e.g. Stomatium) or at night (e.g. Mossia, Aridaria noctíflora). Cross-pollination is required by most groups, but selfing is known, and some cleistogamous species are reported in which flower size is reduced (Gunniopsis, Chinnock 1983; Dorotheanthinae, Ihlenfeldt and Struck 1987, Trianthema polysperma, Tetragonia tetragoni-oides, Hagerup 1932). Andromonoecy is reported only for Tetragonia (Knuth 1904).

Phytochemistry. The possession of betalains and the occurrence of ferulic acid in unlignified walls confirm the position of the Aizoaceae in the order Centro-spermae.

Condensed tannins are present in xeromorphic members of the Ruschioideae and some Tetragonioideae, they are absent in most Mesembryanthemoideae and those member of the Ruschioideae with idioblasts on the leaf surface; Hymenogyne, however, is remarkable in its possession of epidermal tannin idioblasts. Calcium oxalate incrustations in the outer epidermis cell walls are known from Mesembry-anthemoideae to a lesser and from Ruschioideae to a greater extent, they seem to be absent in Aizooideae, Sesuvioideae and Tetragonioideae. The absence of saponins is reported to distinguish Aizoaceae from Molluginaceae (Hegnauer 1964). Phenylalanin-derived alkaloids of the mesembrin type occur in several genera (Hegnauer 1964).

Photosynthesis follows all three known pathways: Kranz anatomy and the related C4 pathway have been demonstrated for members of the Sesuvioideae (Carolin et al. 1978), CAM is widespread in Mesem-bryanthemoideae and Ruschioideae (von Willert 1979), but only one record of CAM in Tetragonia is known (Schutte et al. 1967). For Galenia dregeana C3 has been reported (von Willert 1979), similarly for few Ruschioideae (von Willert et al. 1977). Changes from C3 to CAM have been shown to occur in various cases.

Subdivision of the Family. The five subfamilies fall into two groups. In the first group, comprising the Aizooideae, Sesuvioideae and Tetragonioideae, attraction of pollinators is achieved by colourful internal faces of the tepals, the basic chromosome number is x = 8 (Bittrich and Hartmann 1988). The second group is characterised by the presence of petals and a basic chromosome number of x = 9 (Mesembry-anthemoideae and Ruschioideae). There is no doubt that the last-named group represents a monophylum (Bittrich and Hartmann 1988; Bittrich and Struck 1989). This fact is reflected in the establishment of family rank, Mesembryanthemaceae, for the group (e.g. Ihlenfeldt and Straka 1962). If this decision is followed, the remaining three subfamilies would automatically become a family as well. The mono-phyly of the first group, however, still remains doubtful (Bittrich and Hartmann 1988; Bittrich 1990).

Synapomorphic characters of the family are a leaf epidermis with idioblast bladder cells, and possibly the development of a simple hygrochastic or hygroscopic capsule with septal expanding tissue (Bittrich and Hartmann 1988; Hartmann 1988).

Affinities. The occurrence of many centrospermous characters in the Aizoaceae (Eckardt 1976) leaves no doubt about the inclusion of the family in the order. Closer relations to various other families have been suggested on grounds of morphological similarities (to Molluginaceae, Hofmann 1973, to Phytolacca-ceae, Ehrendorfer 1976), anatomical features (to Ha-lophytaceae, Thorne 1983), or chemical and flower characters (to Cactaceae, Rodman et al. 1984). Bittrich and Hartmann (1988) state that there is at present no evidence to prove a particularly close relationship between Aizoaceae and any other family of the centrosperms. Common characters are in general widespread primitive ones (Ehrendorfer 1976), possibly indicating a rather basal position of the Aizoaceae within the order (Ehrendorfer 1976). None of these characters, however, can be used to describe synapomorphies of a group of families including Aizoaceae.

Distribution and Habitats. The Aizoaceae inhabit the drier parts of the subtropics and tropics, the centre of highest species frequency lies in the winter rainfall region of southern Africa. It seems not unlikely that the centre of origin may have been near this present distribution centre, because adaptation to dry conditions with an expressed seasonality seems to be one of the apomorphic features of the family. Relatively few taxa, mostly of weedy character, occur very widespread (e.g. Mesembryanthemum crystal-linum in Northern America, around the Mediterranean and into Asia, in Southern Africa), many are very localised in distribution (Hartmann 1987). Outside southern Africa, the family occurs in east and northern Africa, around the Mediterranean, Asia minor, Irak, Iran and India; in Australia and New Zealand; along the western coast of southern and northern America and into the Carribean. In general, disturbed areas are invaded easily, and since many genera are used as ornamentals, garden escapes often establish themselves. As a consequence, Aizoaceae present an adventitious floral elements in certain regions (e.g. in southern England or the Channel Islands), and it is often difficult to determine natural distribution (e. g. in the Mediterranean, even in Australia). Most genera occur in summer-dry areas receiving less than 200mm rain in winter, in other regions (e.g. summer rain), locally dry habitats are preferred (azonal occurrence).

The dominance of Ruschioideae and Mesembry-anthemoideae forms the base of the description of the phytochoron "leaf succulent zone" of the drier parts of the winter rainfall region in southwestern Africa (Jürgens 1987). Several associations dominated by Aizoaceae have been described (Jürgens 1987), and investigations into the dynamics of some of them have been started (Yeaton and Esler 1990).

Economic Importance. Only Tetragonia tetragonioi-des is used widely as a vegetable, namely in drier climates, few other species are locally consumed, particularly in Asia and Africa. The predominance of Mesembryanthemoideae and Ruschioideae in extensive Karoo areas makes them locally important for life stock. The same subfamilies are popular as horticultural plants for their bright flowers, special growth forms are appreciated as curiosities in cultivation and collections ("Living Stones"). Although in some species distinct alkaloids are well known (Sceletium, Trianthema), the medical use is restricted (Hegnauer 1964).

Key to the Subfamilies

1. Petals of staminodial origin present 2

- Petals absent; tepals petaloid inside, sepaloid outside 3

2. Plaeentation central; nectaries shell-shaped to tubular (koilo-morphic); petals and stamens mostly basally united into a tube IV. Mesembryanthemoideae (p. 46)

- Plaeentation basal or parietal; nectaries crest-shaped (lopho-morphic); petals mostly free V. Ruschioideae (p.48)

3. Fruit indehiscent, a winged or horned nut; plants often an-dromonoecious III. Tetragonioideae (p.46)

- Fruit a capsule; inflorescences of hermaphrodite flowers only 4

4. Capsule circumscissile; aril present, covering the seed completely n. Sesuvloideae (p. 45)

- Capsule loculicidal or septicidal, often hygrochastic; seeds never añílate I. Aizooideae (p. 44)

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    How are mesem seeds dispersed?
    3 years ago

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