Magnoliaceae

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Magnoliaceae

Description

Trees or shrubs, glabrous or with an indumentum of single hairs. Leaves spirally arranged, simple, entire or 2-10-lobed, penninerved, evergreen or deciduous; Flowers terminal or pseudo-axillary on a short shoot in the axils of the leaves, bisexual, rarely unisexual, pedunculate. Stamens numerous, free, spirally arranged; — The fruits of Magnolioideae principally consist of several to many (in Michelia mon-tana sometimes one) free carpels spirally arranged around the receptacle. The fruiting carpels are woody and entirely free in some taxa while they are, secondarily, more or less connate in others. The young carpels are always free when initiated, and only relatively late in ontogeny they may become concrescent. Dehiscence of the mature carpels is mostly along the dorsal suture, often also along the ventral suture. Sometimes the carpels become bivalved, the 2 valves only adnate to the central axis, shedding their seeds. Sometimes the carpels form a pseudosyncarp, becoming more or less concrescent. But only the outer layers of the exocarpium are really concrescent, giving the fruit the appearance of a syncarp. In the latter case the dorsal (abaxial) parts of the carpels finally fall away leaving their base, which is inbedded in the receptacle, exposed with the seeds mostly hanging from their placenta on the elongate funicle which in that stage consists of spirally thickened vessels. In some taxa the abaxial parts of the carpels when falling also dehisce along the dorsal suture ('Tsoongiodendron', but also in some 'Talaumas' and in 'Manglietiastrum'). In Magnolia nitida and M. kachirachirai the carpels are essentially connate. When maturing they tear apart and dehisce along the dorsal suture thus giving the appearance of a common Magnolia fruit. This condition probably exists in many species with crowded carpels. The concrescence of the carpels has apparently developed independently in different lineages of Magnoliaceae. Species with free and concrescent carpels sometimes are very closely allied, judging from the other characters. In Liriodendron the fruits are free, indehiscent, samaroid (produced at the apex into a wing-like beak) and, contrary to Magnolioideae, caducous.
Fruit apocarpous, sometimes syncarpous; Seed(s) 1 or more in each fruiting carpel, large, in dehiscent carpels hanging from the elongated spiral vessels of the funiculus, with arilloid testa, rarely, when fruit indehiscent adherent to the endocarp;

Distribution

North America: present S. Brazil: present Southern America: temperate and tropical SE. and E. Asia: present
Seven genera in temperate and tropical SE. and E. Asia and from North America southward through the West Indies and Central America to S. Brazil.

Pollination

— THIEN (1974) studied the floral morphology of North American Magnoliaceae. They are all protogynous. The flowers are pollinated by beetles which enter at all stages from large bud to open flower and feed on stigmas, pollen, nectar, and the secretions of the petals. They become covered with pollen when feeding. Because the pollen is shed after the stigmas cease to be receptive, the first flowers that open often are not pollinated. There are indications that if all the flowers blossom in one flush, as rarely happens after a severe winter, no seed is set (TRESEDER, 1978). The timing of the flowers, including opening and closing of petals, the appression of stigmas to the gynoecium (in some species), large quantities of food, and protogyny all suggest that the flowers are highly specialised for exclusive pollination by beetles. Despite the sharing of pollinators few hybrids have been found in nature. Not enough is known about isolating mechanisms in Magnolia to explain fully the lack of natural hybrids. Between several species artificial hybrids easily can be obtained (THIEN, 1974).

Morphology

— Vegetative morphology. All Magnoliaceae are trees or shrubs with deciduous or persistent stipulate leaves. The stipules are first connate, thus forming a cap for the terminal bud and then rupture longitudinally and fall, leaving a circular scar round the twig. They can be free from the petiole or (partly) adnate to it. Growth of the twigs often is faster in the beginning of the season, thus first producing longer internodes than later. In tropical species, however, there may be no conspicuous difference in length of internodes throughout the twig. In Lirioden-droideae and in tribe Magnolieae of Magnolioideae growth is sympodial, and when a flower terminates a twig, the axillary bud of the uppermost leaf or a lower leaf gives rise to the next vegetative shoot which in turn eventually will be terminated by a flower. In tribe Michelieae growth is monopodial, the flowers grow from the axil of a leaf and are mostly borne on a brachyblast that consists of two or several internodes.

Anatomy

— Vegetative anatomy. The epidermal cells are irregular in form, less often polygonal and with sinuous, curved or nearly straight walls which are often thickened. Many members of the family have a modified subepidermal layer (hypodermis) beneath the upper epidermis, some have such a layer associated with both upper and lower epidermis, and few have it only with the lower epidermis. Many species lack specialised subepidermal cells entirely.

Hairs may be present on both the upper and lower epidermis, or only on the lower. In some species the leaves lack hairs altogether. They are uniseriate, consisting of one to several or many cells. Several types of hair base have been distinguished by BARANOVA (1972), of which the most primitive occurs in Manglietia while the most advanced are found in Elmerrillia, Michelia (Tsoon-giodendron), and in some scattered species of Magnolia.

The stomata are paracytic in Magnolioideae (but in rare cases NONG VAN TIEP, 1980: 519, found also anomocytic stomata in Manglietia) and paracytic as well as anomocytic in Liriodendroideae. They are confined to the lower surface. Significant thickenings on the walls of the epidermal and subsidiary cells, as well as strongly developed cuticular thickenings on the outer wall of the guard cell, occur mainly in the tropical sections of Magnolia subg. Magnolia as well as in many Asian species of Magnolia subg. Talauma. Foliar sclereids are present in certain taxa. They vary widely in form, size, wall thickness, pitting, and degree of ramification. They may be found in four tissues or cell assemblages: First, as idioblastic sclereids in the petiole and along the midrib, especially in evergreen leaves; less frequently in the mesophyll, in dermal layers and in the vein sheath system including veinlet endings.

Taxonomically the mentioned characters are of different value. Each genus shows a particular range of features, some of which can be used diagnostically to separate taxa above the genus level. The coriaceous texture of tropical leaves is achieved in various ways arising from the variable form and diverse distribution of the sclereids. Magnolia subg. Talauma species characteristically have the entire vein system of the leaf encased in sclerenchymatous sheaths, and the veinlets terminate in sclerified elements. Manglietia species in contrast have parenchymatous vein sheaths near the terminations and lack sclerified terminal cells, but most of the mesophyll and epidermal cells are sclerified. Manglietia leaves are characterized by sclerified epidermal and hypodermal layers, sclerified arm parenchyma in the mesophyll, unlignified lobate vein-sheath cells, and an absence of either sclerified veinlet terminal cells or a sclerified leaf margin. No other genus combines all those features, although some do show one or more. Sterile specimens of Manglietia can easily be recognized on the basis of cleared leaf segments.

Magnolia subg. Magnolia and subg. Talauma and Michelia exhibit in many of their members sclerified terminal veinlet cells, thick sclerified leaf margins, and stellate sclereids along the midrib. Rarely those features may be diagnostic for species, but in most cases they are too common to be useful. The foliar characteristics of Magnolia subg. Talauma include veinlets terminating in thick-walled stellate sclereids and thinner walled tracheary elements. These cell types are typically absent from temperate zone Magnolia sections and therefore are possibly an adaptation to climatic conditions. In all the Asian species of subg. Talauma examined there is a thick sclerified margin, usually including a vein. In the American species this is absent but in few of them a weak approach to this condition is seen.

The massive marginal ridge helps to identify certain species of Magnolia subg. Talauma and subg. Magnolia, Manglietia (3 spp.), and Michelia (3 spp.).

In Alcimandra, Liriodendron, 'Paramichelia', and most Elmerrillia species foliar sclereids are rare. This may indicate that these taxa have not developed vegetative modifications different from those of the related large genera (mainly after BARANOVA, 1972 and TUCKER, 1977).

Taxonomy

The family can easily be divided into two subfamilies, Magnolioideae and Liriodendroideae, the latter not in Malesia.

Cytology

The basic chromosome number is x = 19. Polyploidy has been demonstrated in Magnolia. In wild species diploidy as well as tetraploidy is found. TRESEDER (1978: 208) reports for cultivated Magnolias besides diploid, also triploid, tetraploid, pentaploid, hexaploid, hepta-ploid, and octoploid numbers, while aneuploid numbers also are recorded.

The chromosomes are characteristically small, short rods (WHITAKER, 1933). The chromosome base number of all Angiosperms is 7, the higher numbers are derived from it by palaeopolyploidy (EHRENDORFER C.S., 1968). The same basic chromosome number of 19 is found in Myristicaceae (also 21) and Monimiaceae (also 22).

Phytochemo

Chemical characters of the family were summarized and analyzed by HEGNAUER (1969). A general occurrence of essential oils deposited in large idioblasts and of alkaloids of the biosynthetic pathway resulting in the so-called benzyltetrahydroisoquinoline family of alkaloids comprising among others benzylisoquinolines, bisbenzylisoquinolines, aporphines, oxoaporphines, and protoberberines was stressed. Leaf phenolics were represented mainly by kaempferol, quercetin, rhamnetin, procyanidins, and caffeic acid. At the same time a total lack of flavonols with trihydroxylated B-ring, of ordinary flavones and of galli- and ellagitannins had been demonstrated. Other characters mentioned are strongly silicified leaves in many taxa, oil-rich seeds without starch, and a strong tendency to store cyclitols in Liriodendron (liriodendritol), Magnolia subg. Magnolia (pinitol) and subg. Talauma (quercitol). The sporadic occurrence of lignans (lirioresinol), neolignans (magnolol), coumarin glycosides (magnolioside), cyanogenic compounds (not identified), and of the sesquiterpene lactones costunolide and parthenolide was noted. The totality of known chemical characters was interpreted as being highly typical of Polycarpicae.

In the meantime much additional phytochemical research was performed with members of the genera Alcimandra, Elmerrillia, Liriodendron, Magnolia subg. Magnolia and Talauma, and Michelia. In each instance alkaloids were isolated, some being new compounds and some already known from other members of Policarpicae. All, however, belong to the phenyltetrahydroiso-quinoline class. The oxoaporphine liriodendrine and the quaternary aporphine magnoflorine seem to be nearly ubiquitous. New features are the rather general occurrence of biologically active sesquiterpene lactones belonging to the germacranolide, eudesmanolide, elemanolide, and guaianolide groups of these constituents, and of lignans and neolignans (together called lignanoids). Hitherto sesquiterpene lactones were isolated from species of Liriodendron, Magnolia, and Michelia. All species investigated for lignanoids were shown to contain such metabolites. Today many individual compounds are known from members of Liriodendron, Magnolia subg. Magnolia and Talauma, and Michelia; they represent at least 12 different structural types. Cyano-genic compounds were detected in Liriodendron tulipifera (taxifolin and triglochinin) and in Magnolia sprengeri cv. 'Diva’ (taxifolin). They belong to the tyrosine-derived class of cyanogens.

From the taxonomic point of view the secondary metabolism of Magnoliaceae can be considered as highly characteristic of Polycarpicae and closely related orders such as Piperales and Aristolochiales. They all have essential oils in idioblasts and comprise members synthesizing ben-zylisoquinolines. Similar lignanoids are presently known, among others, from Aristolochiaceae, Eupomatiaceae, Lauraceae, Myristicaceae, Piperaceae, and Trimeniaceae. Sesquiterpene lactones occur also in Chloranthaceae and Lauraceae. They form one of the arguments for the proposition of an evolutionary line Polycarpicae — Rutales — Umbelliferales s.str. — Asterales. — R. HEGNAUER.

Embryology

— The anther wall at the microspore mother cell stage comprises epidermis, endo-thecium, 2-4 middle layers, and a bilateral glandular tapetum. By the time the cytokinesis is completed in the microspore mother cells, a large number of Ubisch granules line the inner walls of the tapetum. In a fully mature anther the papillate epidermis and endothecium along with 2 or 3 middle layers persist. After meiosis II in the microspore mother cell the cytokinesis takes place by furrowing, resulting in tetrahedral or isobilateral tetrads. The mature pollen is shed at the 2-celled stage. The generative cell is surrounded by a thin sheath of finely granular cytoplasm and a delicate membrane.

The ovules are anatropous, bitegmic, and crassinucellate; the outer integument is vascularised. The hypodermal archesporium is multicellular and of hypodermal origin although ultimately only one cell functions. The primary parietal cell divides repeatedly to form the parietal tissue so that the megaspore mother cell is buried deep in the nucellus. At the end of meiosis II linear or T-shaped megaspore tetrads are formed. The chalazal megaspore functions, giving rise to the Polygonum type of embryo sac. The synergids and antipodal cells are ephemeral. The endosperm is cellular from the beginning, and embryogeny conforms to the Myosurus variation of the Onagra type or is irregular (HAYASHI, 1964, 1966; BHANDARI, 1971; DAVIS, 1966).