magnoliid clade

Diversity of structure

The magnoliids illustrate much of the diversity that characterizes the dicotyledonous plants as a whole. Evergreen and deciduous trees and shrubs are found in the clade, as are perennial herbs and a few annual herbs. Trees, shrubs, and vines are characteristic of Magnoliales, Laurales, and Piperales. The latter also contains herbs. Canellales consists largely of shrubs and trees.

Most magnoliids have features of a relatively archaic nature: in the flowers, the usually unfused carpels are surrounded by either many petals or none at all; the numerous, sometimes leaf-shaped, stamens release two-celled pollen that often contains only a single aperture; the ovules are surrounded by two integuments; and the mature seeds usually contain a small embryo and usually copious endosperm. Biochemically, magnoliids are characterized by the presence of benzylisoquinoline or aporphine alkaloids, which are secondary metabolites with a defensive function and are rare in other groups. Only rarely do the magnoliids produce tanniferous substances, and betalains, iridoid compounds, or mustard oils are not evident. These different classes of defensive agents do, however, occur in some other groups.

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Distribution

The magnoliid clade has members throughout the world, especially in tropical, subtropical, and temperate areas. This is true of Annonaceae, Myristicaceae, and Magnoliaceae, three of the largest families in the order Magnoliales. A few species are found in Australia, New Guinea, and Fiji. Winteraceae (Canellales) is principally found in the southwestern Pacific, including New Guinea, New Caledonia, and Australia. A few species occur in Central and South America. Piperales and Laurales occur almost exclusively in tropical and temperate areas of the world.

Reproduction and life cycles

The reproduction and life cycles of the more primitive extant members of the magnoliids reflect stages in the life histories of the early angiosperms that cannot be found in the fossil record. The more primitive families of the subclass also exhibit some of the basic, primitive features of the angiosperms as a whole.

There are usually two pairs of microspore- (pollen-) producing sacs in an immature, developing stamen, each divided by a partition to make four compartments. The stamens of the most primitive magnoliids have four pollen sacs, although some genera of a few families have only two pollen sacs as a derived condition. The tapetum, the nutritive layer of cells that lines the inner wall of the pollen sac, is of the secretory, or glandular, type in the Magnoliales and other primitive members (see angiosperm: Reproductive structures). The tapetal cells remain intact but become absorbed as they supply nutrients to the developing pollen grains. An amoeboid tapetum, on the other hand, breaks down early, and the contents of the cell (protoplasm) extrude between the young pollen grains, providing a more efficient way of nourishing them. This type of tapetum has been found in the Lauraceae (Laurales), where both types of tapetum occur.

In the more primitive angiosperms and in all magnoliids, pollen grains are released in the two-celled condition (one tube cell, which expands to form the pollen tube at germination, and one generative cell, which divides to form two sperm cells); in advanced subclasses of flowering plants, they are released in a three-celled condition (one tube cell and two sperm cells), because sperm cells are formed before the pollen is released. An exception of sorts occurs in Laurelia (Monimiaceae) and Beilschmiedia (Lauraceae) of the order Laurales: in some of these pollen grains are two-celled, in others they are three-celled, and in still others the sperm cells are in the process of forming as the pollen is liberated.

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In the most primitive magnoliids three of the four megaspores formed from the megakaryocyte (megaspore mother cell) degenerate. A female gametophyte of eight nuclei, including the ovum (egg), develops from the surviving megaspore (see angiosperm: Reproduction). About 70 percent of angiosperms have this type of female gametophyte development. With few exceptions in the subclass, two integuments form the seed coat of the ovule.

In magnoliids the ovules are inverted so that the opening in the ovule through which the pollen tube enters (the micropyle) remains alongside the stalk (funiculus, or funicle) that attaches the ovule to the ovary. This contrasts with the gymnosperms and some advanced groups of angiosperms, where the ovule does not bend back upon itself but rather remains erect, with the micropyle at one end and the funiculus at the other.

The endosperm is a copious embryonic nutritive tissue that occupies much of the mature seed in more primitive angiospermous plants. It provides nourishment for the developing embryo and forms with the embryo during double fertilization, a reproductive process unique to angiosperms.

The more primitive magnoliids have a cellular type of endosperm. In the nuclear type of endosperm, repeated nuclear divisions take place before cell wall formation. Nuclear endosperm occurs in the Myristicaceae (Magnoliales). Both cellular and nuclear endosperm have been found among the Lauraceae (Laurales), lending support to the theory that one type has evolved from the other, and vice versa, many times.

The seed of a primitive angiosperm, such as Winteraceae and Degeneriaceae (Magnoliales), contains a minute, relatively undifferentiated embryo, which occupies only a small part of the seed at maturity. Such plants are at a disadvantage. Because the embryos are so extremely small at the time that the seeds are shed, considerable time is lost while the embryo develops further—i.e., before the actual rupture of the seed coat occurs and a seedling can arise. Also in these primitive plants, the pattern of embryo development is rather irregular and inconsistent—e.g., in Degeneria vitiensis and Drimys winteri. In some of the more advanced magnoliids—for example, Cinnamomum of the Lauraceae (Laurales)—the seeds contain large embryos and little or no endosperm. These angiosperms also have an established pattern of development from an early stage. This means that one can predict not only which cell or cells of a young embryo will be the next to divide but also what their plane of division will be. In all but a few of the primitive magnoliidae groups, embryo development has a set pattern and follows one or the other of the types described for flowering plants in general.

It has been argued that the hypothetical primitive dicotyledonous plant had three or four, rather than two, cotyledons. One of the most primitive angiosperm orders, Magnoliales, has members in which the embryos contain three or four (very rarely two) cotyledons, such as Degeneria (Degeneriaceae). Idiospermum (Calycanthaceae) of the Laurales has three or four cotyledons as well. On the other hand, embryos in some of the more advanced angiosperms also contain more than two cotyledons—e.g., Pittosporaceae (Rosales; Rosid I group), in which it seems certain that the polycotyledonous condition evolved from the dicotyledonous one, because the more primitive relatives of these plants have two cotyledons. Whether angiosperms first had three or four, rather than two, cotyledons remains uncertain.

In the few primitive families that have been investigated (e.g., Winteraceae and Eupomatiaceae), the cotyledons emerge from the seed and are elevated above the surface (epigeal development), where they become green and capable of photosynthetic activity. In an alternate form of seed development, hypogeal germination, the cotyledons remain inside the seed coat. This also occurs within the magnoliids and in some Annonaceae (Magnoliales). There is an intermediate form of seed germination, as seen in some species of Peperomia (Piperaceae; Piperales), in which one cotyledon remains inside the seed and functions as an absorbing organ and the other becomes the first leaf of the seedling.

In some angiosperms the fruits are dispersed whole with their included seeds; in others, the fruit opens to release the seeds. The most primitive angiosperm fruit is often said to be a follicle. This consists of a single carpel that opens along a ventral suture to release individual seeds. Although follicles are found in some primitive members of the magnoliids, such as Magnoliaceae (Magnoliales), and occur in the early fossil history of flowering plants, many other types of fruits are seen in the subclass as well. Most magnoliids, however, have indehiscent fleshy fruits, predominantly berries, which are dispersed by birds.

Ecology and habitats

Magnoliids occupy a wide range of habitats and are found in most countries. They occur in most habitats where other flowering plants exist, except in salt water, where only a few specialized angiosperms can survive. Magnoliids include short, medium, and tall trees; shrubs, some in Alpine regions; scramblers, vines, and climbers, a few of which are root parasites; and herbs. They occur in tropical, subtropical, and temperate forests, in shrublands, on streambanks, in grasslands, in bogs and marshes, and on mountain slopes.

Some of the more primitive angiosperms are considered to be rare and endangered species. One of the most vulnerable is Lactoris fernandeziana (Lactoridaceae; Piperales), a family distantly related to other families of the order Magnoliales. The plant grows on a single island, Nearer Land Island of the Juan Fernández Islands, 650 km (400 miles) west of Chile. A tiny shrub, Lactoris is sparsely distributed in fog-swept forests and grows under the shade of shrubs and ferns. The principal mechanisms of extinction may be grazing animals and competition from hardier plants.

A relict genus may rely on a particular pollinator for its continued existence, the absence of which may mean the extinction of the dependent plant species. The Eupomatiaceae (Magnoliales), another family quite isolated taxonomically from others, contains two species of Eupomatia, both of which occur in eastern Australia and one of which is also in New Guinea. Eupomatia species are pollinated by a single genus of beetles (Elleschodes); if the beetles become extinct, so probably will Eupomatia.

Winteraceae is generally considered to be one of the most primitive groups of the flowering plants and is found farther back in the fossil record than any other known family. Takhtajania is unusual in the family in having a flower with an ovary consisting of two fused carpels with peripheral ovules. Its single species, T. perrieri, is endemic to Madagascar.

The greatest number of species of magnoliids are native to tropical regions, and there probably remain many undescribed members of the subclass in that area. With the rapid clearing of tropical forest for agriculture and with the populations of most tropical countries increasing steadily, many members of this group face extinction in the future.

Vegetative structures

The tracheid is the basic conducting element in the xylem of vascular plants. It is an elongated water-conducting cell, dead at maturity and surrounded by a lignified secondary wall. The vessel-less tracheids of gymnosperms permit relatively slow water movement, making these plants vulnerable to wilting when water transpires from leaves faster than it can be replaced from the roots. Thus, the greatest limitation on gymnosperms has been not in geographic range but on the habitats they can occupy and on the life-forms they can exhibit. (Gymnosperms, for example, are not annuals, and they have adapted small leaves and thick cuticles as protection from desiccation.) Angiosperms, on the other hand, have evolved a vessel system from the tracheids, which has enabled them to occupy the widest possible range of habitats (see angiosperm: Structure and function). The two characteristic conducting structures of the angiosperm tracheary elements are vessel members and the tracheids. Vessel members resemble the tracheids but have perforations usually confined to the end walls of the cell (tracheids have pits, which are thinner parts of the wall but not perforations, along the length of the cell). Several vessel members are joined end to end to form vessels, providing more efficient water conduction.

Vessels have evolved only to the level of conductiveness required by a particular plant. Stages in their evolution are therefore preserved in extant plants, especially the primitive magnoliids. The most primitive vessel members—e.g., in Eupomatia (Eupomatiaceae)—resemble the tracheids from which they evolved, being long and narrow with long, sloping end walls. The only difference is that, in vessel members, perforations replace the membranes inside the many bordered pits, forming scalariform vessels with which to expedite the movement of water through the vessel member.

Fewer than 200 species of angiosperms have the primitive feature of vessel-less wood. With two exceptions, all vessel-less angiosperms occur in the magnoliids. The exceptions, Trochodendron (Trochodendraceae) and Tetracentron (Tetracentraceae), show definite links with the magnoliids but are classified in the Eudicot clade. Of the magnoliids, all Winteraceae (Canellales) and Amborellaceae (Laurales) lack vessels.

As they evolved, vessel members became shorter, wider, and rounder. The many transverse slitlike perforations arranged one above the other like rungs in a ladder (scalariform perforation plates) in the long sloping end walls were gradually replaced by fewer, larger, rounder perforations in more transverse end walls, leading to transverse walls with a single large perforation (simple perforation plates). Such vessels are weaker. Large woody plants that possess them compensate by having many fibres or fibre-tracheids in their wood.

Only in angiosperms are sieve tubes and companion cells found in the phloem (see angiosperm: Vascular tissue). In other vascular plants, parenchyma cells function in the same way as companion cells (that is, as the sieve cell’s living protoplasm), but they are not derived from the same mother cell as the sieve element. The sieve cells of gymnosperms and pteridophytes are less efficient conductors of food materials than the sieve tube members of angiosperms, because they do not have enlarged sieve pores in their more sloping end walls. The only angiosperm to have parenchyma cells with the same function as companion cells is Austrobaileya (Austrobaileyaceae; Austrobaileyales). Austrobaileya seems to retain a stage in the evolution of phloem in angiosperms, for a few companion cells have recently been found in its phloem as well.

A wide range of leaf types are found in the magnoliid clade, some resembling those of early fossils. The types of stomata also range widely, sometimes even within a single family (e.g., Winteraceae). There is, however, no clear way to discern the primitive from the advanced types of stomata. Most magnoliids contain ethereal oil cells, commonly with isoquinoline alkaloids, in their leaves and often in other aerial parts as well. Such ethereal oil cells are not found in other subclasses. It has been suggested that they form a chemical defense mechanism against predators and pathogens.

Reproductive structures

The archetypal angiosperm flower may well have lacked sepals and petals, for some primitive magnoliids seem to lack sepals and petals (a perianth) or tepals. In Eupomatia (Magnoliales), for example, the young flower bud is covered by a floral leaf (bract) that drops off at the time of flowering, exposing a naked flower (with stamens and carpels). Although the flower appears to have petals, it is actually a false, or pseudo-, perianth because it lies between the stamens and carpels rather than surrounding these reproductive structures. The pseudoperianth is thought to have evolved from sterile stamens (staminodes). It releases odours that attract beetle pollinators and is partially eaten by them. Some primitive magnoliids have a dull perianth, while others, such as Magnolia, have a showy petal-like one. In woody magnoliids, however, there is never a clear differentiation into typical sepals and petals.

Early fossil flowers and the vast majority of magnoliids are bisexual, although the fossil record indicates that early flowers could revert to unisexuality. This has occurred in some primitive families. For example, unlike other Winteraceae, most species of Tasmannia have unisexual flowers. The flowers indicate their bisexual origins by the presence of sterile carpels in the centre of the male flowers. In the magnoliids, as in early fossil flowers, the number and arrangement of floral parts varies. In other subclasses these characteristics are more fixed.

The discovery of Degeneria (Degeneriaceae) in Fiji in 1942 by the American botanists Irving W. Bailey, Albert C. Smith, and others renewed researchers’ interest in discovering the most primitive dicotyledonous plant group. Degeneria is an example of a vanulean angiosperm with primitive stamens and carpels. It has leaflike three-veined stamens and carpels rather than the obvious filaments and anthers of more modern groups. A pair of pollen sacs embedded between the midvein and each lateral vein run most of its length. In the leaflike carpels, a row of ovules is embedded on either side of the midvein, and the carpel is folded in along the central axis, with the seams facing the centre of the flower (a conduplicate carpel). This primitive carpel has been found in a Cretaceous fossil and in some of the Winteraceae (Canellales). Conventional carpels—with stigma, style, and ovary—evolved in a number of families of Magnoliales, including the Magnoliaceae. Similarly, there are members in the Magnoliales that exhibit leaflike stamens and stages in evolution of stamens from the leaflike three-veined condition to the conventional single-veined stamens with anther and filament. The number of carpels per flower varies considerably in the subclass. The carpels are not fused in almost all of the Magnoliales, Canellales, and Laurales, but in some Piperales they are fused.

Pollen structure is diverse within the magnoliids. Pollen with three elongated apertures equidistant from one another (tricolpate pollen) and related forms are dominant in the more advanced angiosperm subclasses but are almost absent in monocotyledons as well as in all but a few magnoliids. The most frequent pollen types found in magnoliids and in the earliest angiosperm fossil pollen are monosulcate (in which grains have a single elongated aperture), inaperturate (without apertures), polyforate (with many round apertures), and biaperturate pollen. These types are not fundamentally different, for Trimenia (Laurales) has pollen that is inaperturate, polyforate, or disulculate (a biaperturate type). Such forms would have evolved from one to another many times over.

There is increasing evidence that early flowers were pollinated by insects and that this was one of the early events that triggered the evolution of the flowering plants. Some insect groups were well established before angiosperms evolved, especially beetles (Coleoptera) and flies (Diptera). The Hymenoptera (wasps and bees) appeared in the Triassic Period (about 251.9 million to 201.3 million years ago), although it seems that they and Lepidoptera (butterflies and moths) only came into prominence near the end of the Cretaceous Period (145 million to 66 million years ago). Pollination by beetles is found in many primitive magnoliids, including Magnolia, Eupomatia, Calycanthus, and possibly Degeneria. Pollination in various members of the Winteraceae is carried out by beetles, primitive moths, flies, thrips, and even caterpillars. Plants highly adapted for wind pollination, such as grasses (Poaceae), did not appear until later in the evolution of the angiosperms.