Cone-bearing plants

The cone-bearing plants comprise the biggest group of nonflowering plants—the division Pinophyta (the gymnosperms). This group bridges the gap between the ferns and the angiosperms—the flowering plants. The structure of gymnosperm cones and the details of their reproduction may indicate how angio-sperm flowers evolved; both groups may have had a common ancestor at some stage in their evolution—gymnosperms are known to date from the Permian period, about 275 million years ago.

The welwitschia (Welwitschia mirabilis) of the Namib Desert appears at first sight to be an unlikely relation of the cone-bearing plants. But the dark patches on top of the plant, centrally placed between its two leaves, are groups of cones. These cones are similar to those of conifers in that those of the male plants bear pollen grains and those of the female plants carry an ovule. Pollination is mainly by means of insects.

Gymnosperm characteristics

The name gymnosperm applies to the seed (gymnos is the Greek word for naked), which is not enclosed within an ovary. This means that the extensions of the ovary wall found in angiosperms—the style and stigma—are absent. In most gymnosperms the fertile leaves (sporophylls) grow in tightly grouped formations called strobili, which in many species are conelike.

The gymnosperms can be divided into three classes, based on the anatomy of their wood, the shape of their leaves, and the structure of their seeds: the Cycadopsida, the Gne-topsida, and the Coniferopsida.

The Cycadopsida comprises the seedferns and cycads, together with other fossil orders. These fern- and palmlike plants are described in the previous article.

The class Gnetopsida contains three odd groups—the welwitschias, the ephedras, and the genus Gnetum. Despite being gymnosperms these plants have close affinities to the angiosperms, particularly in their wood structure.

The Coniferopsida consists of the ginkgos, the conifers, and the yews. These plants have secondary wood that is relatively dense with small parenchyma rays. The wood is simpler than angiosperm wood and contains the elongated cells called tracheids but does not in most gymnosperms have true vessels. Pith rays found in the phloem and xylem tissues transport food from the leaves to the trunk for storage and carry water away from the trunk.

Some conifers, such as spruce (Picea sp.), have no fibers.

The wood parenchyma of gymnosperms is usually associated with resin. These cells may form a network from the resin canals, as in pines (Pinus spp.), or a column of cells that fill with resin. The leaves are usually needle- or scalelike, but may be fan- or paddle-shaped. The leaves or veins branch dichotomously— that is, they branch equally into two and then two again, and so on.

The odd group

The plants in the class Gnetopsida differ widely in appearance but all share some features—for example, they are all woody and contain conducting vessels. Also, the microsporangia have a perianth surrounding them, as may the female megasporangia. The megasporangia themselves are arranged in compound strobili, or inflorescences. No fossils (apart from pollen grains) of any of the three living genera have been found, so relationships between them cannot be determined.

The presence of vessels in the secondary xylem may be a characteristic shared with angiosperms but does not indicate a close relationship with them—it is, rather, an example of convergent evolution. They have evolved from pitted tracheids, whereas angiosperm vessels are derived from tracheids with ladderlike (scalariform) thickening.

Found only in the deserts of southwestern Africa is the unique welwitschia (Welwitschia mirabilis), the only species of the order Wel-witschiales. This peculiar plant has a large, globe-shaped, underground stem, with two strap-shaped leaves that can be 2 to 3 feet (61 to 91 centimeters) wide and often twice as long. The leaves continue to grow throughout the life of the plant (more than 100 years), and are continually split into ribbons by the hot, desert winds. Male and female strobili grow on different plants in the form of a cone covered by scales or bracts. The female cones are larger than the male cones and are scarlet when mature. Pollination is mainly by insects.

The order Ephedrales comprises the small, thin xerophytic shrubs, which are found in North and South America and from the Mediterranean eastward to China. The plants have small, scalelike leaves and are dioecious (with male and female organs on different plants). The male strobili are solitary and form a conelike inflorescence. The female strobili grow in groups of two or three and are wind-pollinated. Depending on the species, the seeds are either dry and winged or brightly colored and fleshy; they are wind- or animal-dispersed, respectively.

The species of the order Gnetales are usually found in tropical rain forests. Most are woody climbers (lianas) although some species grow as shrubs or trees. The leaves are remarkable in that they look just like dicotyledon leaves with a central midrib, net venation, and a broad blade (lamina). The cones resemble small upright catkins and are generally dioecious. The females are wind-pollinated and produce seeds in which the embryo is covered with a protective stony layer and surrounded by a fleshy outer layer. The seeds are dispersed by birds.

The ginkgo

The pretty, fan-shaped, deciduous leaves of the ginkgo (Ginkgo biloba) stand in sharp contrast to the needlelike evergreen leaves of other members of the order Coniferopsida. This tall, spreading tree (right) has ovules at the end of short shoots on the female trees and catkins of male sporophylls on the male trees. The photograph above shows young male catkins.

The ginkgo (Ginkgo biloba), or maidenhair tree, has distinctive two-lobed, fan-shaped leaves. The leaves are identical to those that fell from ginkgos millions of years ago, probably in the Triassic period, when some were preserved in mud and fossilized. Then it had a worldwide distribution, whereas now the ginkgo is found naturally only in Asia. It remains the sole representative of the order Ginkgoales.

The mature tree has a broad spreading crown that reaches a height of about 80 feet (24 meters), with deciduous leaves. Male and female reproductive organs are found on separate trees. The male microstrobili, which bear pendulous microsporangia, are like catkins produced at the end of short shoots. The pollen is transferred by the wind to the ovules, which are also produced at the ends of short shoots. The sperm released from the pollen is large with a spiral band of flagella—a primitive feature. Unlike all other members of the class Coniferopsida, the sperm is motile and swims to the female gamete. The mature ovule falls from the tree in autumn and fertilization may take place while it is on the ground. The seed develops a stony inner layer and an outer fleshy one, which gives off a smell like rancid butter. This smell may be attractive to the animals that are potential dispersers of the seeds.

Conifers

The true cone-bearers belong to the order Coniferales. Most living species are trees, and only a few can properly be described as shrubs. Some of the tallest of living trees are included in the order—the Californian redwood (Sequoia sempervirens), for example.

The order contains six families, the largest one being the pines (Pinaceae). The others are the cypresses (Cupressaceae), the plum yews (Cephalotaxaceae), the redwoods and swarpp cypresses (Taxodiaceae), the araucarias (Arau-cariaceae), and the podocarps (Podocar-paceae). Most are found in the Northern Hemisphere, although the last two families occur predominantly in the Southern Hemisphere.

The wood of conifer trunks has tracheids with large pits in their walls, but no vessels or pores. Resin canals are common, found sometimes in the wood but mostly in the leaves and cortex. The branches are generally regularly arranged up the trunk to give most conifers their distinctive pyramidal appearance.

Most conifers are evergreen and in some species, notably the monkey puzzle tree (Araucaria araucana), the leaves may remain on the tree for up to 15 years. A few species drop their leaves every autumn—larch (Larix sp.), for example. The leaves are usually needle-shaped, scalelike, and small, but may be broad, as in podocarps (Podocarpus spp.l, whose leaves are 12 inches (30 centimeters) long and 2 inches (5 centimeters) wide.

Conifers are adapted to growing in boreal zones and on mountains, where there may be a lack of water when the ground is frozen in winter, although there are exceptions—such as the swamp cypress (Taxodium sp.) of the southern United States. In addition, their pyramidal shape ensures that snow is more likely to slide off the branches than collect on them, possibly breaking them. The conifer leaves are constructed to resist drying out, with a thick, waxy cuticle and, usually, a small size. The needle shape is most resistant to frost. These xero-morphic characteristics equally help those conifers that live in arid places such as sand dunes, as do many species of pine (Pinus sp.). The stone pine (Pinuspinea), for example, is a familiar sight on bare stony Mediterranean hills where water is in short supply.

A cross section through a female cone from a conifer (left) reveals the cone scales, or megasporophylls, each containing one ovule. In these ovules one mega-spore develops into a ga-metophyte. In the male cones (right)the micro-sporophylls each contain several microsporangia, which develop two or three microspores; these eventually become pollen grains.

Conifers can be monoecious or dioecious. In all species, however, the cones are unisexual. The female cones, or megastrobili, contain two ovules. In some species of podocarp, however, there is only one ovule in the cone. The cones usually consist of a central axis with large woody bract scales attached to them, each scale carrying a seed. But not all cones take this form; in some species, such as juniper, the bract scales are so reduced that the cones resemble berries (which are used for flavoring gin), whereas in the podocarps the cones look like small plums.

The male cones, or microstrobili, tend to be smaller than the female ones and simpler. In the larch the male cones turn from bright red to dark red to chestnut-brown when ripe. They contain microsporophylls with pollen sacs— each cone may produce up to 10 million pollen grains. Pollination is by means of the wind in all conifers. The pollen grains that land on the ovule develop a pollen tube with one male nucleus, which fertilizes the female gamete.

The roots of conifers are inhabited by my-corrhizal fungi. The fungal hyphae grow either between the cells of the cortex (when the association is known as ectotrophic mycorrhiza) or inside the cells (endotrophic mycorrhiza). In the podocarps the fungi grow in special root nodules. These fungi supply the trees with minerals, especially nitrogen.

Pines (Pinus spp.l, along with most other conifers, are well adapted to living in cold environments. The pyramidal shape of these trees means that heavy snow will slip off the branches, preventing them from breaking.

The yews and associated species

The evergreen shrubs and small trees that make up the order Taxales grow only in the Northern Hemisphere, except for the monospecific genus Austrotaxus, which is confined to the island of New Caledonia in the South Pacific. There are five living genera, the sixth (extinct) genus Palaeotaxus having provided the basis upon which the order was separated from the true conifers (its seeds were not present in cones, but attached to part of a branch). The yews belong to the genus Taxus, and other genera include the torreyas (Torreya spp.), Amentotaxus, and Pseudotaxus.

Redwoods (Sequoia spp.) are among the largest trees in the world, generally reaching a height of about 200 to 275 feet (61 to 84 meters) and a diameter of 8 to 12 feet (2.4 to 3.6 meters).

The plants vary in size from a height of 100 feet (30 meters) with a diameter of only 10 feet (3 meters) as in Torreya to a height of 65 feet (20 meters) and a diameter of 22 feet (7 meters) as in the yews. The massive trunk of yews may not be a single trunk but represents several fused smaller ones.

Yew leaves are flattened and shaped rather like a Roman sword. They are usually about 1 inch (2.5 centimeters) in length, but reach about 3 inches (8 centimeters) in the Californian nutmeg (Torreya californica). The leaves contain one vascular bundle only, which has special transfusion tissue on each side of it that facilitates the movement of materials between the leaf and the bundle. The leaves spread apart in two rows along the stem. The branches grow out horizontally from the main trunk and form a rather dense umbrella under which little will grow.

Cypresses (Cupressus spp.) are a familiar sight among the olive groves of the Mediterranean countries and are one of the few examples of conifers that do not grow in cold, mountainous regions.

Unlike the wood produced by conifers, the wood of these plants does not contain resin canals or wood parenchyma cells. The yew leaves do not have resin canals, but in the other genera resin sacs occur in the leaves and flowers. The tracheids of the secondary wood have abundant spiral thickening and may account for the elasticity of the wood, which made it so popular for bows.

Whereas the ovules of most of the other gymnosperms, except Podocarpus, are grouped, the ovules of these plants are solitary, growing at the end of a small branch and not in a cone. They are surrounded by stony integuments and a succulent cup, or aril. The red fleshy aril is eaten by birds, which consequently disperse the seeds. The aril is the only part of the plant that does not contain the poisonous alkaloid taxin.

Yews are dioecious. Pollen is produced by microstrobili that take the form of cones or scales. It is transmitted to the female flowers by the wind, and the seeds are produced a few months later. The seeds have two cotyledons that persist after germination for about three years; they are similar in shape to the true leaves, but somewhat larger.

The bright red cups, or arils, of yew trees (Taxus spp.) surround the ovules. The color attracts birds that eat the aril and later disperse the seed.