ginkgophyte

▪ plant division

Introduction

      any member of the division Ginkgophyta, a group of gymnospermous plants of particular interest to paleobotanists. Two of the three genera of ginkgophytes, Ginkgoites and Baiera, are extinct. The third genus, Ginkgo, has only one member, Ginkgo biloba, commonly called the ginkgo tree. It is also known as the maidenhair tree because of the resemblance of its bilobed leaves to those of the maidenhair fern. G. biloba may be the oldest living seed plant, and it is regarded by some as one of the wonders of the world.

General features

      Fossil leaves with similar form and venation to the living Ginkgo have been found in the Jurassic period (200 to 145 million years ago). These fossils have been described from such geographically separated areas as Australia, western North America, Mongolia, Alaska, England, and central Europe. The fossils vary greatly in form and are usually described as species of the genus Ginkgoites. Almost the same degree of variation in leaf form can be found on a living Ginkgo tree, however. Some paleobotanists, therefore, have recommended the abandonment of the genus Ginkgoites and the recognition of several species of Ginkgo.

      There is one type of ginkgophyte leaf in the fossil record that is generally regarded as a distinct form and is given the generic designation Baiera. The leaf is deeply lobed into four segments and lacks a stalk (petiole). Following the Mesozoic Era, Ginkgo declined progressively in its distribution, and some botanists believe that remote portions of southeastern China are the last natural home of the maidenhair tree. After the outer fleshy seed coat is removed, the seed kernel is used as food in China and Japan. For 3,000 years or longer, extracts of the Ginkgo leaf have been recommended in Chinese medicine as benefiting the heart and lungs. Ginkgo has been investigated for its effects in the treatment of asthma, toxic shock syndrome, and various circulatory disorders. In addition, it has been used by many individuals for its purported enhancement of memory function, which has been studied in hopes of finding a treatment or preventative for Alzheimer disease.

Form and function

Stem

      At maturity a Ginkgo tree can reach heights of 20 to 30 metres (65 to 100 feet). Young trees often have a central trunk with regular, lateral branching; in older trees the branching is irregular.

      A conspicuous feature of Ginkgo is the possession of long branches and short, or spur, branches. Leaves are produced on long branches during the spring growth. In subsequent years, clusters of leaves are formed on the lateral short branches. Ginkgo is deciduous, and the leaves of some varieties turn a beautiful golden colour in the autumn. The colour varies to some extent among horticultural varieties. There is some plasticity in growth form in that a short branch may become a long branch or the tip of a long branch may be converted to a short branch. The interplay between these two types of branches accounts for the more irregular shape of the older trees. Branching appears to be controlled by the distribution of auxin, a naturally occurring plant hormone.

      The trunk diameters of the older specimens of Ginkgo may become large as a result of secondary growth. The vascular cambium gives rise to secondary phloem and secondary xylem (wood) for the conduction of water and dissolved minerals. The growth activity of the vascular cambium is sustained in the trunk and long shoots and produces a rather hard wood with well-defined growth rings. The activity of the vascular cambium persists in short shoots, but only a limited amount of soft wood is produced each year.

Leaves (leaf)

      One of the most distinctive features of G. biloba is the foliage leaf, which consists of a leaf stalk (petiole) and a fan-shaped dichotomously veined blade, or lamina. Although biloba (or bilobed) correctly describes the form of many Ginkgo leaves, there is a great range of variation in the degree of lobing and dissection among leaves of the same tree. Bilobed and undivided leaves occur on spur or short branches, while most of the leaves on the upper part of a long branch are divided by a deep sinus into two lobes, each of which may be further lobed. Multilobed leaves also occur on new branches (sucker shoots) arising from the tree trunk at ground level.

      The dichotomous venation pattern in a leaf blade is a striking morphological characteristic of Ginkgo. Two vascular bundles extend through the petiole and give rise to two systems of dichotomously branched veins. This type of venation was also present in the leaves of extinct members of the Ginkgoales. Such a system of venation is often referred to as an open type, devoid of vein fusions. It has been shown, however, that vein unions may occur with some regularity.

Reproductive structures and function

      Completion of the entire reproductive cycle, from the advent of pollination to the production of seeds with well-developed embryos, takes about 14 months. Pollination and the development of the sexual, or gametophytic, phase of the life cycle occur in the first year (April to September), but embryo development is not completed until the spring of the following year.

      Ginkgo is dioecious, which means that pollen-producing structures and ovules (ovule) are produced on separate trees. The reproductive structures are restricted to the spur branches, where they are evident in the spring in the axils of bud scales and foliage leaves.

      The pollen-producing strobilus is a loose, pendulous, catkinlike structure consisting of a main axis to which are attached numerous appendages, each of which usually bears two microsporangia at its tip. Meiosis occurs in cells of the microsporangia, giving rise to numerous haploid microspores. Cell divisions take place within the microspores, resulting in the formation of five-celled pollen grains (male gametophytes).

      Ovuliferous structures also arise in the axils of bud scales and the foliage leaves of spur branches. Each consists of a stalk that bears two or sometimes three or more erect ovules. An ovule is composed of an integument (the future seed coat) surrounding a tissue called the nucellus. It is in the nucellus that meiosis occurs, resulting in the formation of four haploid megaspore cells. It is at about this time that pollen grains are released from the microsporangia of male trees. The pollen (male gametophyte) is carried by wind currents and adheres to a pollination droplet, which exudes from the micropyle at the tip of the integument. Retraction of the droplet brings the pollen grains into a pollen chamber in the nucellus, where they develop into multibranched pollen tubes (male gametophytes).

      One of the megaspores in the ovule that results from meiosis enlarges and undergoes a succession of free nuclear divisions (without wall formation). After about 8,000 haploid nuclei are produced, cell walls begin to form. After the female gametophyte becomes cellular, archegonia (normally two) are initiated at the surface toward the micropylar end of the ovule. An archegonium consists of neck cells and a large egg cell.

      The basal end of the filament-like male gametophyte becomes suspended in a cavity above the female gametophyte (called the fertilization chamber). The spermatogenous cell of a male gametophyte divides, resulting in the production of two multiflagellated sperm. The sperm and the contents of the pollen tube are released into the fertilization chamber. The sperm swim in the liquid for a brief period of time. Approximately 1,000 flagella are attached to a spiral band at the anterior end. A sperm enters an archegonium and fuses with the egg nucleus. Ginkgo and the cycads are the only seed-producing plants that have motile sperm.

      The growth of the embryo (embryogenesis) may begin shortly after fertilization but continues after the developing seeds fall to the ground. The embryo grows into the nutritive tissue of the female gametophyte. A seed at maturity consists of a dicotyledonous embryo, nutritive tissue of the female gametophyte, and the seed coat, which is made up of a hard inner layer and a fleshy, orange-coloured outer layer. Because of the presence of butyric acid, upon decay the fleshy layer emits an odour similar to rancid butter.

Classification

      In earlier systems of classification, Ginkgo was placed in the class Coniferopsida, along with conifers (e.g., pine, fir, spruce). In recent years Ginkgo and its fossil allies have been placed in a separate group, the division Ginkgophyta (sometimes classified as the class Ginkgopsida), in recognition of the many characteristics outlined above. The early ancestral stock of Ginkgo extends back in the fossil record to a time coordinate with the ancestors of the conifers, but the two groups appear to have evolved independently.

Division Ginkgophyta

 Large trees; leaves typically fan-shaped and bilobed, or with more lobes, especially in fossil forms; leaves borne mainly on spur (short) branches; male and female trees; seeds with a fleshy outer layer that upon decay, emits an odour of rancid butter; a single order, Ginkgoales; a single family, Ginkgoaceae; a single extant genus, Ginkgo; 2 extinct genera, Ginkgoites and Baiera.Ernest M. Gifford

Additional Reading

Thomas N. Taylor and Edith L. Taylor, The Biology and Evolution of Fossil Plants (1993), provides details on the fossil record of ginkgophytes. Details of the life cycle and morphology of ginkgo are in Harold C. Bold, Constantine J. Alexopoulos, and Theodore Delevoryas, Morphology of Plants and Fungi, 5th ed. (1987).

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