ORCHID biology and cultivation

Orchids are fascinating plants. They depend on a peculiar association with fungi during their young stages; they may take years to reach maturity; they may be exceedingly localized or rare; and above all they have some of the most beautiful, colourful and incredibly bizarre flowers known to man. Some are extremely difficult to cultivate. These characteristics perhaps explain the interest and the excitement that is conjured up by the word ‘orchid’.

As a group the orchid family is one of the largest families of flowering plants in the world with up to 30,000 or more species belonging to nearly 800 genera. Orchids grow all over the world but the majority of species occur in the warm humid tropics and their numbers dwindle towards the poles. Nevertheless a few species occur within the Arctic Circle in Alaska, Greenland and Siberia, while other species occur above the tree line on mountain ranges.

Structure

All orchids are perennial herbs, but they exhibit an astonishing variety of size and form. The most characteristic features are those of flower structure. The flower is composed of six perianth segments in two whorls. The outer whorl represent the sepals, while the inner whorl represent the petals. One petal (the median posterior) is larger and distinctly different from the others and forms the lip (labellum). As a result of the twisting of the ovary through 180° this lip usually points downwards in the centre of the flower. In most orchids the reproductive organs are borne on a single structure called the column, which is formed by the fusion of the stamens and style. The complex structures of the orchid flower are related to the mechanism of pollination.

The orchid fruit is a capsule formed by the fusion of three carpels. When the fruit is ripe longitudinal slits appear, opening from the tip, to release innumerable minute seeds. These seeds are remarkable for their extremely small size (three million seeds weigh only one gram) and their lack of both food reserve (endosperm) and a well-formed embryo plant. An almost incredible four million seeds are produced by a single fruit capsule of theorchids Cynoches and Anguloa.

Habitats and adaptations

The largest number of orchids are epiphytes growing on trees in tropical rainforests. They are most abundant in the extremely humid cloud forests of mountain slopes which create ideal conditions for epiphytes with the trees densely clothed with mosses, an abundance of water and an angle of inclination of the ground which allows a good penetration by sunlight. Temperate species and a few tropical ones are terrestrial herbs that inhabit grasslands, woodlands, marshes and bogs, or rocky mountain ledges. A few woodland species of temperate zones, such as the coral-root orchids, Corallcr-hiza, are saprophytes which lack the green pigment chlorophyll and which cannot produce their own food. As a consequence these plants are dependent on the decaying humus in the soil.

The epiphytic orchids of the tropics grow on the branches of trees for support. The relationship is not a parasitic one for the orchid derives no food from the tree except for substances in the decaying outer bark. These orchids have specialized roots that adapt them for living in such an inhospitable place. Clinging roots attach the plant to its support. The network of roots that forms between the plant and the branch entangles dead leaves and other debris and retains a reservoir of humus into which the absorbing roots grow. There are also aerial roots that hang down in festoons. These roots contain green tissue and serve to assimilate. The fleshy living tissue is covered with spongy dead tissue that absorbs water during periods of rain or heavy dew but which protects the living tissue from dessication during dry conditions. All epiphytes have special arrangements for shedding their leaves with a joint where the leaf blade meets the sheath. In parts of the tropics that experience a pronounced dry season these epiphytes may lose their leaves and hibernate as pscudobulbs. A pseudobulb is formed by the thickened stem internodes which are usually formed each year and which store water and food reserves. Some epiphytes store water in their fleshy leaves. A West Indian orchid, Dendrophylax, has no leaves at all and is reduced to a series of broad fleshy green roots. Similarly the Malaysian Taeniophyllum has a very short stem (about 3cm —I.2in) with fiat green spreading roots on the bark of a tree, and with leaves reduced to tiny brown scales.

Growth

Most orchids, whether they are terrestrial or epiphytic, have the same growth form. Each leafy stem, or pseudobulb, is limited in its growth. The rhizome may be long or short and the pseudobulbs may consist of one or many internodes. Flowerstalks are produced by lateral buds which may arise at the top of the pseudobulb, on its side or at its base. This type of growth with a succession of stems of limited growth is called sympodial growth.

The alternative growth form, termed monopodia!, occurs in the Vanda tribe of Southeast Asia and in other groups. For example, in the Malaysian scorpion orchids, Arachnis, the tip of every stem goes on growing indefinitely, producing roots along its length at intervals, so that the older parts may die while the tip continues to grow. Some of these orchids develop as climbers or vines if their stems are very long; for example the vanilla orchid, which is grown commercially, forms a vinelike plant.

The world’s largest orchid plant is the giant Malaysian Tiger Orchid Grammatophyllum speciosum in which the pseudobulbs are 2m (6.5ft) in length with leaves all along, but which occasionally reach twice that size. The individual flowers are up to 12cm (4.7m) across and are borne in huge inflorescences of numerous flowers. These plants frequently grow in the crowns of large trees with their pseudobulbs arching down in graceful curves. The stiff branched roots grow up and outwards to accumulate debris and dead leaves.

Terrestrial plants may only be a few centimetres high, such as the north temperate bog orchid Hammarbya pahidosa, but other species may be very bizarre in form. The Malaysian bamboo orchid, Arundina, closely resembles a clump of bamboo with the flowers borne on slender solid stems.

Saprophytic orchids

A number of unrelated orchids inhabiting damp woodland situations have evolved as saprophytes. These plants have a yellow or pinkish colouring and have very branched fleshy rhizomes which absorb food material from the decaying humus in which they live. The form of the root gives the name ‘coral root’ to some of these orchids. The evolution of some species as saprophytes is not very surprising when it is realized that all orchids start their lives as saprophytes dependent on a strange relationship with a fungus called a mycorrhiza.

While the microscopic orchid seeds admirably serve the function of wide distribution by wind dispersal, even to inaccessible locations high on forest trees, they sacrifice the food reserve which most plants have to start their development. The orchid seed is dependent on the presence of a fungus growing within its tissues for its germination and subsequent development. The mycorrhizal fungus is a saprophyte which invades the orchid seed and penetrates its tissues. The fungal hyphae are in contact with both the plant tissues and the surrounding soil and provide the orchid seedling with essential nutrients. The relationship between the orchid and the fungus continues throughout the life of the plant but is most essential to the orchid in its young stages before it can produce its own food. Mycorrhizal relationships are widespread in nature and occur also with forest trees and many other woodland plants. The growth and development of the orchid seedling or mycorrhizome is very slow even with the assistance of the fungus. In temperate regions green leaves may not be produced for two or three years and several years may elapse before the plants produce flowers. In the European burnt tip orchid, Orchis ustulata, it may take as long as 14 years from germination of the seed to the production of a flower spike.

Pollination

The flower is the most remarkable feature of an orchid. The flower size may vary considerably from the tiny flowers of the epiphytic orchids Pleurothallis, from the New World tropics, which are barely 2mm (o.oSin) in diameter to the flowers of a tropical American Brassia which may exceed 38cm (15m) in diameter.

The fantastic shapes, colours, textures and scents of orchid flowers are all designed with their respective pollinators in mind. Bees and wasps are the most important pollinators of orchid flowers, flies and other insects follow, with birds such as humming birds pollinating relatively few species. Each type of flower is cleverly designed to attract a certain type of creature to visit the flower. To avoid hybridization many orchids are designed to attract only one species. The colour and/or scent of the flower attracts the pollinator, while the complicated flower structure guides and manipulates the pollinator so that the pollination mechanism may work with precision.

The fascinating and intricate processes of pollination in orchids were first investigated and understood by Charles Darwin who published a book on the subject in 1862 (The various contrivances by which orchids are fertilized by insects). In orchid flowers the pollen grains are generally in the form of a pollen mass or pollinium. This pollen mass is detached by a visiting pollinator and is transferred to the stigmatic surface of another flower. The pollen mass ensures that when the fruit is formed it will contain a large number of fertile seeds.

Pollination mechanisms

A simple example of the pollination mechanism is given by the European twayblade, Listera ovata. The greenish flowers attract ichneumons and other insects that alight on the strap shaped lip (labellum). The insect crawls up the lip to feed on the nectar secreted by a groove running up the centre of the lip. The pollinia are free in the open flower and are supported by a part of the column called the rostellum. When the insect touches the rostellum it explosively produces a drop of sticky liquid which contacts the tips of the pollinia and the insect and sets hard in a few seconds. The pollinia are then cemented to the head of the insect. After removal of the pollinia the flower alters by the straightening of the rostellum so that the stig-matic surfaces are exposed to the next insect visitor. If it bears pollinia from another flower it will deposit them on the stigma.

Orchid flowers generally attract insects by the presence of nectar, but nearly onethird of the family have nectarless flowers and use other means of attracting insects. One of the most unusual types of attractant is found in the waxy flowers of the tropical American orchid Coryanthes that exploit the sexual response of certain bees. The lip of the flower is in the form of a bucket which accumulates liquid secreted by the column. The flowers attract male euglossine bees of the genus Eulaema by the production of scent which the bees collect with the tarsal brushes of their front feet. In the process of collecting this scent from the base of the lip the bees become intoxicated and fall into the bucket of liquid. The only escape is via a narrow passage in which the bees pick up the pollinia. After removal of the pollinia subsequent bees can escape much more easily. Those carrying pollinia pollinate the flower in the process. This mechanism appears to be designed to control the movements and reduce the activity of the bees so that they can be guided by the flower and the pollinia can be applied and removed with a high degree of accuracy.

To increase the chances of cross-fertilization the epiphytic Malaysian pigeon orchid Dendro-bium crumenatum produces spectacular displays of simultaneous flowering. Research has shown that the flower buds developing in the axils of the leaf sheaths stop growing at a certain stage and await a suitable stimulus. Flowering is triggered by a storm which causes a sudden drop in the day temperature. The flower buds then resume development and nine days later the fragrant flowers open just before dawn. Bees visit the flowers in the morning and emerge with pollinia attached to their heads to deposit them on the stigmas of the next flower. The flowers close up by the afternoon and are over by the next morning. The synchronization of the flowering is important for a flower which is so short-lived while the rain, which provides the necessary stimulus of falling temperature, also provides an ample supply of water for flower formation and subsequent evaporation loss from the flowers.

The European orchids of the genus Ophrys exhibit some of the most remarkable pollination mechanisms found in any type of flower. The mirror orchid, Ophrys speculum, from the western Mediterranean provides a good example. The lip of the orchid flower is shining blue in colour with a yellow border fringed with red hairs. The flowers superficially resemble female wasps of Campsoscolia ciliata. The males of this species emerge some while before the females and are then attracted to the flowers by their scent and appearance. Male wasps alight on the flowers with the head below the rostel-lum and make repeated and vigorous attempts to copulate with the flowers. This activity nearly always dislodges the pollinia, which become attached to the insect’s head. Other Ophrys species attract the males of other insects in the same way and sometimes the scent of the orchid flower may be more attractive to the male insects than their own females.

In general orchids pollinated by bees have fragrant flowers with bright colours; those that attract moths are scented and light coloured with long tubular nectaries; butterfly-pollinated flowers are upright with fragrant colourful flowers; while bird-pollinated flowers are usually scentless. Orchids that attract flies often have dull brown or purple coloured flowers and produce odours that resemble decaying flesh.

Flowers that fail to attract any pollinators may resort to self-pollination. A mechanism in the flower ensures that the pollinia fall down into contact with the stigmatic surfaces, eg in the European bee orchid, Ophrys apifera. Species with small insignificant flowers such as the small white orchid, Leucorchis albida, are regularly self-pollinated in the bud with the pollen masses breaking up inside the flowers before they open.

Economic importance to man

Cultivated varieties of orchids may produce very spectacular sprays of beautifully coloured flowers. About 50 different genera are commonly cultivated such as the tropical American Cattleya species and the Cymbidium, Vanda and Dendrobium species of Southeast Asia.

The only species of economic importance is the Mexican vanilla orchid, Vanilla plauifrons. This vinelike plant is grown in tropical countries which have a regular dry season such as Madagascar, the Seychelles and Reunion. The pods are picked and carefully cured by a process of slow drying and fermentation to bring out the full flavour which is imparted by crystals of vanillin that form in the tissues of the pod. The vanilla ‘beans’ are used intact for flavouring or for making vanilla essence.

Sorry, comments are closed for this post.