What are orchids?
Orchids are a family of monocotyledons. They have, like other monocots, a single seedling leaf and their floral parts in threes. Orchids are one of the largest families of flowering plants, challenging the daisy or sunflower family, the Asteraceae (Compositae), for the title of the largest of all families. Currently there are known to be 25,000 species of orchids and estimates, based on the current rate at which new species are being discovered and described, suggest that the number may reach 30,000.
Orchids are a cosmopolitan family found all the way from within the Arctic Circle to Tierra del Fuego and the islands south of Australia. They are absent only from open water and from true deserts.
The smallest orchid is thought to be Bulbophyllum minutissimum (Fig. 1) which is 3-4 mm tall, but may other orchids approach it in size. The largest orchids are probably vanillas which are lianas that reach into the crowns of rainforest trees and may be 20 m or more in length (Fig. 2). Some tropical orchids also form very large clumps on rocks or in trees. Grammatophyllum speciosum plants of several hundred kg have been reported from Southeast Asia (Fig. 3).
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Some orchids live on the ground (terrestrial, Fig. 4) while others grow perched on trees (epiphytic, Fig. 5) or rocks (lithophytic, Fig. 6). Vanilla and some of its allies scramble up shrubs and trees or are lianas that grow up forest trees, using its roots for support.
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What then unites these diverse plants into the family called the Orchidaceae? The distinctive features of orchids, which separate them from other flowering plants lie primarily in their flowers.
Orchid flowers are simple in structure and yet highly modified from the more typical monocotyledon flower as exemplified by a Trillium or Lilium, to which orchids are very distantly allied. These characteristically have their floral parts arranged in threes or multiples of three. Orchids are no exception. This can most easily be seen in the two outer whorls of the flower. Let us take, as an example, the common Asian and Pacific Island orchid Phaius tankervilleae (Fig. 7), which is similar in general floral structure to the majority of orchids.
Its floral parts are situated at the apex of the ovary, which itself can be seen to be tripartite in cross section. The outermost whorl of the flower is the calyx, which consists of three sepals, which are petal-like and coloured yellow with a red stripe in the middle. The two lateral sepals differ slightly from the third, called the dorsal or median sepal. In some orchids, such as dendrobiums and bulbophyllums, the lateral sepals form at the base a more or less conical chin called a mentum.
The corolla of P. tankervilleae comprises three petals, which are brightly coloured. The two lateral petals, resembling the dorsal sepal in colouration and shape, are uppermost in the flower and differ markedly from the third petal, which lies at the bottom of the flower. The third petal, called the lip (or labellum), is highly modified, 3-lobed and with a short spur or nectary at the base (Fig. 7). The spur can be longer or more sack-like in other orchids and can contain callosities (ridges or keels) that are diagnostic for some species. In some orchids the upper surface of the lip may be adorned with a callus of raised ridges or lamellae or tufts or areas of hairs or glands. The lip is an important adaptation of the orchid to facilitate cross-pollination. It can be imagined as a brightly coloured flag to attract potential and specific pollinators, which are then guided towards the pollen and stigmatic surface by the form of the callus. The lip, therefore, can be supposed to act as a landing platform and the callus structure as a guidance system for the pollinator.
The central part of the orchid flower shows the greatest modifications to the basic monocotyledon pattern. The major evolutionary forces at work in orchids have been reduction in the number of floral parts and fusion of the male and female organs into a single structure. The fused organ in the centre of an orchid flower is called the column (Fig. 7). In this species and in most Pacific Island orchids, a single anther lies at the apex of the column. The pollen in the anther is not powdery as in most plants, but is borne in eight discrete masses, called pollinia (single pollinium). The pollinia are attached to a sticky mass called a viscidium. In other species the number of pollinia may be two, four or rarely six and these are attached to the viscidium either directly or by a stalk called a stipe in most epiphytic orchids and a caudicle in most terrestrial ones.
The stigma, the receptive surface on which pollen alights and germinates, is also positioned on the column in the centre of the orchid flower, on its ventral surface. The stigma is a sticky lobed depression situated below and behind the anther in most orchids, but in some terrestrial genera such as Habenaria and Peristylus the stigma is bilobed with the receptive surfaces at the apex of each lobe. In many species the pollen masses are transferred to the stigmatic surface by a modified lobe of the stigma called the rostellum. This is developed in P. tankervilleae as a projecting flap that catches the pollen masses as the pollinator passes beneath on its way out of the flower (Fig. 8).
An interesting feature of the development of most orchid flowers is the phenomenon of resupination. In bud, the lip lies uppermost in the flower while the column lies lowermost. In species with a pendent inflorescence the lip will, therefore, naturally lie lowermost in the flower when it opens. However, this would not be the case in the many species with erect inflorescences, such as P. tankervilleae. Here the opening of the flower would naturally lead to the lip assuming a place at the top of the flower above the column. In most species this is not the case, and the lip is lowermost in the flower. This position is achieved by means of a twisting of the flower stalk or ovary through 180 degrees as the bud develops. This twisting is termed resupination.
Orchids carry their flowers in a variety of ways. Even within the same genus different species have different ways of presenting the flowers. Most orchids in the Pacific Islands have inflorescences bearing two or more flowers, usually borne on a more or less elongate floral axis comprising a stalk called the peduncle and a portion bearing the flowers, the rhachis. In P. tankervilleae the flowers are borne in an elongate erect raceme, which is unbranched with the flowers arranged in a lax spiral around the rhachis. In a raceme the individual flowers are attached to the floral axis by a stalk called the pedicel. In some species pedicels are virtually absent and the flowers sessile on the axis; such inflorescences are termed a spike.
We find some interesting variations on the multi-flowered inflorescence. In several species the flowers are borne all facing to the same side of the rhachis, this being called a secund inflorescence. The most spectacular group, however, are those in which the rhachis is so contracted that the flowers all appear to come from the top of the flower stalk in an umbel, with the inflorescence rather resembling the head of a daisy. Branching inflorescences with many flowers are found in some orchids and are termed panicles. In many species the flowers are borne one-at-a-time either sessile or on a shorter or longer stalks.
The orchid plant
The vegetative features of orchids are, if anything, more variable than their floral ones. This is scarcely surprising when the variety of habitats in which orchids are found is considered. Orchids grow in almost every situation: on the permanently moist floor of the lowland tropical rain forest, in the uppermost branches of tall forest trees where heavy rainfall is followed by scorching sun for hours on end, on rocks, and in the grassy areas found on landslips and roadsides. The major adaptations seen in orchid vegetative morphology allow them to withstand adverse environmental conditions, in particular, the problems of water conservation on a daily and seasonal basis.
That tropical orchids might suffer from periodic water deficits is not immediately obvious. However, rainfall is not continuous; even the wettest habitats and in many places in the tropics the rainfall patterns are markedly seasonal. Furthermore, most tropical orchids are epiphytic, growing on the trunks, branches and twigs of the trees, or lithophytic on rocks. In these situations water run-off is rapid, and the orchids will dry quickly in the sunshine that follows the rain.
Many orchids have marked adaptations of one or more organs, which allow them to survive these periodic droughts. Some of these adaptations are as dramatic as those encountered in the Cactaceae. The stem can develop into a water-storage organ. This is so common in tropical orchids that the resulting structure has been given a technical name, a pseudobulb. In Dendrobium the pseudobulbs comprise several internodes (Fig. 9) while in Bulbophyllum they are of one internode only (Fig. 10). Pseudobulbs are also found in many terrestrial orchids and can grow either above the ground as in Calanthe or underground as in Geodorum.
Many terrestrial orchids, such as Orchis, Ophrys and Disa, lack pseudobulbs and have underground tubers, which survive drought (Fig. 11). The new growth grows from one end of the tuber in suitable conditions. In others such as jewel orchids and the creeping lady's tresses, Zeuxine and Goodyera, the stems are succulent but not swollen (Fig. 12). The horizontal stem or rhizome creeps along the ground in the leaf litter, and erect shoots bearing the leaves are sent up periodically.
The leaf is another organ that has undergone dramatic modification in the orchids. Fleshy or leathery leaves with restricted stomata, such as those of Dendrobium and Bulbophyllum species, are common.
A number of orchids have no green leaves. In some epiphytic orchids, such as Chiloschista, Dendrophylax, and Microcoelia, the leaves have been reduced to scales and photosynthesis takes place in the flattened green roots (Fig. 13).
Some terrestrial orchids, such as the Eurasian ghost orchid Epipogium aphyllum, are leafless and lack chlorophyll altogether (Fig. 14). They are called mycoheterotrophs. Lacking chlorophyll, they cannot photosynthesize and must obtain all of their nutrition from the mycorrhizal fungus with which they are associated.
Orchid species with green leaves with which to photosynthesise are termed autotrophic. The terrestrial species usually have leaves of a much thinner texture than their epiphytic cousins. In lowland forest, the perpetually moist atmosphere and lack of direct sunlight means that such leaves are not vulnerable to drought. Some of the terrestrial species of the forest floor have beautifully marked leaves. In Goodyera, Zeuxine, Erythrodes and their relatives, the leaves can range from green to deep purple or black and may be mottled or reticulately veined with silver (Fig. 15).
The roots themselves are much modified in most epiphytic orchids. They provide both attachment to the substrate and also uptake of water and nutrients in a periodically dry environment. The roots have an actively growing tip; the older parts are covered by an envelope of dead empty cells called a velamen. The velamen protects the inner conductive tissue of the roots and may also aid the uptake of moisture from the atmosphere, acting almost as blotting paper for the orchid.
Life in the tropics can be inhospitable even for orchids. In those regions with a more marked seasonality conditions may be positively hostile for orchids at certain times of the year. Even tropical forests can have periods of relative drought where the orchids have to survive days or even weeks without rain. In these conditions, tropical orchids without water-storage capabilities in their stems or leaves can drop their leaves and survive on the moisture stored in their roots, which are protected by their cover of velamen.