In this article we will discuss about:- 1. General Features of the Orchidaceae 2. Floral Range in the Orchidaceae 3. Divisions 4. Position and Affinity 5. Number and Distribution 6. Commonly Occurring Plants 7. Economic Aspects.
General Features of the Orchidaceae:
Habit – Perennial herbs, terrestrial or epiphytic or saprophytic.
Roots – Main roots always absent; instead, three types of adventitious roots arise from the nodes: aerial roots, tuberous and normal cylindrical earth roots.
Leaves – Thick half-alternate with a closed sheath; ligules and stipules lacking.
Flowers – Showy, bisexual, irregular, epigynous.
Perianth – Segments 6, in 2 whorls, free; outer segments consist of nearly equal 3 sepals and inner of somewhat dissimilar 3 petals; 2 lateral petals alike and simulate sepals, but median petal larger and constitutes a labellum; labellum of diverse shape, size and colour, normally posterior but rendered anterior by resupination, i.e., twisting of pedicel or ovary through 180°.
Androecium – Stamens 1 or 2; filament adheres to style (gynandrous) and forms a column or gymnostemium which springs from top of ovary and ends in a beak known as rostellum; pollen grains united into masses termed pollinia’, each pollinium bears a cord (caudicle or translator) which terminates in a sticky disc or gland called retinaculum or viscidium.
Gynoecium – Carpels 3, united; ovary inferior, twisted, unilocular with many ovules; placentation parietal; stigmas 3, discoid, glutinous, situated below rostellum and facing labellum.
Fruit – Capsule.
Seeds – Very minute, exalbuminous; embryo undifferentiated.
Floral Range in the Orchidaceae:
The Orchidaceae is characterised by a wide range of variations in the floral structure.
The sepals and petals may be nearly alike or somewhat different in form. In general, the sepals are smaller and inconspicuous; but they may become larger and prominent, e.g., Masdevallia. The sepals are free or somewhat united. The odd sepal is spurred in Disa, but joins with the lateral petals to form a hood-like structure in Haemaria.
In Coryanthes, the lateral petals are lost. The petals are usually filiform in Epicranthes and smaller than the labellum, but sometimes become larger in Oncidium. The petals may assume the shape of a ribbon, as in Cypripedium. In some species of Paphiopedilum, the petals may attain a length of 30 cm.
The petals may or may not be spurred. The labellum is sometimes minute and narrowed or curved in Cattleyd or large with spreading limbs in Odontoglossum or may look like a slipper in Cypripedium or may form a cage in Stanhopea or may assume the form of a bucket in Coryanthes. The labellum is simple in Thelymitra or trifid or tripartite or variously incised.
The outer whorl of stamens generally becomes fertile, as in Arudina pentandra; but they may form a fleshy staminode in Diuris. The staminodes are represented by small auricles in Cypripedium and related genera and form a leaf-like staminode in Orchis. The lateral pair of the inner whorl of stamens is fertile in Cypripedium, but foliaceous in Thelymitra or form staminodes of diverse shape in Epipactis.
The anther is generally bent over the inner free of the column, but the anther may become erect and free on the top of the column (Ophrys) or the anther may hang downwards vertically from the top (Coelogyne).
The anthers are 2-, 4- or 6-chambered with the corresponding number of pollen masses. The pollen masses are free and granular in Cephalanthera or united into packets by elastic nets in Orchis or waxy pollinia cohere in a chamber in Cattleya. The pollinia are provided with a caudicle, but sessile in Malaxis.
All the three stigmas are fertile in Cypripedium, Paphiopedilum and other genera. The ovary is generally spindle-shaped or cylindrical, but furnished with lines, ridges or wings.
Divisions of the Orchidaceae:
The Orchidaceae is divided into two subfamilies, one with two tribes and the other with four tribes.
Subfamily I. Diandroideae:
Stamens 2 or rarely 3. Functional stigma 3.
Tribe (i) Apostasieae:
Flowers with an almost radical symmetry. Stamens 2 or 3, fertile. Ovary trilocular, Examples- Apostasia, Neuwiedia, etc.
Tribe (ii) Cypripedieae:
Flowers with a well-marked median symmetry. Stamens 2, fertile; odd stamen of outer whorl forms a large staminode Ovary unilocular or incompletely or completely trilocular. Examples- Cypripedium, Paphiopedilum, etc.
Subfamily II. Monandroideae:
Stamen 1, fertile. Median stigma rudimentry or forms a rostellum.
Tribe (i) Epidendreae:
Stamen lifting up like a little cap. Pollinia soft and waxy in texture, ends sometimes drawn out into sticky portions but with no distinct sticky discs. Examples- Coelogyne, Dendrobium, etc.
Tribe (ii) Neottieae:
Stamen attached to back of a column, but easily lifted. Pollen soft and easily breaking up. Examples- Neottia, Spiranthes, etc.
Tribe (ii) Ophrydeae:
Stamen firmly attached to column and immovable. Pollinia consist of small masses attached to a central stalk with one or two sticky discs. Examples- Habenaria, Orchis, etc.
Tribe (iii) Vandeae:
Stamen lifting up like a little cap. Pollinia attached to one or two distinct sticky discs and removed with them, texture more frequently hard and bony but sometimes softer and waxy. Examples- Cymbidium, Vanda, etc.
Position and Affinity of the Orchidaceae:
The Orchidaceae was included by Bentham-Hooker as the third family of the Microspermae at the beginning of the monocotyledons. Engler placed the family in the suborder Gynandrae of the order Microspermae which was at the end of the monocot class. In Hutchinson’s arrangement, the family appeared under the Orchidales which marked the end of his second division Corolliferae of the monocots.
The Orchidaceae is related to the epigynous components of the Liliiflorae (Liliales). Hutchinson thought that the family originated either from the Haemodoraceae or Amaryllidaceae as a result of complex development of floral parts and reduction of stamens.
Furthermore, the Burmanniaceae and Apostasiaceae of his Orchidales form a connecting link with the families of the epigynous Liliales. The Orchidaceae has also been believed to arise from the Musaceae or its ancestor; this suggestion is based on the similar structure and construction of flowers of most genera of the Orchidaceae and Orchidantha, a typical Musaceae.
The Orchidaceae is agreed upon by almost all botanists to represent the most advanced family in the monocots for the following reasons:
1. The orchids comprise an enormously richly differentiated group of plants in terms of habit, size, vegetative features and floral peculiarities.
2. All the orchids are herbaceous plants, there being rarely shrubs or no tree forms. They exist ordinary rooted herbs, climbing vines, epiphytes or saprophytes.
3. “The Orchidaceae,… together with the Asteraceae or Compositae comprise the largest family of flowering plants as regards number of species. They are chiefly tropical but numerous are subtropical, temperate, and boreal, and some species even arctic …”.
4. “In the reproductive region, the specialization is very diverse. The inflorescence is racemose and the flowers thus lateral which condition is often connected with zygomorphy. The tepals are petaline and the labellum may or may not be supplied with a spur”.
5. “The flowers are always epigynous. Epigyny may have evolved as a means of protection of the gynoecium and possibly also greater stability of the perigone. Nectar production or probably more frequently the production of various volatile substances lead to the attraction of particular groups of insects….. Specialization in the shape, size, colour and pubescence of the perigone indicates a long period of co-evolution with the pollen vector”.
6. “Further adaptations of style, stigma and androecial parts have secured a position of the functioning anther and the stigmatic areas to facilitate pollination”.
7. “The pollen grains are numerous and usually where in massulae or pollinia, which are further adapted for dispersal by the occurrence of stipes, caudicle and a retinaculum which serves to keep the pollen aggregated. As the pollen is often sticky there is no need for a thick, sculptured exine or any distinct apertures”.
8. In the Orchidaceae, “… the above character states are operating in interaction and are logical consequences of the specialized mode of pollination”.
Number and Distribution of the Orchidaceae:
The Orchidaceae comprises about 750 genera and 18,000 species. The members of this family are of wide distribution, but most abundant in tropical forests. They are concentrated in three areas, notably, tropical America, Indo-Malaysia and eastern Himalaya.
About the orchids, Cronquist (1968) commented thus:
“… they are not among the most abundant plants in terms of number of individuals, and not many of the species have a wide geographical range of any ecological amplitude”. “Orchid species with surprisingly similar flowers can be found from the damp mountain sides of Norway and New Zealand to the dry savannahs of tropical Africa and South America, and from the coastal strands of the South Pacific to rocky ledges in the Himalayas”.
Commonly Occurring Plants of the Orchidaceae:
Acampe praemorsa (Roxb.) Blatt. & McCann and Vanda tessellata (Roxb.) Hook, ex G Don are epiphytic on trees.
Ania viridifusca (Hook.) Tang & Wang occurs in Assam hills and Anoectochilus rotundifolia (Blatt.) Balakr. in South India.
Calanthe veratrifolia (Willd.) R. Br. occurs in the hilly tracts of South India.
Dendrobium nobile Lindl. is seen in the hills.
Didymoplexis pallens Griff., Pogonia carinata Lindl. and P. plicata Lindl. are found in shady thickets.
Geodorum dilatatum R. Br. is a terrestrial herb of grassy lawns.
Zeuxine strateumatica (L.) Schlecht. is a grass-like herb which is ubiquitous in the plains.
Economic Aspects of the Orchidaceae:
“For use in corsages and as an ornamental the orchids are more sought after than any other type of plant, and not only is an industry built upon orchid culture but growing orchids is also a widespread hobby”. Genera commonly grown in glasshouses include Cymbidium, Dendrobium, Epidendrum, Miltonia, Odontoglossum, Oncidium, Paphiopedilum and Phalaenopsis.
The leaves of Calanthe veratrifolia contain a glycoside, ‘indican’, which on hydrolysis, yields ‘indigo blue’. The fibres from the stemcortex of Dendrobium crumenatum are used as a braiding material for hats. The tubers of Eulophia epidendraea are used as vermifuge.
The rootstocks of Geodorum densiflora are crushed and rubbed on cattle to kill flies. The dried leaves of Jumellea fragrans constitute ‘Faham tea’. The roots of Vanda tessellata are employed as a remedy for scorpionbites and for rheumatism. The capsules of Vanilla planifolia yield commercial ‘vanilla’, a flavouring agent.
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