In this article we will discuss about:- 1. General Characters of Orchidaceae 2. Floral Formula of Orchidaceae 3. Number and Distribution 4. Range of Floral Structures 5. Common Plants 6. Affinity and Economic Importance.
General Characters of Orchidaceae:
Plants—perennial herbs, terrestrial or epiphytic, a few saprophytic (e.g. Didymoplexis) deriving their nourishment from humus since their roots or rhizomes are associated with endotropic mycorrhiza rarely climbers (e.g. Bulbophyllum).
Roots—the main root is always absent, its place being taken by adventitious roots which arise especially from the nodes; they are generally of three kinds:
(1) Normal cylindrical earth-roots.
(2) Tuberous roots which serve for storage of reserve food materials and
(3) Aerial roots in which the epidermis is specially developed to form the velamen, special water-absorbing tissue, rarely absent.
Leaves—thick, usually alternate with closed sheaths. Inflorescence—raceme or spike. Flowers—bisexual, epigynous, medianly zygomorphic, usually very showy. Perianth—free, 3 + 3; the outer consisting of 3 sepals, more or less similar, and the inner of 3 petals, more or less dissimilar; the two lateral petals are alike and resemble the sepals but the median petal is usually much larger and known as the labellum or lip; the labellum is provided with a spur, variously shaped and normally posterior in position but becomes anterior by the twisting of the ovary.
Stamen—1 (in Monandrae) or 2 (in Diandrae) in Monandrae the column bears an anther at the top and just under it there is a more less projecting beak, the rostellum and under this there lies two fertile stigmas which are more or less united into one; but in Diandrae the three stigmas are united into one, on which the two anthers are placed and there is no rostellum. (The rostellum is only the projecting portion of the stigma; it is itself functionless but bends over and conceals the receptive portion of the stigmatic surface.)
The filament is united with the style (gynandrous) forming a column or gynostegium; pollen grains granular, or powdery, more frequently united into masses called pollinia, 2-8 in number and each pollinium bears a cord, the caudicle, which ends in sticky discs or glands known as retinacula.
Carpels—(3); ovary inferior, usually twisted, 1-celled with three double parietal placentae bearing many ovules; stigmas 3, sticky, situated below the rostellum and facing the Iabellum. Fruit—capsule. Seed— very numerous, minute, exalbuminous. Embryo—undifferentiated.
Floral Formula of Orchidaceae:
Number and Distribution of Orchidaceae:
This family consists of about 450 genera and 10,000 ̶ 15,000 species widely distributed in the tropical and temperate regions of the world. This is the largest family of flowering plants in India. It is only in the Eastern Himalayas, Burma and Malaya Peninsula that the family predominates.
Range of Floral Structures in Orchidaceae:
A wide variation occurs in the floral structures of this family.
The sepals and petals may be quite different in form or they seem to be almost alike. The sepals are generally minute and indistinct but sometimes become somewhat conspicuous and larger, as in Masdevallia. The sepals may be free or more or less united. The odd sepal becomes spurred in Disa, but in Haemaria, it joins with the lateral petals forming hood-like structure.
Variation in the form of petals is also noticeable. The lateral petal is obliterated in Coryanthes, but in Epicranthes the petals are usually filiform. The petals become usually smaller than the lip, but sometimes become larger, as in Oncidium. In Cypripedium, the petals look like narrow ribbon and occasionally they are one yard long.
In Disa, the labellum becomes sometimes minute and narrow or becomes large with spreading limbs, as in Odontoglossum, or they are curved, as in Cattleya. The labellum becomes slipper-like in Cypripedium, but in Coryanthes it assumes a bucket-like structure. The spurred petals may or may not be present. The origin of the spur is supposed to have been derived from the lip (labellum) or partly from the floral axis.
The incision of the lip may be various; it becomes either trifid or tripartite or the middle segment of the lip becomes 4-partite when re-divide or it may often represent the simplest type of lip which conform to the structure of the lateral petals, such as, in Thelymitra.
The odd member of the outer whorl of stamens, which is situated opposite the sepal, becomes generally fertile but a fleshy staminode is formed by that very stamen, as in Cypripedium and allied genera. In Arundina pentandra, they are found to be present and fertile, but in Diuris, they assume the form of leaf-like staminodes. In Orchis, they are represented by small auricles.
The lateral pair of the inner whorl of stamens becomes fertile, as in Cypripedium, but in others they form staminodes of different shapes, such as Epipactis, or are foliaceous, as in Thelymitra.
The anther may become erect and free on the top of the column, as in Ophrys, Orchis and others, but generally it becomes bent over the inner face of the latter. In Coelogyne, the anther seems to be hanged downwards vertically from the apex.
The union of column with anther may also vary. In the tribe Ophrydeae, it is provided with a broad, short filament from which it is quite inseparable. The anther remains strongly coherent with the column and it is not pulled off with the removal of pollen.
The anthers are either 2-, 4- or 6-chambered which correspond to the number of pollen masses. Pollen masses become either granular or powdery, as in Cephalanthera, or they are found to be united in packets by some sort of elastic net, as in Orchis, or waxy pollinia being united in a chamber are formed in Cattleya. A slender process is often formed by the drawing out of the lower part of the anther on each side and ultimately this process covers the lower sterile portion (caudicle) of the pollinium.
All three stigmas become functional in Cypripedeae and Apostasieae, but in some cases the lateral pair, the third being sterile and finally forms the rostellum. The stigmas originate as smooth, viscous, flat or cushion-like spots on the inner side, or sometimes on the end of the column, or often forms special processes that are carried up on the style-like projection, as in Habenaria.
The ovary manifests wide variations, but it is usually cylindrical or spindle-shaped. The ovary is often provided with longitudinal lines, ridges or wings which later on become more prominent with the ripening of the fruits.
The Orchids represent inferior and 1-locular ovary which bears numerous small ovules borne on three double parietal placentae.
Common Plants of Orchidaceae:
(1) Vanda roxburghii R. Br., an epiphytic herb commonly found on mango trees.
(2) Zeuxine sulcata Lindl., a common terrestrial grass-like herb.
(3) Dendrobium is found on the hills as well as in the Sundribans; D. nobile Lindl. is one of the noblest and finest specimens of the genus.
(4) Vanilla planifolia L., cultivated for the fruits.
(5) Saccolabium papillosum Lindl., epiphytic on trees.
(6) Pogonia plicata Lindl. and P. carinata Lindl., common in shady thickets.
(7) Geodorum dilatum R. Br., a terrestrial herb on grassy lawns.
(8) Didymoplexus pallens Griff., commonly found in shady thickets.
Affinity and Economic Importance of Orchidaceae:
Orchidaceae established a link with that of epigynous members of Liliflorae. Hutchinson considers the family to be derived from Haemodoraceae or Amaryllidaceae for higher and complex development of floral parts and the reduction of stamens during evolution. Moreover, the families Burmaniaceae and Apostasiaceae of the order Orchidales resemble closely with those of epigynous Liliflorae.
Orchidaceae is also supposed to have been derived from Musaceae based on the similar structure and construction of flowers of Orchidantha (a member of Musaceae) and a few genera of Orchids. On the whole, Orchidaceae represents the highest evolved family among the monocotyledons in the presence of floral complexity and general plant of construction.
There are certain reasons for considering Orchidaceae as one of the highest evolved family of monocotyledons.
These are as follows:
(1) Zygomorphy of flowers accompanied with different types of labellum-formations along with spur containing nectar.
(2) Epigynous flowers.
(3) Constant reduction and suppression of the members of androecium.
(4) Reduction in the stigmatic lobes which often develop into rostellum.
(5) Diversity in shape and size of the flowers which may be of various types.
Economic Importance of Orchidaceae:
This family is less important economically. The roots of Vanda are used as curative for rheumatism and as an antidote for scorpion stings. The dried pulpy fruit of Vanilla planifolia furnishes the ‘vanilla’ of commerce, used for flavouring chocolate and confectionary. Many species are cultivated as ornamentals, such as, Cypripedium, Cattleya, Cymbidium, Dendrobium, etc.
No comments yet.