ORCHIDACEAE

            1. SYSTEMATIC POSITION:

            Bentham & Hooker

            Division: Phanerogames (Seed Plants)

                Class: Monocotyledones

                      Series: Microspermae

                           Family: Orchidaceae

           

            2. MORPHOLOGICAL CHARACTERS:

            Habit – Perennial herbs or sometimes shrubs, epiphytic or terrestrial or saprophytic.

            Root – Adventitious, fleshy, climbing or aerial.

            Stem – Usually erect but sometimes climbing or trailing, annual in terrestrial forms (aerial shoot), perennial in epiphytic forms, often thickened into rhizome and pseudobulbs.

            Leaves – Simple, alternate, sometimes opposite or whorls, entire, often oval or linear with parallel venation, sheathing leaf base, often fleshy, sometimes reduced to scales in saprophytic and parasitic species.

            Inflorescence – Racemose, spike, panicle or solitary. Generally the inflorescences are short lived, but in some species, it becomes perennial producing new leaves round the year.

            Flower – Often very colourful and attractive, but in some species, they are colourless and inconspicuous, hermaphrodite, sometimes unisexual, zygomorphic, bracteate, sessile or pedicellate, epigynous.

            Perianth – Tepals 6, arranged in two whorls of 3 each, outer whorl calyx like while the inner forms corolla. Tepals free or sometimes united. The median tepal of each whorl, different in size, shaped and often coloured. One of the segments of inner whorl becomes often enlarged and modified and is known as labellum or lip. Labellum is strictly the uppermost tepal but looks as if located on the lower side of the flower in most orchids. This is caused by twisting the ovary through 180⁰. This phenomenon is known as ‘resupination’. It enables the labellum to work as a landing place for pollinating insects. Sometimes the labellum is prolonged backwards into a spur or sac, which collects nectar or acts as nectar secreting tissue.

            Androecium – Stamens 3+3, which are never all present. Only 1 or 2 stamens present, which unite together with style and stigma to form a single and highly complex structure called column or gynandrium. Anthers dithecous, introtse and dehisce by longitudinal slits. Pollen grains granular and usually bound together by viscin threads into masses, called pollinia.

            Gynoecium – Tricarpellary, syncarpous, inferior, unilocular, rarely trilocular, placentation parietal, rarely axile. Stigma 3, functional, but in some species more often the anterior lobe of the stigma become sterile and develops into a small pocket known as the rostellum. The rostellum is situated between anther and the functional stigmatic lobes of the column.

            Fruit – Capsule.

               4. COMPARATIVE SYSTEMATIC POSITION AND AFFINITIES:

            Bentham & Hooker placed the family Orchidaceae as a third family of Microspermae at the beginning of monocotyledons. Engler & Prantl have treated the Orchidaceae to be the highest evolved monocot, but this view is not supported by the phylogenists. Orchidaceae was placed under a separate order Orchidales by majority of the taxonomists including Hutchinson, Cronquist and Takhtajan. Thorne, however, placed Orchidaceae under the order Liliales.

            Several taxonomists consider Orchidaceae to be the most advanced and highest evolved among monocotyledons. The characters which support this view are – reduction in the number of stamens, resupinate epigynous ovary, presence of rostellum, non-endospermic seeds, herbaceous habit and presence of several epiphytes.

            According to majority of taxonomists, Orchidaceae evolved from Liliaceae and Amaryllidaceous stocks. Hutchinson opined that Orchidaceae originated from Liliaceae through Hypoxidaceae and Apostaciaceae. Some, however, also trace another line of parallel evolution of Orchidaceae from Musaceae.

                     5. ECONOMIC IMPORTANCE:

            1. The economic importance of this family is concerned with numerous beautiful ornamental plants like species of Dendrobium, Cypripedilium, Cymbidium, Paphiopedilium, etc. Such plants are cultivated in green houses for their beautiful sweet scented flowers with labellum of various hues and colours.

            2. A dried pulpy fruits of Vanila planifolia yields “vanilla”, a scent for flavouring chocolate and confectionary.

            3. Some orchids are medicinal such as Eulophia epidendrae, Geodrum densiflora, Vanda tessellata, etc.

            4. A blue dye is obtained from the leaves of Calanthe veratriflora.

           

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