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In this article we will discuss about the Classification of Microspermae. According to Engler and Prantl, Microspermae consists of two families:- 1. Burmanniaceae 2. Orchidaceae.
Family # 1. Burmanniaceae:
Burmanniaceae are saprophytic herbs with a small slender stem and underground rhizome or tubers; the rhizome is associated with endotrophic mycorrhiza. Leaves are small scale-like or linear and green. Inflorescence is terminal and scorpioid cyme. Flowers small, bisexual, usually actinomorphic, rarely medianly zygomorphic, epigynous.
Perianth in 2 whorls of 3 segments in each, all petaloid and united in a cup or tube, persistent. Stamens 6 in 2 whorls, adnate to the perianth tube, often the outer whorl absent; filaments very short or absent and then the anthers are sessile; connective appendaged or winged or very broad; anthers bilocular, dehiscing transversely or longitudinally.
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Ovary inferior, 3- or 1-locular, ovules numerous, minute, on axile or parietal placenta; style short with 3 stigmatic lobes. Fruit a capsule; seeds many, minute, with an undifferentiated embryo and scanty endosperm; testa thin and membranous.
The family contains a little over a 100 species under 20 genera found in most tropical countries. The family is represented in India by a few species of Burmannia.
The family is considered to be allied to Orchidaceae in having minute seeds, undifferentiated embryo and epigynous flowers and is placed in the order Orchidales by Engler and also by Cronquist.
Hutchinson places Burmanniaceae in a different order Burmanniales where he includes besides Burmanniaceae two other families, viz. Tkismiaceae and Corsicaceae. Takhtajan places Burmanniaceae in Iridales along with Iridaceae, Geosiridaceae and Corsicaceae with which families he considers Burmanniaceae is more allied than with Orchidaceae.
Family # 2. Orchidaceae:
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Orchidaceae are perennial herbs, terrestrials or epiphytes, rarely saprophytes. The stem is monopodial or sympodial; in case of sympodial growth the successive annual shoots may or may not end in an inflorescence. The terrestrials usually have a rhizome. The inter- nodes in many are thick and form a pseudobulb, and in some cases a fleshy tuber is formed at the base of the growing shoot.
In the case of epiphytes there are 3 types of roots:
(i) The clinging roots which are negatively heliotropic, fasten the plant to its support, the network of such roots form a mass which act as a reservoir for humus;
(ii) The absorbing roots which are negatively geotropic penetrate into the network of clinging roots and absorb moisture;
(iii) The true aerial roots which hang down in long festoons have a layer of dead spongy cells below the epidermis known as velamen.
The roots of saprophytic orchids are associated with endotrophic or endophytic mycorrhiza. Didymoplexis and Gastrodia are Indian examples. Neottia nidus-avis is a peculiar saphrophytic orchid where the roots coming out from the rhizome from a birds-nest like mass. Corallorhiza has a rhizome which bifurcates repeatedly and a number of lobular branchlets lie closely crowded together forming a coral-like body.
Leaves are usually thick and fleshy, sessile or petioled, in some terrestrial orchids the leaves are thinner and plicate; often with closed sheath; whorled on top of pseudo- bulb or distichous along the short stem.
The inflorescence is a spike or raceme, sometimes a panicle, rarely flower solitary. Flowers hermaphrodite, strongly medianly zygomorphic, epigynous, usually showy and colourful, with a complicated morphology. The perianth in 2 whorls, segments all petaloid, either free or variously united, imbricate or sub-valvate, usually differentiated into calyx and corolla. The median or posterior sepal and petal are unlike the laterals.
This petal called labellum, is larger and by the twisting of the ovary through 180° it comes to the anterior side (resupination). The labellum is produced into a sac or spur-like structure containing within it nectar secreting glands. Rarely the labellum is formed of the median sepal; often lateral sepals unite to form a sac-like structure known as mentum or chin.
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The filaments of stamens and the style of the ovary unite and form a column (which according to some is nothing but a prolongation of the floral axis). In majority of the genera the column bears one anther and 2 fertile stigmas (which sometimes unite) and a special structure called the rostellum representing the third stigma. The single anther is that of the anterior stamen of the outer whorl.
The other 2 of this whorl and the entire inner whorl are absent. The anther is bilocular and each cell contains a pollinium where the pollens are tied together by fine threads which unite at the base forming a caudicle. In the other group there are 2 anthers and a simple stigma. These anthers are of 2 stamens of the inner whorl.
There is a large staminode representing the outer whorl of the androecium. The pollens are in tetrads in a sticky fluid; rarely pollens granular. The stigma is not sticky. Ovary inferior, of 3 united carpels, 1-celled with ovules on 3 double rowed parietal placentas.
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The fruit is a capsule with a large number of minute and light seeds. The seed is enclosed in a membrane and contains no endosperm; embryo is an undifferentiated mass of cells. The seeds are dispersed by wind.
In many orchid flowers the arrangement is very complicated; the labellum may be spurred, there may be one axial protuberance bearing the labellum and sepals. The column may also show great variety in structure. The various complexities in flowers are adaptations for insect-pollination.
This is a large family with over 2,000 genera and 18,000 species mostly found in damp tropical and temperate regions of the world. The flowers are very showy and many species are cultivated. Few are fragrant and Vanilla planifolia is important for the essential oil. A few are used medicinally.
As regards the structure of the androecium and gynoecium of flowers the family is divisible into 2 clear groups, one having 2 stamens and the other with one stamen.
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The following is an outline of the classification of the family by Schlechter:
I. Subfamily. Diandrae:
Stamens 2, representing the lateral 2 of the inner whorl, a third stamen transformed into a large staminode placed above the anthers and more or less covering the style; 3 stigmatic lobes fertile; pollen granular, not united into masses. Tribe 1. Cypripedieae
II. Subfamily. Monandrae:
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Stamen solitary, representing the abaxial member of the outer whorl, the lateral completely abortive or forming small staminodes; 2 lateral stigmatic lobes fertile, the middle one extended into a usually small rostellum:
Division I Basitonae:
Caudicle and viscidium arising from the base of the pollinia; anther erect or more or less resupinate, very closely adnate to the broad-based column, never deciduous after flowering; pollinia always granular. Tribe 2. Ophrydoideae.
Division II Acrotonae:
Caudicle and viscidium arising from the apex of the pollinia; anther erect or incumbent, the filament short, slender generally narrowly jointed to the column, usually deciduous, but if persistent soon withering:
(a) Pollen grains soft; anther mostly persistent; inflorescence always terminal. Tribe 3. Polychondreae
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(b) Pollen waxy or bony; anther generally soon deciduous; inflorescence terminal or lateral. Tribe 4. Kerosphaereae.
The family Orchidaceae is considered to be the most advanced and highest evolved among the Moncotyledonae.
The most diverse and complex development of flowers and reduction in the number of stamens, the resupinate epigynous ovary reduction in stigmatic lobes and formation of the rostellum, absence of endosperm in the seeds and undifferentiated embryo all point towards great advancement in the course of evolution.
The habit of the plants, all herbs with a large number of epiphytes and a few saprophytes also support this view.
Most authors agree that the family originated from Liliaceous stock. According to Hutchinson it has been derived from Liliaceae through Hypoxidaceae and Apostaciaceae the latter included in Orchidaceae by earlier authors.
Takhtajan and Cronquist also consider that Orchidaceae although does not have close affinity with Liliaceae has been derived from this family through others with epigynous flowers. Another line of parallel evolution is traced from Musaceae through Lowiaceae which has resupi-nate epigynous ovary and a distinct labellum.
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About 700 species are found in India and two zones rich in Orchids can be demarcated, viz.
(1) The eastern zone comprising E. Himalaya and the hilly area of eastern India and
(2) The south-western zone comprising Western Ghats and Nilgiri hills.
Species of Paphiopedilium, Dendrobium, Cymbidium, Coelogyne, etc. are among others having large beautiful flowers and are much cultivated. Cattleya-flowers are most beautiful among Orchids and are to be found in almost all Indian nurseries but this is an American genus.
The description of the family as given here is as understood by the modern authors. Orchidaceae of the older authors included a few genera which have more or less regular flowers having 2-3 stamens. These are now placed in a distinct family Apostaciaceae, Apostasia being an Indian example.
