Orchids : Introduction
Discover the enchanting world of orchids. Learn about their diverse beauty and fascinating biology.
Orchids
Orchidaceae is second largest family of flowering plants in India
70 different species are found in Sindhudurg district
Orchids are having incredible diversity in their forms, structure & colors of flowers
Orchids can be Epiphytic that is growing on trunks & branches of trees, Lithophytic that is growing on rocks and cliffs or they are Terrestrial growing on forest floor on soil
Some epiphytic orchids are found growing on rocks i.e. become lithophytic as there is ample amount of rainfall in Sindhudurg district for e.g. Dendrobium barbatulum
Stem:
It is reduced in epiphytes In some orchids it is in the form of flattened disc or pseudobulbs which are storage organs while in terrestrial & lithophytes they are in the form of rhizomes growing horizontally over soil or tubers growing vertically or irregularly below soil both rhizomes & tubers act as a storage organ, it is survival mechanism of orchids
Growth in orchids is either monopodial with apical bud or sympodial by axillary bud as apical bus terminates into inflorescence
Growth pattern & type of velamen tissue forms criteria of classification of orchids
Leaves:
Leaves are Cauline i.e. borne along the stem & generally arranged in two rows alternating on opposite sides of stem or pseudobulbs distichous in most orchids like Aerides, Dendrobium, Vanda in some orchids leaves are spirally arranged on stem or reduced to single termianal leaf as in Nerivilia Leaves are prostrate on soil like in Habenaria diphylla Leaves are spirally arranged in several orchids like Pecteilis, Habenaria Zeuxine & Cheirostylis.
Sometimes leaves are totally absent in holomycotrophic orchids like Epipogium, Galeola, Didymoplexis.
Flower:
Orchid flowers shows diverse morphology & structural complexities. Unique feature in development of most of orchid flowers is phenomenon of resupination. In buds lip normally lies uppermost in the flower & column lowermost. In most species, however lip is lowermost in flower. This is achieved by twisting of flower stalk or ovarythrough 180 degree as bud develops or by bending pedicel in solitary flowered genera
Flowers may be inconspicuous 2-3mm in size in Oberonia, Cheirostylis parviflora, Peristylus lawii, Peristylus densus or large 10cm across in Pecteilis gigantea
Sepals & petals are similar in colour in many orchids except one of the median petal which is modified, variously designed & attractive to insect called labellum or lip it is highly specialised floral part of orchid flower which is complex & variable. Basal middle & distal parts of labellum can be distinguished in many orchids & termed as epichile, mesochile & hypochile
Surface of labellum bears callus of raised ridges, lamellae or tufts of hair or glands forms landing platform for insects Labellum is single or three lobed structure In many species of orchids labellum is extended on lower side into saccate or filiform spur, spur is variable in shape globose in Peristylus plantaginea, Clavate in Habenaria marginata, elongated in Habenaria lingicorniculata , Pecteilis gigantea or tublar & inflexed in Aerides crispa however spur is absent in some orchids, In some orchids like Dendrobium lateral sepals are fused with column foot forming spur like mentum
Column:
Male & female reproductive organs i.e., stamens & carpels are fused in orchids forming column; Tip of column where anther is embedded forms clinandrium, clinandrium may be flat & inconspicuous or specious, it usually has apicular stalk or filament on opposite side to hold the anther
Tooth like horn like projections from upper margin of column is called stelidia synonym stelids Acampe & Bulbophyllum
Orchids do not bear powdery pollen unlike most other plants; pollens are forming discrete mass called pollinia which are attached to viscidium by a stalk called caudicle. Pollinia, caudicle & viscidium form pollination unit termed Pollinarium
Aggregation of pollen is less defined Apostasioideae & Cypripedioideae ,Soft pollen may be packed relatively uniformly into masses or pollen is aggregated into packets or in massulae. Pollinium structure may be solid or granular
Pollinium number vary by form of anther & number of loculi In number of orchids there are four pollinia representing 4 anther cells. the ancestral number is eight like in Porpax ,reduction in series from eight to four to two can be traced in Aerides
Pollen stalk that attaches pollen masses to viscidium has different origins:
Those originated from anther are called caudicles, it also consists of aborted pollen mother cells as well as tapetum & pollen tetrad (F. N. Rasmussen) These are found in Orchidoideae some Arethuseae & some Epidendreae most vandoid and few other orchids have trap of nonsticky tissue called the stipe derived from rostellar tissue instead of anther tissue. Most common type of stipe is the tegula. It is formed by periclinal walls of epidermis of rostellum. Second type of stipe is hamulus is the recurved apex of rostellum.
Stigma & rostellum: The stigma is sticky lobed depression situated below & behind anther Dendrobium while in some terrestrial genara like Habenaria the stigma is bilobed with receptive surfaces borne distinctly on projection benath anther In Pecteilis stigma lobes are not very prominent In many species the pollen masses are transferred to the stigmatic surface by modified lobe of the stigma called rostellum Rostellum is thin short truncate or subtruncate & broad as in Cymbidium, recurved in Dendrobium & horse shoe shaped in Cleisostoma Vanda In Habenaria & Peristylus rostellum is distinct three lobed mid lobe is broad while lateral lobes are generally narrow.
Fruit:
Orchid fruits are casules developed from tricarpellary ovary which dehisces along midline of each carpel Young & developing fruits are green & photosynthetic Mature capsules of terrestrial orchids are erect, thin walled, dry & papery while those of epiphytic orchids are pendent, thick walled & somewhat fleshy
Roots:
Like most of the plants orchid roots are used for absorption & anchorage In epiphytes epidermis of roots is modified into a special types of tissue called velamen tissue used for absorption of atmospheric moisture along with nutrients, some roots become photosynthetic by developing chlorophylls while in terrestrial orchids roots become storage organ called tuber
Spur:
In many species of orchids labellum is extended on lower side into saccate or filiform spur, spur is variable in shape globose in Peristylus plantaginea, Clavate in Habenaria marginata, elongated in Habenaria lingicorniculata , Pecteilis gigantea or tublar & inflexed in Aerides crispa however spur is absent in some orchids, In some orchids like Dendrobium lateral sepals are fused with column foot forming spur like mentum