Orchid Flower

As in their vegetative habits , in flower size , shape and colour also , the orchids exhibit a wide range of variations. As Lindley remarked ,”……..flowers are produced so unusual and so grotesque in form that it is no longer with the vegetable kingdom that they can be compared , but we are forced to seek resemblances in the animal world .” In most cases flowers are smallish , to seek resemblances in the animal world.” In most cases flowers are smallish , but the range of size may vary from that of some species of Oberonia where it is only 0. 15 cm. Across to Pecteilis susannae which may be as much as 10 cm . across. Platystele ornate of Venezuela has flowers which are smaller than a pin-head . The predominant shades are white , yellow , green and purple seen in the pure state or occurring in every conceivable combination . The beautiful azure blue of species like Vanda coerulea , valued among orchid breeders occur among South Indian orchids only in the lip of Vanda tessellate . So also is the bright orange red which made species like Ascocentrum ampullaceum very popular among orchids lovers , seen only in the lip of Vanda tessellata. So also is the bright orange red which made species like Ascocentrum ampullaceum very popular among orchid lovers, seen only in the pendant species of Schoenorchis filiformis which , in spite of its smallish flowers , looks very dainty and elegant .

Notwithstanding this great range of variety in form ,the general structure of the flower in orchid is remarkably uniform . Except in a few cases like Malaxis, Oberonia , Satyrium , etc., the flower , as it appears to us , is in an upside down position , having through 180 degree on its pedicel.

The three outermost segments or sepals are most often similar or at least subsimilar in appearance , the laterals sometimes becoming falcate. But in a few cases like some Bulbophyllums , Renanthera, many Oncidiums etc., the odd sepal is of a different size from the laterals. In species of Porpax, Cryptochilus, Masdevallia etc., the sepals unite into a tube with only the ends free of each other . In Paphiopedilum , Cypripedium , Rodriguezia etc., the lateral sepals cohere along their entire length . In the tropical American Cryptochilus , Masdevallia etc. , the sepals unite into a tube with only the ends free of each other . In Paphiopedilum , Cypripedium , Rodriguezia etc., the lateral sepals cohere along their entire length. In the tropical American Cryptophoranthes these fused lateral sepals also unite with the odd sepal at the base and the tip, leaving the middle portion free, so that the flower never opens . This peculiar construction has gained for this orchid the name window orchid. In Fabenaria perrottetiana, Brachycorythis etc. the odd sepal unites with the petals to form a hood over the column.

The petals are , as a rule, much more brightly coloured than the sepals. Of the three segments , the odd one is modified into the labellum or the lip , which , in most cases , is larger , three-lobed and more brightly coloured than the laterals . The petals may be similar to the sepals in size, but in a few cases as in some Habenaria and Coelogyne species of the old genus Cirrhopetalum they are much smaller than the sepals . In Lichenora , Cryptochilus , Masdevallia etc. they are hidden by the sepaline tube. But in Cypripedium sanderianum , C. caudatum ,C. caricinum etc., they are very conspicuous by their twisted ribbon-like form and several inches of length.

The labellum is the most prominent of all perianth parts , being endowec with arresting colours , unusual forms and peculiar markings , ridges and grooves . Except in a few instances like Bulbophyllum , Eria etc. the lip is trilobed , with the central lobe much more prominent than the side lobes . In Dendrobium brymerianum and Brassavola digbyana the fimbriation of the lip attains extreme proportions . Odontoglossum and Miltonia show very peculiar markings on their lip. In the latter, perfect reproductions of common numerals are sometimes seen, imprinted in contrasting colours .

The lip is attached to the base of the column. The mode of attachment may be rigid of loose. In the South Indian Bulbophyllum tremulum and the East African B. distons , the lip is provided by several movable hairs, which catch even the slightest movement of the wind.

But the most fascinating aspect of the lip is its habit of mimicry . More often than not , this serves to promote pollination by insects . In such cases , the lip is modeled as an exact replica of the female of the insect-pollinator . The Mediterranean wasp orchid has a lip which resembles a female wasp and emits a similar odour, and is believed to attract the male. But what is even more remarkable than these are the instances where the lip gives the unmistakable impression of comic imitations of distinct animal forms. Such pranks appear to serve no purpose at all, except perhaps, to add to the element of the unusual which already is part of orchid architecture .

The orchid flower may or may not be spurred . This appendage of the lip is meant for storing nectar and is possessed by quite a number of species . It may vary in shape and length in the different species. In the African Angraecum sesquipedale, it reaches a length of one foot. In many Peristylus species , it is scroriform or globular , whereas in Gastrochilus , it is wide and cup-shaped . There are two spurs instead of one in satyrium and diplocentrum.

Situated in the centre of the flower opposite the lip and facing it is the unique structure ,the column or the gynostegium ,which is peculiar to the orchids and their chief identifying character. The gynostegium is formed by the fusion of the filaments ,styles and stigmas of the flower . The only fertile anther of the flower is borne on top of the column. In diandrae , where two anthers are fertile , they are borne subteminally on the two sides of the column . In orchideae and Neottieae ,the anther is persistent ,whereas in the advanced Epidendreae they are deciduous . The anther cap which is easily removable in the deciduous forms , provides a protective covering over the pollen grains, which are collocted in masses called the pollinia . The pollinia are contained in a cavity called the clinandrium . There is a great amount of variation in the construction of the pollen apparatus in the various species. In the primitive Diandrae pollinia are not formed ; instead pollen grains adhere together by a viscous substance. In Neottieae and Orchideae pollinia are organized but are of soft consistency. In the former they separate easily into a powdery mass and are termed granular of mealy pollinia are organized but are of soft consistency . In the former they separate easily into a powdery mass and are termed granular or mealy pollinia while in the latter they cohere in packets and are termed sectile pollinia . In the advanced Epidendreae the pollinia are hard and waxy .

The pollinia may or may not have appendages . In Neottieae and Orchideae and advanced subtribes of Epidendreae , like Oncidiinae and Sarcanthinae , these appendages are present and the whole structure is called the pollen apparatus , being consistuted by the pollinia , the stipe and viscidium to caudicle , which is a tail-like prolongation of the pollinia . The stipe is of columnar derivation and is often hygrophilous . The caudicle and disc are part of the pollinial apparatus in Orchideae also . But here they are derived from the pollinium itself and are thereby morphologically distinct from the stipe and disc of the Sarcanthinae.

In the primitive subtribes of the Epidendreae pollinia are prolonged into a tail as is seen in Calanthe , Eria etc. In Cattleya the pollinia are lenticular and have a distinct ribbon-shaped appendage which lie inside the clinandrium and is called as ‘ appendicula’ by Bentham.

The number of pollinia may vary from 2,4 and 6 to 8 . Six pollinia are of rare occurrence and are found only in a few genera like Hexadesmia , Leptotes, Tetramicra etc.

In Orchideae the various packets of pollinia are drawn together by elastic threads into one club-shaped mass with a stalk-like caudicle ending in a disc. The pollinia , two in number , lie one in each half of the anther which is made up of two erect bag-like containers or loculi. The disc is kept hidden in a flap of brusicle and is always moist with the secretion of a sticky substance . The pollinia cab be dragged out of their containers , which is what insects do unknowingly , during visits to the flowers.

Just beneath the anther is the rostellum which is morphologically one of the three stigmas modified . It extends a s a horizontal beaked plate between the anther and the functional stigma and it serves the purpose of preventing the pollen from failing on the stigma of the same flower . It is also supposed to secrete a viscid substance which holds the pollinia till it is mature to be disbursed . The viscid portion then dries up and releases the pollinia

On the ventral side of the column, beneath the rostellum is a hollow cavity which is the functional stigma of the flower . Actually it is the two fertile stigmas of the flower fused together . It is covered by a viscous substance which is very dear to the insects . This substance helps to hold the pollinia which are depostited on it during the process of pollination. For the same purpose , the stigma may also develop papillae on its surface . In species of Habenaria , stigmatic lobes are in the form of club-shaped processes projecting beneath the anther . Here when cross pollination fails , the pollen falls on the stigma and self pollination takes place.

In many of the Oncidiums , the column extends beyond the anther giving rise to cucullate of hooded anthers. In Catasetum the column has two extremely sensitive , bristle-like appendages below the stigma, which aid in releasing the pollen on to the insect’s back during pollination.

In genera like Eria , Dendrobium , Bulbophyllum , etc. and some genera of the Vanda tribe, the column is extended below beyond its attachment to the stalk of the flower , into a structure called the foot ,and together they form a sac-like ‘mentum’ which is characteristic of the above genera .

The ovary in Orchidaceae is generally tricarpellary , one-called , with three pariental placentations and innumerable number of ovules. In the primitive Apostasieae and Selenipedium of Cypripedieae , ovary is three celled with axile placentations.