Pollination in Orchids

The various modifications of the floral parts in orchids facilitate cross pollination. Due to this fact there is a popular belief that cross pollination is the rule in Orchidaceae . But a considerable number of species have now been revealed as regularly self pollinated , even though they possess all adaptations for pollination by insects . Cattleya aurantiaca , Paphiopedilum mastersianum ,P. lowii etc., have long been known as self pollinated . In cleistogamous species like Chysis laevis , Spathoglottis microchilina , Epipactis cleistogama, Pterygodium newdigatae , Encyclina species etc., and in those species which complete their life cycle underground like Rhizanthella gardneri and Cryptanthemis slatterii , self pollination is what naturally occurs . Summerhayes (1951) has reported that in many of the British orchids such as the bee orchid , the Halleborines etc., self pollination takes place, in cases where cross pollination fails. The stalk of the pollinium shrinks, bringing its club-shaped head forward and downward to touch the stigma of the same flower or the pollination fails. The stalk of the pollinium shrinks , bringing its club-shaped head forward and downward to touch the stigma of the same flower of the pollinia may simply slip out of its bursicle and fall on to the stigma which is projecting below. The latter has been found to occur frequently in a few South Indian species like Habenaria longicorniculata , H. Montana etc. In some species flowers are so constructed that it is impossible for the insect to enter it eg. Epipactis pendula (Summerhayes , 1951), where flowers are pendulous . Here self pollination is the rule , although occasionally apomixes is also reported.

Pollination by insects in orchids is such a complicated process which involves a high degree of delicate balance which could be easily upset by interference from several agencies . Such being the case, cross pollination can easily fail to take place. Nature seems to have provided self pollination as an alternative to cross pollination in cases where the latter has failed .

The insects that visit the orchid flower are the various bees, moths, fruit-flies, mosquitoes and wasps. The flowers offer them a variety of refreshments- to drink and to eat. The nectar or honey is usually contained inside the spur or at the base of the column and the edible albuminous tissue is located either on the surface of the lip of in the stigmatic cavity . The insect , attracted to the flower , either by its arresting colours or its strong scent , alights on the lip and proceeds in search of the edible ingredient . During this exploration some part of its anatomy rubs against the rostllar beak .The pressure applied against this part pushes off the anther cap exposing the pollinia, which promptly attaches itself on to the head or tail or back of the insect . When the insect visits another flower of the same kind , the pollinium gets entangled in the sticky substance in the stigmatic cavity of that flower and pollination is effected .

The mechanisms of pollination in the several entomophilous species of orchids are merely ingenious improvisations upon the above basic pattern. In Listera ovata which is a primitive orchid, pollen are in tetrads held together loosely. When mature , this pollen mass is deposited on the rostellum . The insect in its attempt to get at the nectar, which is stored in a groove in the lip, brushes against the rostellum causing a tiny explosion. As a result of this, the rostellum unloads on to the insect’s back first a drop of mucilage followed by the mass of pollen tetrads, while the startled insect immediately takes to flight . During its visit to another flower the pollen is deposited on the stigma of that flower .

In the tribe Orchideae , the pollinia are included in special bags called bursicles ,which slit open upon maturation . As soon as the insect enters the flower, the two viscidia attach themselves to the two sides of the insect’s head. When the insect leaves the flower , the two pollinia are drawn from the bursicles and carried away on the insect’s head. By a special hygrophilous quality of the caudicle ,the pollinia which are at first held upright on the insect’s head slowly bend forward , till it comes to occupy a position at right angles to the insect’s head . In this position ,when the insect visits the next flower , the pollinia immediately come in contact with the stigma . This hygrophilous nature of the caudicle is characteristic of some of the advanced Sarcanthine orchids such as Vanda teres , Luisia sp.etc. Here the caudicle is transparent and ribbon-shaped . Immediately after removal from the anther it appears straight . But while it is being carried on the insect’s head, it absorbs it appears straight . But while it is being carried on the insect’s head , it absorbs water and curves in an S-shaped manner bringing the pollinia forward , so that they are deposited on the stigma of the flower which the insect visits next .

The most complicated and ingenious methods by which cross pollination is effected in orchids, are met with in South American species of Cucnoches , Catasetum etc. In Catasetum, flowers are unisexual . The column has two tentacle-like prolongations which lie in contact with the membranous stalk of the pollinia . The membrane is stretched at a high degree of tension. The tentacles are extremely sensitive and act like a trigger . Upon being touched by the insect ,they release the membrane with the sticky disc at one end and the pollinia at the other , with such force that the pollinarium is practically shot at the insect and instantly plastered on to its back .When the insect eventually visits a female flower , pollination takes place.

Apart from all these , a most unusual form of pollination is effected in orchids by a method called pseudocopulation . This extraordinary process was noticed by a Fenchman Pouyanne in 1928 in Ophrys speculum pollinated by the insect Scolia ciliate .The lip of this orchid resembles the female of the insect at rest ,even to minute details . As a rule the male Scolia emerges from the pupal stage ahead of the female. This time the plants of Ophrys speculum open their flowers . The male going about in search of suitable mates are attracted by the lip of the flower . They alight on the lip and go through the motions of copulation , during which process the pollinia are dislocated and attached to the insect’s body. Upon the insect entering another flower with the same instinctive urge, the pollinia get deposited on its stigma . There are a few orchids whch achieve pollination by this most interesting method .

In one species of Ophrys , the lip resembles a small insect which is the natural prey of the common wasp. Mistaking the lip for the real insect the wasp makes a dive for its prey and in the process away the pollinium of the flower .

The South Indian Cottonia peduncularis which is a member of the Sarcanthoid group , has a lip resembling an oversize bee. But eventhough the plant is quite common on the Western Ghats at elevations of 600-900 m and flowers freely under cultural conditions , a curious lack of fruitset has been observed in this plant ; no insect has so far been observed to visit this flower under artificial cultural conditions .Whether this is due to the lack of co-ordination in the time of emergence of the insect and the time of flowering , or whether it is a matter of extinction of the suitable insect is not clear .

The terrestrial orchid Habenaria obtusata is reported to be regularly pollinated by mosquitoes , which carry the pollinia on their eyes.