In this article, we will discuss about the three agents for pollination. They are: (1) Anemophily (2) Zoophily and (3) Hydrophily.
It has already been stated that pollination, in a vast majority of cases, involves some agent. The usual agents are wind, animals and water.
Accordingly, the types of pollination are anemophily or anemogamy (anemos==wind), zoophily or zoidiophily or zoogamy or zoidiogamy and hydrophily or hydrogamy. The terms zoogamy and zoidiogamy should better not be used as they are of ambiguous meanings. Zoophily is further subdivided according to the type of animal involved.
Anemophilous or wind-pollinated flowers are inconspicuous and not showy. They are also devoid of scent, nectar, etc. On the other hand, they produce a very large quantity of dusty pollens so that in great forests, the air is often charged with these.
Such air may be healthy as in the proximity of pine forests where the abundant pollens are described as ‘sulphur showers’, or, the pollen-laden air may give rise to an epidemic of allergic hay fever.
Another adaptation of anemophilous flowers is the branched bushy stigma (Figs. 374C, 374E & 393) capable of catching pollens from air easily as is seen in cereals. Such flowers are often unisexual and occur in bunches.
The anthers are often versatile, swinging freely in air (Fig. 334) and the pollens are dry, light and smooth-walled. Some wind-pollinated flowers like Urtica dioica show an explosive mechanism by which the anthers burst and surcharge the air with pollens.
In many anemophilous trees, flowers appear when they, are bare of all leaves so that wind can play in full. In evergreen plants or herbs also the anemophilous flowers are well exposed to wind.
Anemophily is found in most cereals and palms and in a number of Archichlamydeous families like Salicaceae (poplar, willow), Betulaceae (alder, hazel, birch), Fagaceae (oak, beech), Ulmaceae (elm), Urticaceae (Urtica) etc.
According to some, anemophily is a primitive character in the evolution of plants.
Zoophilous flowers are again subdivided into (A) Entomophilous (insect-pollinated), (B) Ornithophilous (bird-pollinated), ‘(C) Chiropteriphilous (bat-pollinated) and (D) Malacophilous (slug- and snail-pollinated) important.
A great majority of the flowers that we see about us today are insect-pollinated. It has been said that the evolution of flowering plants has gone on side by side with the evolution of insects.
Insect-pollinated flowers are made attractive to insects in different ways and the pollens are sticky with a rough surface so that they may easily stick to insect limbs. The stigma also is similarly sticky to be able to receive the pollens more easily.
To attract insects the following qualities have passed the test of evolution:
A flower may be rendered conspicuous by the bright colour of its petals or perianth leaves. Sometimes bracts, sepals or even stamens may become petaloid and serve the same function.
The conspicuousness may be due to the individual flower or to the aggregation of flowers forming such inflorescences as capitula. Miiller’s suggestion that certain insects show preference to certain colours has been found to be true. Thus, bees seem to prefer blue flowers, butterflies red and so on.
Flies seem to be attracted by the nauseous odour (indoloid odour) emanating from certain flowers like Rafflesia and some aroids. But how far insects can distinguish scents is not known so that it cannot be said definitely whether the flies are attracted by the smell or by the fleshy appearance of the flowers.
The actual role of odours in pollination is not yet clear. However, certain flowers, inodorous during day-time, emit a strong scent during night and are found to be pollinated by night-flying moths.
This suggests a definite role of scent in pollination. Many essential oils are present in different flower petals being produced in internal glandular cavities.
Nectar glands are situated in different positions of flowers secreting a sugary fluid called nectar or honey much sought after by insects. As a matter of fact, nectar is the most valuable thing for which insects visit flowers.
Bees collect nectar and elaborate it into commercial honey. Nectar glands may be located on the thalamus, on the outside of sepals, at the bases of sepals, petals, carpels and even in the septa within ovaries.
The nectar is often concealed within corolla tubes or spurs so that insects have to do much searching for this and thereby get thoroughly dusted with pollens.
Often there are spots and lines on the petals which converge towards the nectar glands and are called nectar-guides as they are supposed to guide insects towards the nectaries.
Sometimes nectaries are present on extrafloral parts as on leaves, stipules, etc. These are supposed to divert unwanted insects.
(4) Edible sap:
Some flowers (e.g., some orchids like Orchis morio) do not have nectaries but secrete an edible sap.
(5) Edible pollen:
Pollens of very few flowers are sought after by insects. Nevertheless, bees collect a large amount of pollens to nourish their youngs and, at the breeding season, collect more pollen than nectar. Bees also utilise the wax on the pollens to build their combs.
(6) Special mechanisms:
Certain entomophilous flowers show special mechanisms favouring pollen dissemination by insects:
It is a feature shown by the anthers and stigmas of certain flowers. Bi-fid stigmas of Bignonia, etc., close the two flaps on being touched by insects. If not pollinated, they open again. Corn-flower (Centaurea of Compositae) pistil bends and exposes the stigma on being touched (Fig. 394).
(ii) Explosive Mechanism:
Some flower anthers burst and discharge the pollens on being touched.
Versatile stamens and other balancing features (viz., Salvia described below) often greatly help the dusting of insects with pollens. .
According to the principal attraction in the flower, two groups may be recognised within entomophilous flowers:
I. Pollen Flowers:
Flowers sought after mainly for pollens are usually actinomorphic with a large number of stamens. Flowers of Argemone and other poppies, Anemone, Magnolia, Anona, water- lily, potato and other Solanaceae, etc., fall under this group.
II. Nectar Flowers:
A large majority of flowers fall under this group. The nectaries may be fully exposed, partly concealed, fully concealed or the flowers may be borne in compact groups (social flowers). The position of the nectary is the most important factor in determining which type of insect visits the flower.
The chief insects that visit flowers belong to the groups Hymenoptera (bees and wasps), Lepidoptera (butterflies and moths), Diptera (flies), Coleoptera (beetles), Thysanoptera (thrips) and Hemiptera (bugs).
It is found that usually certain groups of flowers are visited by certain groups of insects. This adaptation is regulated by the advantage that the insect gains (pollen, nectar, etc.), the season and the hour when the plant flowers, habits of the insects and, above all, the accessibility of the nectary, etc., to the particular, type of insects.
Of these insects, the Lepidoptera have the longest tongues, next come the Hymenoptera while the other groups have rather short tongues. Accordingly, flowers can be arranged in groups in respect of its adaptation to types of insects.
(i) Flowers adapted to short-tongued insects, i.e., Coleoptera (beetles), Thysanoptera (thrips) and Hemiptera (bugs). Naturally, these insects are limited to flowers which have either (a) the nectary fully exposed as in Rutaceae (nectaries in exposed disc—Fig. 310), Vitaceae (disc—Fig. 311), Umbelliferae, Euphorbiaceae, etc., or (b) partially concealed nectaries as in some Cruciferae, Ranunculaceae, Berberidaceae, Rosaceae, etc.
(ii) Diptera flowers:
Diptera are small feeble flies which carry on pollination by crawling into flowers. These flies do not seem to gain much by their visits except, possibly, a shelter. Probably they are attracted by nauseous smells.
Among these we may consider two instances:
(a) Nauseous Flowers:
These include the common aroids, e.g., Arum maculatum. (Fig. 267), Typhonium trilobatum, Amorphophallus campanulatus (Fig. 395) Alocasia indica, Colocasia esculentum, etc., which emit strong fetid odour during night.
On the spadix female flowers are located below and male flowers on top. Flies crawl up and down and thereby pollinate them. Rafflesia arnoldi and similar evil-smelling flowers also fall in this group. The odour or the fleshy appearance of such flowers attracts carrion flies.
(b) Pitfall Flowers:
These possess special traps for Diptera flies. Aristolochia clematitis flower (Fig. 396) serves as a good example. The flower is protogynous. In the young stage it is upright and flies can crawl down the corolla tube pushing the downward pointed hairs but they cannot come up until these hairs wither away.
By that time the stigmas wither, the anthers mature bursting out the pollens and the flower droops down. So, the flies come out pollen-smeared. They can now enter a second younger flower, pollinate its stigma and again remain trapped until the anthers of that flower mature.
(iii) Hymenopterous Flowers:
Flowers of this group are pollinated by members of the Hymenoptera (bees, wasps, etc.). While a varied and large number of flowers are included here, red, blue or violet coloured zygomorphic flowers predominate.
While these insects may visit flowers wherein the nectaries are more or less exposed (e.g., orange flowers), their range also includes (a) flowers with fully concealed nectar as in Papilionaceae, Labiatae, Scrophulariaceae, Orchidaceae, etc.; (b) social flowers with concealed nectar as the aggregated flowers of Compositae.
These two groups are pollinated by Hymenoptera with shorter tongues (up to 6 mm). (c) Flowers with long tubes with the nectar hidden below so that they may be pollinated only by insects with proboscis 6 to 15 mm long.
Within this group, again, one may distinguish some flowers favoured by special types of Hymenopiera. Some of these flowers are also visited by butterflies.
Within this group the interesting mode of pollination of a few flowers may be discussed:
(1) Salvia of Labiatae (The Sage Flower):
Salvia has got a bilabiate corolla with two fertile epipetalous stamens. The stamens and pistil remain hidden under the upper lip (Fig. 397A).
The flower is protandrous and the short epipetalous filament of each stamen is connected to the peculiar distractile connective (Fig. 345) which is long and lever-like, its two unequal arms separating the two anther lobes.
The basal lobe of the anther is sterile while the upper lobe is fertile. A slight pressure on the lower anther lobe brings the upper lobe down (Fig. 397B). Salvia is bee-pollinated.
Bees alight on the lower lip of the corolla and enter the flower to reach the nectary at the end of the corolla tube. In so doing, they push against the united lower anther lobes (Fig. 397C) thereby bringing down the fertile anther lobes which dust the bee’s back with pollens.
When the pistil of the flower becomes ripe, the stigmas protrude out of the upper lip (Fig. 397D) so that any bee entering the flower brushes against the stigma thereby pollinating it with pollen already on its back.
(2) Ficus carica and other Ficus spp. of Moraceae:
Flowers of Ficus plants are enclosed within the hollow pear-shaped hypantho-dium inflorescences. There is a narrow orifice for entering the receptacle (Fig. 398) within which there are three types of flowers male, female and gall.
The male flowers are situated on the top near the orifice while lower down are the long-styled female flowers and the short-styled ‘gall flowers’ which also are female.
Figs are pollinated by the gall wasp (Blastophaga) which crawls into the receptacle and lays eggs inside the ovules of the ‘gall flowers’ which it can easily reach by its ovipositor because of the shortness of the style.
The eggs develop larvae which feed on the ovules and form galls. After passing the pupa stage the larvae develop into mature wasps and crawl out of the fig. In so doing they brush against the male flowers near the orifice carrying away pollens on their bodies.
These pollen-laden insects then enter fresh figs where they pollinate the long-styled female flowers and lay eggs within the ‘gall flowers’ in their turn.
(3) Orchids of Orchidaceae:
Orchids are peculiarly adapted for bee pollination with their pollens aggregated in pollinia (Fig. 399). The bee alights on the conspicuous labellum of the perianth and, being unable to enter the opening in front of the gynostemium, sends its tongue into the spur to reach the nectar below.
In so doing, its head pushes against the stigmatic surface with the rostellum (which is also a part of stigma) which latter gives way so that the pollinia on the top of it come out and their sticky discs get stuck to the forehead of the bee.
The pollinium at first remains erect but soon droops down on the bee head. When the bee goes to a second flower and similarly brushes against a stigmatic surface, the bent and sticky pollinium gets stuck to the stigma which is thereby pollinated.
(4) Calotropis of Asclepiadaceae:
Flowers of Asclepiadaceae are provided with gynostegiums formed by the union of the prismatic stigma with the androecium (Fig. 400). The pollens are in pollinia as in Orchidaceae.
Two pollinia (from two different anthers) are attached to a glandular adhesive disc at the stigma angle (translator mechanism). These sticky discs get stuck to the proboscis or legs of visiting bees so that the pollinia are pulled out when the bee moves. When the bee visits another flower, the latter gets pollinated with the sticky pollinia.
(iv) Lepidopterous Flowers:
These flowers are provided with the longest corolla tubes, even longer than in the previous group so that the nectar can be reached only by the Lepidoptera (butterflies and moths) which have very long tongues (20 mm or longer).
Many sweet scented flowers come under this group—lily, lavender, Nyctanthes, Polyanthes, jasmine, Nicotiana, pink, etc. Of these, the night- flowering ones are pollinated by moths and the day-flowering ones by butterflies.
Yucca flowers (Fig. 401) are visited by the small moth Pronuba yuccasella for laying eggs within ovaries just in the same manner as figs are visited by Blastophaga wasps.
While about 20% of ovules are destroyed in the process there is no seed-setting in Yucca plants without the visit of Pronuba moths.
Bird-pollinated flowers are not many in number. Tiny birds like the humming-birds and the honey-thrushes (birds only an inch or so long) feed on the nectar of flowers like Bignonia capreolata (Fig. 402) and thereby pollinate them.
Large flowers of Strelitzia (Musaceae) are pollinated by a honey bird called Nectarina afra. In our country, silk-cotton (Salmalia or Bombax), Erythrina and a few other trees are visited by crows and mynas when in flower. These birds may play some part in the pollination.
Bauhinia megalandra of Java, Eperua falcata (Leguminosae) and a few other trees are known to be pollinated by bats. Several trees in this country, like Anthocephalus chinensis (Rubiaceae) and silk-cotton, are also visited by bats which possibly help in the pollination although their role is not definitely known.
D. Malacophily and Pollination by Other Animals:
Snails and slugs visit certain flowers and may have a role in their pollination. Many aroids, which are usually pollinated by Diptera flies are also visited by snails.
Squirrels visit a large number of flowering trees and may have some role in their pollination.
Certain plants, as those of the families Naiadaceae, Ceratophyllaceae, Potamogetonaceae, Hydrocharitaceae, etc., are completely aquatic so that their pollination is adapted to such conditions. Pollination takes place completely under water (hypohydrogamous) in Naias and Ceratophyllum while it takes place on the water surface (epihydrogamous) in the common water weeds Vallisneria, Hydrilla and Elodea of Hydrocharitaceae.
The dioecious plant Vallisneria having strap-shaped leaves grow in the mud at the bottom of stagnant water (Fig. 403). The male flowers are borne low down amongst the radical leaves on short-stalked spadix inflorescences out of which the individual flowers get detached and float freely in large numbers on the water surface.
These flowers open on the surface and the three perianth leaves open widely exposing the two stamens vertically. The female flowers are borne singly on long wiry stalks which grow in such a manner that the flowers float on the water surface when mature.
As the female flower is somewhat waxy, it causes a slight depression in the film of water because of surface tension. This depression, as well as wind, causes the detached male flowers to cluster around the floating female flower and, when the anthers burst, sticky pollens get attached to the stigma. Soon after pollination, the long stalk of the female flower begins to coil bringing the female flower again below water level until it reaches almost the tank base where the fruit matures.