POLLINATION and plant reproduction

Pollination is the process whereby pollen grains are transferred from the anthers of a stamen, their site of origin, to the receptive part of a carpel, ie the sticky surface of a stigma. The male sex cells, which are contained in the pollen grains, are still separated from the female sex cells in the ovules, enclosed in the ovary, by the style. Thus further processes are necessary before fertilization can occur. Fundamentally, pollination is merely a preliminary to fertilization, but a very necessary preliminary.

Cross-pollination versus self-pollination

Pollen may pass from the anther of a flower to the stigma of the same flower or another flower on the same plant, resulting in self-pollination; or it can be transported to the stigma of a flower on another plant of the same species — cross-pollination.

From the many and varied devices, some amazingly elaborate, which plants have adopted to secure cross-pollination, it can be assumed that for some species it is advantageous, and for some obligatory since they will not set seed with their own pollen. Seed produced from the union of two different parents will combine some of the individual features of both, thus bringing about a greater possibility of variation in the offspring. This variation is necessary because without it there can be no further adaptation to a different or changing environment. Stronger or betteradapted individuals will tend to survive, while the weaker ones will be eliminated in the course of competition for survival.

However, too much variation can create instability which for the species as a whole can mean a failure of adaptation to either a relatively stable environment or one that is changing slowly, in addition to a possible loss of a vigorous and well-balanced constitution already achieved. This is probably why there are still today a large number of plants which are able to resort to self-pollination should cross-pollination fail to take place. This can be achieved with the aid of structural and behavioural devices favouring cross-pollination in the early stages of flower opening, followed by the possibility of self-pollination if cross-pollination has failed. For example, love-in-a-mist, Nigella, is usually cross-pollinated. The styles of its carpels are much longer than the stamens, so that it is impossible for the ripe pollen to fall on the stigmas and bring about self-pollination. But if, when the pollen is ripe, cross-pollination has not occurred, the styles gradually bend backwards until they come into contact with the exposed pollen on the anthers so effecting self-pollination.

Before the variety of devices which plants have evolved through the ages in order to further the possibility of cross-pollination and obstruct self-pollination are examined, how pollination is actually effected (that is what agencies are involved) should be discussed. Ovules are fixed and stationary inside ovaries. Pollen grains when mature are exposed freely on the anthers either as separate little dustlike particles or stuck together in small groups, but they are not themselves able to move. At its simplest, pollination takes the form of pollen grains falling by the gravitational pull from an anther onto a stigma, or of the stigma and anther drawn into contact with each other by their manner of growth or by alteration of relative positions at maturity. More frequently, outside agencies are needed to effect transfer. To further this end, plants have made use of almost every thing that moves in their environ- ment, both animate and inanimate. Thus these agencies include a variety of ‘handy’ animals, including birds, bats, slugs and snails as well as insects, the wind, and even water. Of these agencies, insects and the wind tend to predominate, especially in temperate climates.

Bird-pollination

Bird-pollinated flowers are common in warmer countries, the bird species most particularly involved being the humming birds, honey-eaters, honey-creepers and certain kinds of parrots. A flower adapted to bird-pollination is often apparent by its vivid red or orange colouration and by the fact that it contains a very plentiful supply of nectar within it in order to make the visit worthwhile from the bird’s point of view. The habitually bird-pollinated American flower, Erythrina cristagalli, is known as the cry-baby flower because it contains so much nectar that it drips out of its inflorescence, while species of Banksia in Australia produce so much nectar that it is used as food by the aboriginals. Reds and oranges prevail because birds have a preference for harsh colours; some bird-flowers have rather peculiar colour combinations (’parrot colouration’) such as green, yellow and scarlet mixed together, a combination frequently found in the pineapple family. No scents are produced because birds have very little sense of smell.

Although humming birds can be very tiny and some bird-flowers can be extremely large, birds usually are larger than the flowers they pollinate and tend to be rather clumsy in their dealings with them. To counteract this, many bird-pollinated flowers have stamens strengthened with woody filaments to minimize damage and the ovaries are frequently protected from accidental pecks either by being inferior (that is below the corolla, anthers and stigmas) or by being separated from the nectar-secreting part. Sometimes, as in Fuchsia, the flowers have flexible stalks so that they get pushed away if the bird probes too forcibly, and thus they escape damage. Often the plants provide portions of bare stem, leafstalks or some such place for birds to alight upon, though some Australian plants, for example Brachysema, are pollinated by birds standing on the ground. American bird-flowers tend to have longer flower tubes than Asiatic ones because American pollinating birds tend to have longer bills. Occasionally bird-flowers are pollinated by rats, squirrels or, in Australia, by small arboreal marsupials, too.

Bat-pollination

Pollination by bats is quite important in the tropics. Because bats tend to be crepuscular in their activities, we find that the flowers which they pollinate are night-opening. Such a flower is Crescentia cujete, the calabash tree. Bat-flowers often have dingy colours and a sour musty scent (rather a ‘cabbagey smell’). Although bat-flowers have evolved independently in the Old and the New World, the scents of both groups are similar; the smell being similar to the peculiar odour of the glandular secretion which most bats produce for recognition purposes. Bat-flowers evolved by imitating this smell. Generally bat-flowers produce a lot of sticky nectar, though sometimes fleshy sugary bracts are provided for food as in the Indonesian plant, Freycinetia insignis, which is pollinated by fruit bats. Bats visiting flowers often cause damage with their claws: such flowers tend to be large and strong to help guard against this.

Slugs and snails as pollinators

Creeping animals such as slugs and snails unwittingly pollinate some flowers by picking up pollen grains in their slime and depositing them on stigmas encountered along their way as they move over plants. Some flowers are in fact specifically designed for pollination by slugs and snails; Rhodea japonica of the lily family has fawn-coloured flowers smelling of bad bread which provide fleshy tissues for their visitors. The aspidistra, with soil-level flowers, is also pollinated by snails.

Insect-pollinators

Other animals may inadvertently help disperse pollen grains too, but long ago plants discovered that it was the insects which were to become their most reliable allies, particularly the bees, wasps, butterflies, moths, flies and beetles. These are useful pollinators because with their small size they are able to alight upon and enter flowers without damaging them, and they can easily collect pollen on their hairy backs and legs when visiting the flower for food. As the insect passes from flower to flower, the pollen picked up from the stamens of the first is passed onto the stigma of the next.

Insects are attracted initially to the flowers by the bright colours and delicate perfumes of the petals and/or sepals, and come to associate these with the presence of nectar for their taking. Insects appear to have developed their acute sense of smell in close association with the evolution of the flowering plants, since the insect ancestors 250 million years ago, long before the flowering plants had appeared, had little sense of smell, having no need for it then. Many insect-pollinators have an acute sense of taste also; the butterfly for instance contains its organs of taste in its feet and if a single hair on one of the feet touches a nectar solution, it is sufficient to bring the long tubular tongue into action. Many plants show maximum nectar-secretion around midday to coincide with the peak of insect activity at that time, while many bees will organize their flower-visiting routines to coincide with flower-opening times.

Some insects visit flowers not for nectar but for the pollen which is their food. Flowers which are visited in this way include various species of poppy (Papaver), and species of Helianthemum, the rock-roses. Pollen is often found in the honey stored by insects—hive-bee honey owes its vitamin content to the pollen which it contains. In some beetle-pollinated flowers, food is provided in the form of special tissues which are filled with starch and oil, usually attached to the stamens. Sometimes the beetles can stay there for days, eating away.

A simple open flower with spreading petals, such as may be found in the buttercup family, the Ranunculaceae, attracts and provides a landing stage for a wide variety of insects, both large and small; butterflies, moths, bees, wasps, bugs, beetles and flies are all to be found on buttercup flowers. So the buttercup is not specialized in any particular way towards a selected type of insect pollinator. Many flowers however are precisely adapted to pollination by only one kind (or by very few kinds) of insect and some flowers evolved a tubelike form to conceal the nectar. This can only be reached by long-tongued insects with good three-dimen- sional perception, such as bees which are more efficient pollinators than short-tongucd beetles and others. The periwinkles (Vinca species) have the corolla in the form of a slender tube, with a flat part on which the visiting insects can alight, and many flowers in the olive family (Oleaceae) are similarly constructed.

Other flowers have become even more specific in their pollinators. Strange cactuslike plants (Stapelia species) of Africa and southern Asia, with large hairy dark purplish-red flowers which look and smell like bad meat, attract only flies. The Old World orchid, Bulbophyllutn macranthum, smells of cloves and attracts only a single species of fly. In Southeast Asia, each species of fig-tree is pollinated by its own particular species of fig-wasp. The beeorchids (Oplirys species) have a petal lip which is shaped, coloured, sized and even smells the same as the body of the female of certain kinds of humble-bee and related insects. These flowers attract the male insects which try to mate with the orchid lip. The pollen in orchid flowers is clumped together in a pair of club-shaped pollinia (the plant in effect ‘puts all its eggs in one basket’) and these stick to the head of the humble-bee during its activities and get carried away to pollinate another beeorchid.

The monkshood (Aconitum), too, is dependent on humble-bees, for only these can reach the nectar underneath the plant’s ‘hood’ and in so doing effect pollination. A. lycoctonum of the Alps in fact has nectaries some 20mm (o-Xin) long which can be exploited only by a few of the very longest-tongucd humble-bees. The monkshood has become so dependent on humble-bees that it is not found outside the insect’s range. This is the disadvantage of specialized adaptations towards one kind of insect pollinator; although it makes for greater precision in the pollination act, the plant becomes so dependent on this insect that pollination will suffer should the local insect population be adversely affected in any way by factors beyond the control of the plant, such as bad weather, disease or insecticides.

A curious relationship exists between pollinator and flower in the case of the American yucca plant. This flower is pollinated by a single species of moth Tegeticula yuccasella which is attracted by the large creamy white night-scented flowers of the yucca. The female moth climbs up a stamen, bends her head closely over the top of the anther, steadies herself with its uncoiled tongue, and then scrapes all the pollen off the anther, into a big lump which she carries under her head. She may do this to as many as four stamens. The yucca-moth then flies to another flower and investigates the condition of the ovary carefully. If she considers the flower suitable, the moth again climbs up the stamens, but this time goes between them to the ovary into which she bores a hole and lays an egg. Immediately after doing this, she climbs up the stigmas, which are united in a tube, and thrusts some of the parcel of pollen down the tube. This behaviour is repeated with each egg that is laid. This procedure ensures that there will be food, provided by abnormal ovule growth next to each moth’s egg, for the larvae; an unpollinated yucca flower soon dies. In this relationship, the moth ensures the seed-production of the food-plant, while the plant provides food and shelter for the young of its pollinator.

Wind-pollination

Another important agency for pollination is the wind. Plants specially adapted to this method include many trees, grasses and grasslike species, and many sea-side plants. Conifers come into this category, their clouds of pollen blowing in the wind like yellow smoke, being a common sight in early summer. This method has the advantage of freedom from the unpredictable activities of insects; it is effective when insects are scarce or absent. However the chances of a pollen grain reaching an appropriate stigma by wind are so remote that many thousands of times more pollen grains must be produced than can actually be used for fertilization. It has been estimated that a single birch catkin produces about fiveand-a-half million pollen grains and a single floret of rye over fifty thousand grains. An incidental effect of all this pollen released into the air is that it may set up allergies in some people causing the common condition of’hay-fever’.

Insect-pollinated species tend to have rather sticky and highly sculptured pollen grains, to attach themselves to insects bodies, whereas in wind-pollinated plants the grains have smooth dry surfaces and are dispersed singly or in two’s or three’s. Pollen grains so constructed can be carried great distances; they have been found in the air even in midatlantic!

Wind-pollinated flowers share several features. They often have large stamens which hang out of the flower on long filaments or in catkins so that they are freely exposed to the air. Stigmas too are generally large and well ex-serted; and they may be finely feathered so that they can ‘net in’ pollen from moving air. Petals and sepals are inconspicuous often reduced or absent altogether since they cannot ‘attract’ the wind; in fact they are a hindrance by obstructing the air flow and free transfer of the pollen. Often the sexes are separated in different flowers to minimize the chances of self-pollination. Many wind-pollinated deciduous forest trees flower very early in the year. At this time, the trees are bare of leaves so that the movement of pollen grains meets with least obstruction. At this time too, very few insects are active; perhaps in temperate regions, effective pollination of these dominant plants would require a greater population of insects than such a climate could support.

Water-pollination

Pollination by water is rather rare. Most water plants have their flowers above the surface where they can be insect-pollinated. Species that are water-pollinated mostly show a relatively recent ancestry of insect- or wind-pollination. As with wind-pollinated plants, water-pollinated flowers tend to have reduced petals and sepals, a single ovule and flowers which are commonly unisexual. Stigmas tend to be large, but rigid and simple, whilst pollen grains are often elongated; it is interesting that some aquatic fungi (the Hyphomycetes) also show a trend towards elongation in their spores.

Water-pollination may take place at the water surface or completely submerged. A good example of the former is the ribbon-weed, Vallisneria spiralis, with male flowers which break free and float to the water surface where they open and the stamens release their globular masses of pollen. The separate female flowers are borne to the surface on slender flexible stalks where they open in a little depression of the surface-film. They disclose three large fleshy stigmas which are unwettable due to a covering of water-repellent hairs forming a dense velvety pile. The projecting stamens of the male flowers are brought into contact with the stigmas of the female flowers at the water surface by water currents, the male flower sliding down the little depression in the surface-film.

In the eel-grasses (Zostera), the pollen ‘grains’ are threadlike, about 025mm (ooiin) long and have the same density as sea water so that they may move freely at any depth until caught on the submerged feathery stigmas of the female flowers, round which they rapidly curl.

Mechanisms to ensure cross-pollination

Cross-pollination is a more effective method of producing new variants than self-pollination. Plant breeders can produce vigorous ‘hybrids’ by using a little brush to transfer pollen in a controlled method of cross-pollination, but in nature many diverse mechanisms exist to favour cross-pollination and obstruct self-pollination. Selfsterility is not uncommon among flower- ing plants; if pollen reaches the stigma of the same flower, it is unable to develop, or can only do so very slowly, because the stigma inhibits its growth. Buttercups are markedly selfsterile. This ‘prepotency of pollen from a distinct individual over that of the same individual’ was discovered by Darwin in 1876.

Lots of plants have their anthers maturing long before the stigmas of the same flower are sufficiently developed to accept pollen, so that pollen from the flower has already gone before it has the chance of pollinating its own stigma. These flowers are called protandrous, for example the dandelion (Taraxacum) and daisy (Bellis). Some flowers have stigmas which are mature before the stamens, but these are less common, though the mechanism can be found in the meadow rue (Thalktrum flavum) and the woodrush (Luzula), and these are referred to as protogynous flowers. This condition in which anther maturity and stigma receptiveness do not occur together is known as dichogamy.

Cross-pollination is the only possibility if individuals of a species produce gametes of one kind only. Most animals are unisexual, and some plants are too, having separate male and female flowers either on the same plant or on separate ones. Many wind-pollinated flowers are unisexual since the stamens here produce such vast quantities of pollen that if stigmas were close by, they would be so thickly covered with their own pollen that there would be little chance of fertilization by pollen from other individuals.

Heterostyly is another device by which some plants seek to avoid self-pollination and the primrose Primula vulgaris is a fine example of this. Primrose flowers from different plants are of two kinds; those with the top of the stigma protruding like a little pin-head up through the throat of the corolla tube and those with five anthers clustered around the neck of the tube and the stigma invisible beneath them. The former flowers are called pin-eyed, the latter thrum-eyed, and the difference is essentially due to the style being long and the anther filaments short in the pin-eyed flower and vice versa in the thrum-eyed. The primrose is pollinated by the honey-bee which when pushing its long tongue into the corolla tube of a thrum-eyed flower will receive pollen in the right position for pollinating the stigma of a pin-eyed flower, and vice versa if it visits a pin-eyed flower. Also the pollen grains of the pin-eyed flower are smaller than those of the thrum-eyed, and its stigma has coarser surface papillae. The larger pollen grains of the thrum-eyed flower tend to stick more readily to these coarser papillae, whilst the smaller pin-eyed pollen grains are better suited to the thrum-eyed stigma and its finer papillae.

Some insect-pollinated flowers have devices which detain the visiting insect longer, so that pollination can be more effective. Classic-examples of this are to be found in the bindweeds and trumpet gentians which have been called ‘revolver flowers’ because upon entering them, the insect is faced with a ring of narrow tubes like the barrels of a revolver, through which the insect must probe to get its nectar. Other flowers actually trap their insects. This happens in many members oi the birthwort family (Aristolochiaceae) and in the Araceae to which Arum belongs. Essentially the mechanisms are the same in that lured insects slip down the smooth-sided flower tube which has smooth-surfaced cells and downward-directed papillae coated with a layer of oil, and often downward-pointing hairs too, all of which prevent upward escape. A day or two later changes take place which enable the insect’s liberation — the trap hairs and papillae shrivel and the tube may

The temperate Wild Arum or Cuckoo Pint Arum maculatum bears a curious inflorescence which attracts and traps flies in order to ensure pollination. The plant produces flowers in the spring which consist of a central ‘spadix’ and an enclosing leafy sheath or ‘spathe’. The spadix bears simple change to a horizontal position as in the midge-trapping Ceropegia, a genus occurring in Africa, Asia and Australia. In the South American birthwort Aristohchia lindtieri, flics are trapped on the first day of the flower’s opening, on the second day no luring (to the fly) smell is produced and the stigmas bend together so they can no longer receive pollen, and just before the insects are released the anthers burst and dust the prisoners with pollen. In Arum, pollen deposited on the stigmas on the first day produce rapidly-growing pollen-tubes so that the stigmas are withered by the time the inflorescence sheds its own pollen. By these means self-pollination is prevented.

The insect-pollinated flowers as a group in fact show some truly amazing mechanisms for securing cross-pollination. One such flower, an orchid called Gongora maculata, has a very slippery corolla, plus an intoxicating (to a bee) scent which combine to make a visiting bee lose its feet and slide down a curved and downward-hanging structure called a column. As the bee slides down the column it picks up on its way the pollinia pollen-clumps conveniently hanging in its path. Should the bee now visit a female flower, the bee is exposed to a sticky stigma which rubs the pollinia off its body as it slips down the slide.

Other orchids—a group which is highly specialized for insect pollination—show some equally fantastic pollination methods. The massive waxy-looking flowers of Coryanthes macrantha have part of the corolla lip forming a ‘bucket’ into which fall drops of water secreted by a pair of knobs on the column. Male bees of one particular genus (Euclema) are attracted to this flower by its strong scent which leads them to a special place on the flower lip where they endeavour to collect the liquid scent by scratching at it with their forelegs. This fluid which they collect affects special sense organs on the bee’s feet, causing intoxication, so that it loses its grip and falls into the ‘bucket’ of water in which it swims around, but it cannot climb out again. The bee has one exit, however, through a narrow tunnel just beneath the stamens and stigma. As it wanders drunkenly out through the passage, the bee picks up or deposits pollinia. By the time the bee has escaped, the flower’s scent has vanished, so that the bee cannot be induced to reenter the flower, though the scent returns the following day. By this prevention of reentry of the same flower, self-pollination is avoided.

One group of plants are, however, habitually self-pollinating. These are the annuals, small plants often living in unstable habitats, which also tend to be rather restricted and precisely defined. Self-pollination in this case is advantageous in that it maintains their precise adaptation to demanding habitats of this type. Also because their habitats are unstable and large proportions of annuals can be wiped out in a bad year, self-pollination helps numbers to recover more readily and quickly. Cross-pollination may occasionally occur, helping to provide some variants.

Some plant species make the best of both kinds of pollination in the same individual plants by producing two different kinds of flowers. The violets (Viola species) have conspicuous coloured flowers which are in every way adapted to cross-pollination by insect visitors, also possessing devices to ob- struct self-pollination. Seed-setting is however erratic and often poor. If the plants are examined following flowering, little things which appear to be flower-buds on short stalks can be found. These are actually flowers in their own right; they remain small, never open and are self-pollinating, with seeds being set in abundance.

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