Articles

Pollination

by Anonymous

The pollination of flowers is a phenomenon which, like many of the wonders of Creation, is often taken for granted. Little thought is usually given to the miraculous intricacy and split second timing of some of the mechanisms involved. The transference of pollen-grains from the stamen to the stigma is effected in a bewildering variety of ways. Cross-pollination is the general rule, causing out-breeding and vigour, although self-pollination (autogamy) is also found in certain plants.

The most familiar method of pollination is that of insects transferring the pollen grains; principally those of the bees and wasps (Hymenoptera), the flies (Diptera), the butterflies and moths (Lepidoptera), and the beetles (Coleoptera).

There are many species of bee involved in pollination, solitary as well as social, and each one has its own favourite food plant. The amazing thing is that these various kinds of bee emerge when their favourite plant is flowering. Any lack of correspondence here and these plants would go unpollinated, but the accurate timing works every time! The honey-bee scout transmits information about the distance and direction of a suitable pollen source to other bees in the hive using an elaborate dance (see pamphlet 264). This instinct is programed into the scout bee rather like software into a computer. Information cannot arise by chance.

The mouth-parts of a bee are extremely complex, and perfectly structured for the gathering of nectar. There is a hairy tongue which absorbs the nectar; this is drawn back into the space between two maxillary plates and a third plate known as a mentum, thence pressure and suction pull the nectar back into the mouth. Different species have tongues of different lengths, enabling them to obtain nectar from flowers of many different types.

Whilst nectar is being obtained by the mouth-parts, pollen is being stored either on special brushes on the abdomen, in the crop, or else in baskets on the legs, specially designed for this purpose.

The sense of smell in bees in confined to the antennae, and bees can detect scents 100 times more dilute than those that we can detect. Vision is important, however, in guiding bees to a food source from a distance, but smell takes over when the bees are near the food source.

Once a flower has been visited by a honey-bee, any pollen on the antennae is scraped off by the forelegs by means of a notch on one of the segments. Then the middle legs scrape the fore, and the hind legs scrape the middle by means of a special rake situated in a gap in the joint between the tarsus and metatarsus. These two segments of the hind leg are then pressed together, packing the pollen in the gap into a compact mass, and at the same time plastering it into the pollen basket on the tarsus just above the articulation. A spike in the basket serves to hold the mass in position. Each and all of these structures needed to be fully developed for the process to work. A gradually evolving system could not function and the plant would not be pollinated. There would be no second generation!

As we have seen, there are those species of bee which have favourite flowers to pollinate. Other species will pollinate a number of different flowers, but generally these also exhibit the remarkable phenomenon of 'flower constancy', whereby they stick to one species of flower on each flight, passing over others even if they are growing nearby. This is found not only among bees but among other pollinators too, for example, flies, moths and butterflies.

Cross-pollination does not always entirely depend on the pollinating insect . Certain plants have built in mechanisms to ensure pollination. For example, with the bi-sexual inflorescences of Euphorbias (spurges) the female flower matures before the male flowers, so that by the time the latter are ready with their pollen, the female flower will already have been pollinated from an inflorescence on another plant. This mechanism is known as dichogamy' which means separate maturity'. Heterostyly is another mechanism, seen for example in primroses, where there are two types of flower per species - a pin-eyed' type with long styles and anthers halfway up the tube, and a thrum-eyed' type with short styles and anthers at the top of the tube. Any one plant has flowers of either one or the other, never both. The nature of the pollen and style surfaces of each is different too, so that pollen from the pin' can only find lodgement upon the stigmas of the thrum', and vice-versa, both by virtue of position and nature. There could be no evolving of these complementary structures.

A very remarkable cross-pollination mechanism is exhibited by members of the Arum family. For example, in a certain Mediterranean species the spathe opens overnight, and the spadix begins to produce evil-smelling chemicals attractive to flies. As the chemicals form, heat is generated, and this serves to volatilize them. Dung-flies and beetles are attracted to the plant in the morning. If they land on the spadix or the inside of the spathe, they lose their foothold and fall into the flower chamber because the cells have slippery surfaces. In doing so, they fall past a ring of stiff hairs which allows their entry but prevents their egress. If they bear pollen from another plant, they may pollinate the receptive first-day stigmas of the female flowers by climbing over them in their attempts to escape. By the second night, these have been fertilised, so that when the inflorescence produces its own pollen, selfing is impossible. The pollen is abundant, thoroughly covering the trapped insects. On the second morning the spadix has ceased to smell, so no more insects are attracted; the trap-hairs shrivel, allowing the insects to escape from the chamber. The smooth cell surfaces of spadix and spathe roughen up, enabling the pollen covered insects to climb up and get away probably to pollinate another first day stage inflorescence.

Modifications of this process occur in other members of the Arum family. In the Himalayan variety there are bright translucent zones resembling window panes at the back of the spathe, which induce insects to settle on them, only to slide in! Similar mechanisms also operate in the Aristolochia family and in the Ceropegia genus. The ways in which the needs of both plant and insect here are dovetailed into each other clearly demonstrate design and purpose in creation.

In the sage plant the mechanism is based upon the principle of the lever. Each stamen has only one fertile anther lobe, the other being modified into a pedal which is pushed back and up by a visiting insect. Between the pedal and the fertile lobe is a pivot, and as the pedal goes back and up so the lobe comes down and deposits pollen on the visitor's back. In the younger flowers with matured pollen the style keeps out of the way, but in older flowers it bends down to the position occupied by a levered anther and scoops pollen off the visitor's back!

Where insects clean the pollen off themselves between visits, as in the case of the bee which pollinates the Canadian Adapted from.”Science v Evolution” by Malcolm Bowden lousewort, the pollen is deposited on and collected from the area between the head and thorax where the bee cannot easily reach; thus ensuring that some pollen will be left for pollination.

It is, however, among the orchids that the most fantastic mechanisms are found, and the author has discussed these in pamphlet 216. The amazing interdependence of the yucca plant and yucca moths are considered in pamphlet 100.

Insects are not the only agents involved. Wind, water, rain, birds, bats and even molluscs are also agents, or vectors, in cross pollination.

Wind is the agent for many trees, both coniferous and deciduous, and for grasses and sedges. Wind pollinated plants usually have large stamens hanging freely, and large feathery stigmas to trap the pollen. Wind pollinated plants do not have the gaudy petals to attract insects. Many of them are dioecious - that is, separate sexes on different plants. This avoids self-pollination. The deciduous trees often flower early, when there are no leaves to hinder pollen movement. It is noteworthy that some of the dominant species of northern landscapes are wind pollinated. If they were pollinated by insects they would require a greater population of insects than a temperate climate can support. Here is further evidence of the harmony and order of the creation.

The vallisneria plant, among others, uses and female plants are very different. The male flowers are borne under water and become detached from the plant in the bud stage. They surface and burst open, exposing masses of pollen which are held aloft. The female flowers are on a flexible staLk which allows them to remain at surface level despite fluctuating water levels. The stigmas are un-wettable. The male flower 'boats' are carried about by water currents and when one comes adjacent to a female flower, it slides down the slight depression in the surface water film, coming to rest with the pollen masses in contact with the stigmas, thereby effecting cross pollination.

Bird-pollination is not found in Europe or N. Asia, but is common elsewhere. In contrast to insect pollinated flowers, those pollinated by birds are scentless, for birds have no sense of smell!

Bat pollination is entirely confined to the tropics. Bat pollinated flowers have a disagreeable musty odour. They are tough and able to bear the weight of the bat: on long stalks below the leafy branches they are readily accessible to the bats.

An interesting example of mollusc pollination is afforded by the Aspidistra which produces dull brown flowers at ground level. They have a flat platform in the centre over which a snail may crawl, thereby effecting pollination!

Evolution theory would require the flower and its pollinating agent to evolve in a complementary manner, and at the same rate. This is improbable in the extreme. The Written Record that flowers and pollinating agents were all made within one week of Creation fits in with what we observe. The first flowers needed pollinating agents in order to survive. In this process of pollination we have one of the most convincing demonstrations of the fact of Divine Creation that could possibly be provided. "For the invisible things of Him from the creation of the world are clearly seen, being understood by the things that are made, even His eternal power and Godhead." (Rom.1.v20)

Pamphlet 211

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