Cast your mind back 145 million years to when the Scottish Highlands protruded from warm, shallow seas. Elsewhere in this unfamiliar world of giant reptiles, the first true flower, the ancestor of all today’s flowering plants, emerged.
Reproduction is of course essential for all life – where would we be without it? The flowering plants, or angiosperms, have got sex down to a fine art and are one of evolution’s great success stories. A flower is a fascinating structure. If we take a close look at a typical flower, we see that it is supported by a circle of green sepals, which make up the calyx. Within this is the corolla – the circle of petals, which in turn surround the sexual organs. The male parts of the flowers are called stamens. These have anthers, containing pollen, at their tips. Pollen is a fine, tough-coated powder containing the male sex cells. The female parts are called carpels. The carpel is made up of an ovary, a style protruding from it, and a pollen-receiving stigma situated at the tip of the style. There are variations on this theme, but this is the archetypal flower. When pollen reaches the stigma, it germinates, sending a tube through the style to fertilise the ovule.
Even more venerable than the angiosperms are the gymnosperms. These are different from the flowering plants in that while they still produce pollen, their sexual organs are cones rather than flowers, and their seeds, unlike the angiosperms, are not enclosed within an ovary. The conifers, including Scots pine (Pinus sylvestris) and juniper (Juniperus communis), are gymnosperms. Conifers have done very well, especially in colder climates, but flowering plants are far more diverse and widespread. Yet all of these seed-bearing plants face the challenge of needing to breed while remaining rooted to the spot. They obviously cannot roam to find a mate, as animals can, so they have evolved some astonishing ways of transferring pollen from anther to stigma.
The anthers are blowing in the wind
Many flowers make use of the wind to carry their pollen to the carpels of other flowers. This can be something of a lottery; after all, once the pollen is carried aloft on the breeze, the plant has no control over where it will land. To improve its chances, it invests in numbers – large numbers. Grasses are wind-pollinated, and a single flower head of an average grass can produce ten million pollen grains! Any one of those only has a miniscule chance of landing on the stigma of one of is own kind, so while the pollen may be carried incredible distances, the majority of the grains tend to land within just a few metres of the plant. Therefore wind-pollinated plants usually grow closely together, to increase the likelihood of pollination.
Another adaptation to improve their chances of successful fertilisation is in the structures of the flowers themselves. The stamens of many wind-pollinated flowers stick out quite a distance from the flower, giving them plenty of exposure to the breeze, and the styles of grasses are often feathered, to help them ‘capture’ pollen grains from the air. Remarkably, some grasses have evolved to release pollen between around 5 am and 9 am, the time when morning breezes are strongest.
Many wind-pollinated trees, such as birch (Betula spp.) and hazel (Corylus avellana) have catkins, which dangle from the branch so that pollen is easily shaken loose in the wind. Interestingly, hazel catkins emerge before the leaves, allowing the pollen to travel further away from the parent without the obstruction of foliage.
Water and pollination
Pollination by water is much less common, but it does occur in some of the pondweeds (Potamogeton spp.). When pollen is transported on the surface of the water it has the advantage that it is only travelling in two, rather than three dimensions. This improves its chances of landing on the flowers, which are at the water’s surface.
In contrast, for some insect-pollinated flowers it is a distinct disadvantage to become flooded with water. The bell-shaped flowers of heathers such as ling (Calluna vulgaris) are adapted to help shed rain, and it is no coincidence that heathers tend to be most abundant in wet countries such as Scotland!
Insects and pollen
Using insects for pollination is a bit more of a targeted approach than wind-pollination. Nevertheless, flowers that rely on insects need to make an investment to ensure successful fertilisation. They have to advertise themselves, reward the insect, provide a suitable landing spot and, crucially, they must make sure that pollen is transferred onto the insect.
There are a huge number of insects that pollinate flowers. The most important ones are flies, beetles, moths and butterflies and particularly the order known as Hymenoptera, which includes bees. Flies and beetles are more usually seen on open flowers, such as hogweed (Heracleum sphondylium), while longer-tongued bees and butterflies are better adapted to, and relied upon by, deeper flowers including devil’s-bit scabious (Succisa pratensis).
Humans have always appreciated the fantastic visual display of the showier flowers, and these colours send out an advertisement to passing insects. But there is literally more to the hue of flowers than meets the eye. Bees see a higher part of the colour spectrum than we do, so some flowers that appear to be a uniform colour to our eyes actually have markings known as ‘honey-guides’, which are revealed when viewed under an ultraviolet lamp. Honey guides function like landing lights and help to direct the bee to just the right spot for gathering nectar. On certain flowers these guides are visible to human eyes. Speedwells (Veronica spp.) and forget-me-nots (Myosotis spp.) have a highlighted ring around the hole that contains the nectar, and foxgloves have dots leading up into the flower.
Flowers, of course, also use scent to attract insects, and these fragrances are tailored to attract a preferred pollinator. Bee flowers such as heather smell sweet and honey-like; moth-pollinated flowers, like honeysuckle (Lonicera periclymenum), are rich and heavy; and those that are pollinated by flies can be cloying and even slightly unpleasant; examples include hawthorn (Crataegus monogyna), which is carrion-like, and ivy (Hedera helix), which is also visited by wasps (Vespula vulgaris).
Insects need an incentive to visit flowers, so the plant produces nectar – a simple sugar solution – as a reward. It has to get the amount just right. If the flower provides too much nectar in one go, the would-be pollen transporter may become sated too soon and will fly away without visiting another flower (although heather has a trick up its sleeve: if it is not pollinated by bees, its stamens extend so it can resort to using the wind).
Pollen itself is also sometimes offered as the main reward, as happens in wood anemones (Anemone nemorosa). Bees eat both nectar and pollen and bumblebees have pollen baskets on their legs. These are small containers, fringed with hairs, that enable them to transport pollen back to their nests to feed to their larvae, clearly illustrating how intimately the evolution of flowers and insects are intertwined.
The flower must ensure that the insect picks up its pollen, so grains of insect-borne pollen usually have a rough or spiky surface, helping them to adhere to the pollinator. The position of the nectar and the anthers are key to making sure that the insect positions itself in the right place to pick up pollen.
Some flowers are quite open, and tend to attract a range of different insects. Others are more complex in their structure and will attract a specialised clientele. There are some astonishing mechanisms and varied structures used by flowers to transfer their precious load onto insects.
Orchids are perhaps the most sophisticated of all. They actually deposit small packets of pollen, which are glued to the back of the bee and aligned perfectly for them to be deposited on the stigma of the next orchid it visits.
There are pros and cons to developing such specialised relationships with specific pollinators. The advantages are that the pollen is more likely to be taken to the right flower with less pollen being wasted on visits to different species of plant. The drawback is that if for some reason the pollinator goes into decline, then the plant as a species will suffer, and vice versa. This demonstrates the often-fragile interdependence that exists between species. Indeed this scenario is apparent in many parts of the over-grazed Highlands, where high herbivore numbers not only inhibit tree regeneration, they also suppress ground flora, leading to a reduction in insect diversity.
In the tropics, certain birds and mammals are important pollinators of some flowers, but this is not the case in Britain. Even so, it is interesting that blue tits (Parus caeruleus) can sometimes be seen feeding on the male flowers of goat and grey willow (Salix caprea and S. cinerea). It is thought that they like feeding on the nectar and therefore may play a role in pollination.
It is well known that inbreeding is generally not a good thing as certain quirks or faults may become compounded, to the detriment of the overall fitness of the species. So how do flowers avoid self-pollination? Well the short answer is that they don’t always manage to avoid it, but they generally try hard to do so!
The flowers on a stem of rosebay willowherb (Epilobium angustifolium) or foxglove (Digitalis purpurea) emerge in succession, rather than all at the same time, thus there is less chance of insects visiting other flowers on the same plant. Sometimes the stamens and the stigma are positioned in a way that helps reduce the chances of pollination within the flower, while some species have separate male and female flowers (i.e. flowers containing only stamens or stigmas) on the same plant.
Others still are strictly separated, with individual plants being either male or female. Such plants are described as being ‘dioecious’, from Greek ‘di’ meaning two, and ‘oikos’ meaning home (which is also the root of the word ‘ecology’). However the price a plant pays for strictly avoiding self-pollination is the risk of not being pollinated at all. Aspen (Populus tremula) is a notable example of a dioecious tree. It rarely flowers, and the male and female plants are often so far apart from one another (as a result of forest fragmentation) that they have little chance of producing seed. To compensate, dioeceous species are usually good at vegetative propagation. This is very effective when the plant is well adapted to a particular spot, and a lot of aspen stands in the Highlands could well be many thousands of years old. Such a species may be at risk however if local conditions change, which is why aspen tends to flower when stressed.
In contrast, some plants produce flowers that are closed over so that they deliberately self-pollinate. As with vegetative propagation, the advantages of self-pollination are that if the plant is very well-adapted to a specific location, those genes are maintained. Plants that are annuals and readily colonise new areas frequently use self-pollination. This is because they can easily find themselves isolated with no chance of fertilising or being fertilised, so self-pollination is better than no pollination at all.
The outer coating of pollen is extremely tough, and can remain intact, buried in layers of peat for thousands of years. When looked at through a microscope, pollen grains vary hugely in shape and size. They are so distinctive that scientists can often identify which species of plant were present in an area at a given point in time. This can help to paint a picture of the history of our forests. While useful, this approach does have its limitations however. For example some species such as aspen don’t flower very frequently, so may be overlooked. This method also tends to favour wind-pollinated species such as birch, and is not so well-suited for detecting bird cherry for example, which is pollinated by insects.
Pollination and evolution
Pollination clearly demonstrates some key features of healthy, evolving ecosystems. By developing a range of specialised relationships with particular insects, plants can avoid competing with each other for pollinators. This avoidance of competition through diversity and specialism is known as ‘resource partitioning’.
The interactions between insect pollinators and flowers are a good example of what is known as a symbiotic relationship, in which the lives of two organisms are intimately intertwined. When they are linked in a win-win situation, as flowers and their pollinators are, it is called a mutualism. The insect wins food and the plant gets the chance to breed. Of course, once successfully pollinated, the seed develops and is then dispersed – but that’s another story.
Sources and further reading
- Baker, N. 2004. The New Amateur Naturalist. Collins: London.
- Chinery, M. (2005). Complete Guide to British Insects.. Collins: London.
- Fitter, A. (1987). New Generation Guide to the Wild Flowers of Britain and Northern Europe. Collins: Glasgow.
- Kay, Q. O. N.(1985). Nectar from willow catkins as a food source for Blue Tits, Bird Study, 32 (1), 40 – 44.
- www.snh.org.uk/pdfs/publications/geology/bennevisandglencoe.pdf (Accessed 2nd July 2010)
- Tipping, R. (1998) The application of palaeoecology to native woodland restoration: Carrifran as a case-study. In: Newton, A.C. & Ashmole, P. (Eds.) Native woodland restoration in southern Scotland: principles and practice. Occasional paper no. 2, Borders Forest Trust, Ancrum, Jedburgh. Cited in www.snh.org.uk/publications/on-line/heritagemanagement/nativewoodland/modelling.asp (Accessed 2nd July 2010)
- Tudge, C. (2006). The Secret Life of Trees: How They Live and Why They Matter. Penguin: London.