Why are many wild flowers so pretty to look at? Plants are certainly not investing energy in looking attractive for our benefit. Just as plants have to be able to cope with whatever the environment throws at them, because they cannot move away, so too they need elaborate mechanisms to promote the sexual reproduction which is so important in generating genetic variability – they can’t go chasing after a mate, after all!
Plants come with all sorts of arrangements to allow sexual reproduction; some have separate male and female plants (dioecious), some have separate male and female flowers on the same plant (monoecious) and others have hermaphrodite flowers, with male and female parts within a single flower. Hermaphrodite flowers often have a self-incompatibility mechanism to discourage self-fertilisation. All these flower types, however, are made up of the same basic male and female components. The male part of the flower produces pollen grains in structures known as anthers, held on stalks known as stamens. The female part has a stigma to catch pollen grains. When the pollen grains germinate on the stigma they produce a tube which grows down through the supporting style to the ovary, where the ovules are fertilised. These fertilised ovules will eventually develop into seeds.
Crocus flower with feathery stigma and anthers which have split to reveal pollen grains on their surface
For cross-pollination, plants have to rely on some sort of vector to convey the pollen from one flower to another – this may be the wind, or some sort of insect or bird. If an insect or bird, the vector has to be both attracted repeatedly to the same species and able to transfer pollen between flowers to be a reliable pollinator. Often this pollen transfer is accidental, i.e. the pollinator brushes against the anthers in one flower and the stigma in the next. To attract a pollinating insect or bird the plant may provide a reward – often nectar – and will advertise this by the shape and patterning of its flowers. Other plants provide excess pollen which can be used as a foodstuff by the pollinator or provide shelter or a place to raise and feed young. And then there are the tricksters – plants which pretend to be offering a reward but are doing no such thing. Maybe the best known of these are the bee orchids, where an apparently-receptive female mate awaits the unsuspecting male…
Bee orchid (Ophrys melifera) in Crowtrees local nature reserve, Durham
The pollinia dangling over the back of the ‘female bee’ deposit pollen on the back of an unsuspecting male, which he will then deposit on the stigma of another flower when his efforts to mate with this one prove frustrating!
The match between a plant and the way in which it is pollinated is called a pollination syndrome – more on this to come. Flowers have traits which help them attract and/or accommodate particular types of pollen carriers, whilst insects and other animals have traits which allow them to exploit flowers with those particular traits, often obtaining some sort of reward for carrying pollen from one plant to another. Different plant families tend to be pollinated by different types of vector and so have characteristic flower morphologies to match.
The simplest way for pollen to get from one plant to another is, arguably, to be blown there on the wind. This has the virtue of needing no help from a particular animal and no need to provide a nectar reward to attract such a pollinator but wind pollination is rather a hit-and-miss business so plants have to produce very large amounts of pollen, held in a way that it can be easily dispersed by the wind. Such plants tend to hold their flowers up high to ensure unimpeded air flow – male catkins on trees such as willow and hornbeam are obvious examples. Grasses are another.
Immature male goat willow (Salix caprea) catkin before anthers emerge and release their pollen
Male hornbeam (Carpinus betulus) catkin
The flowers of wind pollinated plants tend to be relatively small and inconspicuous, often greenish in colour. There is no point in the plant investing energy in producing something showy.
In contrast, many plants have to compete to attract their pollinators, especially where these are in short supply. There is a biological trade-off here – is it good for the plant to be a generalist, assuming that several different types of insect, for example, can transfer pollen between its flowers? The risk associated with this is that the insects will probably visit a range of plant species and so may deposit a plant’s valuable pollen on the stigma of a plant of a different species, where it will be wasted. The other strategy involves co-evolution of plant and pollinator so that they are a good match for one another. This way, any pollen taken from a plant is likely to be deposited on another of the same species. This time the risk for the plant is that there may not be sufficient numbers of the correct pollinator around to ensure good rates of pollination.
One thing we plan to investigate in Kashmir and Ladakh this summer is how plants are pollinated at different altitudes. I mentioned in ‘A case study in evolution – Podophyllum hexandrum’ that pollinating insects are in short supply at higher altitudes, forcing some plants to rely on self-pollination. It will be interesting to see whether some types of pollinators are more prevalent than others and whether this has affected the types of flowers which are found.