Smart plants or the magical interaction between flowers and pollinators

Close-up of bee on a yellow flower.

The interconnectedness of everything in nature is astounding. Plants have evolved side by side with their pollinators creating intricate and fascinating relationships that benefit plants and their pollinating partners. During a special tHRive HRI (Horticultural Research Institute) webinar Professor Robert Geneve from the University of Kentucky gave such incredible insight that one will never look at flowers in quite the same way again, writes Jean Vernon for FCI.

Professor Robert Geneve’s in-depth treasure trail through the weird and wonderful world of plants was mind-blowing, presenting plant after plant, unravelling their trickery, flower morphology and incredible techniques to effect cross-pollination.

Pollination basics

First, it’s important to understand the basics of pollination, i.e., the process of transferring pollen from the stamens (male) to the stigmatic surface (female); commonly, the ‘vehicle’ used to do this is an insect, or sometimes a bird.

Pollination can occur in two different ways. The most successful system is self-pollination, where a flower is pollinated with pollen within the same flower. Many of the pea family fall into this category. It’s successful but doesn’t generate much genetic diversity in the produced seed. And it doesn’t rely on a secondary pollination vehicle like an insect. Dicentra is another group of plants where the flowers never open and are self-pollinated.

The more ‘sought after’ system is cross-pollination. At the risk of anthropomorphising plants, they have evolved to ensure that they are less likely to self-pollinate. Their flowers are pollinated by pollen from a different flower, usually from a different plant. This means the offspring have a higher degree of genetic variability, which is important to ensure environmental fitness.
And that brings us to the wild and wonderful ways plants enforce or effect cross-pollination in preference to self-pollination.

Separation techniques

Some plants like holly (Ilex spp) have what are called dioecious plants. That means that they have female and male flowers on separate plants. The female holly flowers have stamen-like structures inside the flowers, but these are non-functional. So, the female flowers are always pollinated with pollen from a different holly plant, affecting cross-pollination.

Other plants like the chestnut (Castanea spp) are monoecious. This means that they have separate male and female flowers on the same plant. In the chestnut example, there are many more male flowers than female flowers on the tree, and the flowers are physically separated to reduce the chance of self-pollination.

But there are other more dramatic examples of this, like the toad flower (Tricyrtis hirta) – if you look closely at the flowers, you can see that the stigma and stamens are physically separated so that when the stamens, which hang down, shed pollen it doesn’t usually reach the stigmatic surface within the same flower. The female parts are above the male parts, so pollen is brought to them by a visiting pollinator. These are separations in terms of their space or position, but some plants control the timing of when they shed pollen or when their stigmas are receptive, and indeed others separate in terms of time and space.

Polymorphism

And then it gets even more fascinating. Some flowers have something called pollen presenters, which are additional flower parts, other than the anthers, that present pollen to the visiting pollinator. These virtual billboards, proclaiming a limitless buffet, are particularly attractive to the bees that collect protein-rich pollen for their larvae. If you look closely inside Dahlia flowers, you will see rings of pollen presenters above the floral centre. This also occurs in most Aster flowers too.

Almost any insect visiting for a nectar reward becomes an unsuspecting pollinator. As the insect lands onto the proffered landing pad and probes the central flowers for nectar rewards, they are dusted with pollen. It’s a form of polymorphism where the plant exhibits different mechanisms within the flower to affect pollination. It’s something that famous evolutionary biologist Charles Darwin noticed and studied in Primula flowers. He described his findings as heterostyly because the flowers, on closer inspection, had two different arrangements. ‘Pin’ – where the stigma was elongated and held at the throat of the flower above shorter anther filaments. And ‘Thrum’ – where the stigma was on a shorter stem with the anthers above on longer filaments.

Together they create an insurance package for the plant that ensures a good degree of cross-pollination. It’s an easy plant to examine and see such a highly evolved mechanism within the flowers.

Evolution

The incredible adaptations in flowers tracked the success of the rise of insects as pollinators through the Cretaceous period. Unlike animals and insects, plants are immobile; they don’t move to find their mate; instead, they generally rely on flying insects to transfer pollen, sometimes over long distances, from flower to flower.

In effect, the plants employ the insects, and the payment is nectar and some protein-rich pollen too. The result rewards the plant with pollination and the pollinator with an easy food source.
Plants literally filter out some pollinators with the shape and size of the flowers. It’s one of the most profound things to understand when choosing plants for pollinators. Different pollinators have varying lengths of tongues, proboscis, and mouthparts, and they can’t all access the nectar in the same flowers.

Some plants use adaptations in their flowers to attract specific types of pollinators. But even within a pollinator group, there is often a wide variation in the morphology of their mouthparts.

Floral adaptations

Just changing the shape of a flower can make a difference in how an insect interacts with it. If you consider the cone flowers (Echinacea spp), there are virtual dinner plates for many of our pollinating friends; the insect can alight on the landing pad and then walk around the cone probing each individual flower for its nectar. Many insects see in the UV spectrum, seeing a completely different petal colour, shape, and markings to what we see ourselves. Flowers adapted to be bee pollinated often have coloured nectar guides, marking the way to the nectary.

Butterflies aren’t good at hovering and prefer flowers where they can land and feed on many smaller flowers, so open dinner plate flowers like daisies or racemes of many flowers, as in Buddleia, are excellent butterfly plants. And the long tubular flowers like those of Aloes and red-hot pokers are more likely hummingbird pollinated or attract the longer-tongued moths that can hover and feed.

Some plants like Delphiniums and Aquilegia have nectar spurs behind the flowers that have evolved with long-tongued butterflies, moths, or hummingbirds as their pollinating partner.
Nectar glands are often at the base of long tubular flowers, so an insect or a bird must push past the anthers and stigma to get their sugar reward.

Nicotiana is a good example of a plant pollinated by long-tongued, night-active moths attracted by the evening fragrance.

Floral scents and fetid odours

Most of us associate flowers with sweet floral perfumes. Plants like Polianthes have a rich, sweet fragrance, which is a pollinator attractant.

But the opposite extreme exists too. Some plants, namely the carrion flower (Stapelia spp) and skunk cabbage (Lysichiton americanus), produce a fetid odour that attracts carrion flies or dung beetles. These creatures are fooled into thinking the red-tinged flowers are rotting meat or worse and lay their eggs nearby for the larvae to feed on the ‘flesh’. In the absence of a food source, the movement of the grubs affects pollination, rewarding the flower but not benefitting the duped pollinator at all. This is also referred to as brood site mimicry, where the flower resembles and smells putrid and appears to be a good place to lay eggs.

In addition to pollen and nectar, some pollinators visit flowers to collect resins. These have antibacterial and antifungal properties that help to waterproof and protect their nests. Others collect oils to attract a mate or to feed their larva.

Nectar thieves

Early spring can be a tough time for pollinators, so plants with easy-to-access nectar glands, like the hellebores, are very important for the early emerging bumblebee and solitary bee species. The nodding heads are virtual umbrellas offering open access for many species, and the flowers produce nectar generously.

Insects with shorter mouthparts that cannot reach the nectar deep inside tubular flowers chew holes above the nectaries to access the high-energy resource. This is quite common in spring when there is less diversity of floral forage for early emerging pollinators, like the shorter-tongued buff-tail and white-tail bumblebees.

Ants are also nectar thieves, stealing nectar without affecting pollination. Some plants, like Plumbago, have flowers that have evolved and adapted to restrict access by small crawling insects by using sticky trichomes to deter them.

Opening times

Other flowers have diverse ways that indicate to insects the position of the nectar when nectar is flowing and when pollen is ripe. Plants in the borage family have flowers that change from pink to blue following pollination, indicating which flowers are still rich with rewards and those where the food cupboard is bare. Other plants use different mechanisms for different insects, like the Rangoon creeper (Quisqualis indica), which has scented white flowers attracting the attention of evening flying hawk moths, and red flowers that hang down for day flying bees and flies. The flower of Texas (Lupinus texensis), the blue lupin, when flowering en masse, uses colour signals to indicate which flowers are the most lucrative in terms of food for pollinating insects to visit. A white-centred lupin flower is the freshest and offers the pollinators rich rewards, whereas a red centre flower is aged and already pollinated.

Horse chestnut (Aesculus hippocastanum) flowers use coloured nectar guides that change from yellow to orange as the flower ages. These techniques assist pollination and save the insects precious time and resources.

Guarded treasure

Other plants make pollen harder to access, holding onto the prized protein until a pollinator (usually a bee) uses a technique called buzz pollination to release the golden dust. The insect vibrates at a higher frequency by detaching its wing muscles and vibrating its body instead of its wings. Bumblebees do this on tomatoes, which is why commercial tomato growers purchase bumblebee nests to use in their glasshouse production.

Other flowers, like the snapdragon (Antirrhinum), have hinged flowers that specific insects can only access; this creates a selective process that means only particular pollinators can get inside the flowers to effect pollination, filtering out insects that might enter to eat and leave without paying (pollinating).

Bigger pollinators

Of course, it’s not just butterflies, moths and bees that are pollinators. Some plants rely on larger creatures like hummingbirds and even bats to pollinate their flowers and have adapted mechanisms to suit them. Flowers like the bird of paradise (Strelitzia regineae) and the Jade vine (Strongylodon macrobotrys) have a hinged mechanism that is triggered when the bird or bat (respectively) lands on the flower. A lever mechanism is triggered that pushes the anther-laden stamens into contact with the pollinator so that it acts as a vehicle to move the pollen from flower to flower.

Floral animation

Plants that move or have moving parts have long fascinated young and old. But in addition to the more commonly known Venus fly traps (Dionaea muscipula) and sensitive plant (Mimosa pudica), there are quite a few other examples that relate to the flowers themselves. For example, mountain laurel (Kalmia latifolia) flowers have spring-loaded stamens that jump out like an ambush when an insect lands. Once out, they drop pollen onto the visitor. Unlike the Venus flytraps and the sensitive plant that reset to pre-trigger stance, the Kalmia stamens can’t reset.

But there’s another common garden plant, barberry (Berberis spp), where the stamens react to insects, moving towards them when they land. It’s called stamen irritability and is a positive movement designed to rub pollen onto the pollinator. The stamens reset after about 20 minutes.

Deceit pollination

The diversity of pollination techniques is simply incredible. Some flowers deceive the insect into visiting and offer no reward at all. One-third of the 30 thousand orchid species employ some form of this pollination deceit strategy, and some have exploited this to a high degree.

A basic tactic is a plant with flowers that mimic another plant’s flower, offering a rich reward and suggesting food when there is none. The floral features that can be successfully imitated include the flower’s colour spectrum, shape, form and even scent. For example, the butterfly vine (Mascagnia macroptera) and Oncidium orchid flowers look similar to an insect, but only the Mascagnia offers a reward; the orchid still gets visited and pollinated by the duped pollinator.

In Begonia, the flowers use Bakerian mimicry, where the female flower mimics the male flower, but only the male flower offers a reward. The insect picks up the pollen from the male flowers and deposits it in the female flowers, which it visits to look for food.

Another technique uses pseudoanthers that look like anthers but are not. They attract the pollinators into the flower, but there is no reward. And then there is pseudo pollen, which can be seen in bearded irises; the iris beard is pseudo pollen and deceives the insects to visit the flower.

Another trick that some plants use is prey deceit. This can be seen in some orchids where the flowers resemble something else. So, the spider orchids have petals that are arranged in a way that resemble spiders and attract predatory wasps that are tricked into believing the flowers are prey. As they probe the flowers to lay their eggs, they inadvertently pollinate them.

But possibly the most bizarre and cruel adaptations are the sexual deceit tactics employed by flowers such as the bee orchids (Ophyrs spp), where the flowers are bee size, bee shape and really do look like a female bee. They attract the male bee, which attempts to mate with the flower affecting pollination. It is estimated that over 1,000 species of orchids employ sexual deceit.

Floral traps

Some flowers, like the rosary vine (Ceropegia ampliata), actually enclose the insect, think escape room scenario, and as it attempts to escape, it facilitates pollination.

Spathe (Spathiphyllum spp) flowers prevent beetles from exiting because the spathe is too slippery for them to grip. Flowers of Dutchman’s pipe (Aristolochia macrophylla) have an S-shaped floral tube where insects can become trapped. In arums, the female flowers are at the bottom in a restricted area, so when the insects become trapped, they drop pollen onto the female parts.

Slipper orchids (Paphiopedilum spp) can attract hoverflies with floral spots, bumps, hairs, colouration, or scents. Spots on the petals mimic aphids, so the female hoverfly lays her eggs nearby the perceived food source and falls into the trap, pollinating the flower.

The author, Jean Vernon, is an award-winning writer and author of the best-selling book The Secret Lives of Garden Bees. www.TheGreenJeanie.com

This article was first published in the June 2023 edition of FloraCulture International.