You open your eyes to bright, hot, unforgiving sun beating down on you. As you try to turn away, you realise that the lower half of your body is buried in the ground. You are stuck, unable to move from your place, but for some reason you don’t seem to care. The only thing on your mind: sex.

For context, this is what I imagine it’s like to be a plant. It is an intriguing thought exercise, even just to get a sense of how different their lives are from us. From an evolutionary perspective, it is an incredibly interesting problem to be rooted to the ground and unable to move around. This is particularly pertinent to the endeavour of producing progeny, for which the plants must somehow get to others of their kind. Over the hundreds of millions of years that plants have been around, nature has come up with some fascinatingly creative solutions to this challenge. 

Before we go any further, a quick crash course on sexual reproduction – many plants, most animals, and of course… people, need two special cells called gametes (one from the dad and one from the mum) to fuse in order to create an offspring. Pollen, the same stuff that causes your seasonal allergies, is the male gamete in the case of many plants and needs to make its way to the female gamete somewhere on another plant. The problem is that when you are stuck where you are, it is hard to get your gametes anywhere, let alone onto a specific part of another plant that also has to be of the same species as you. 

Older types of plants solved the problem of gamete transfer by harnessing the wind and the waterways. Mosses, cycads, and ginkgo (which is abundant around campus!) actually produce gametes that can move on their own, teeny little cells propelled by even teenier tails or hairs. Imagine a plant cell, swimming between plants through puddles and raindrops. This mode of movement only really works when individuals are growing close together and in areas with a lot of water. Many conifers like pine or spruce produce a large amount of extremely light pollen that can get picked up by the wind until maybe, just maybe, a couple of them happen to land on another plant. A game of chance sure, but such is the pursuit of love.

The true innovation in plant reproduction came about in the flowering plants, aka angiosperms. Through evolutionary trial and error, they figured out how to hire animals that are able to move to carry their pollen for them, sort of like an express delivery service… for your sperm? In the millions of years since this breakthrough, plants have entered into all sorts of partnerships with bees, bats, birds, beetles, butterflies, and more, to help with pollination. This key evolutionary advantage has allowed them to make less pollen and colonise new areas (because there is a higher probability their pollen will end up where it needs to go). It made it so the plants no longer had to put their faith in wind, water, and chance to reproduce, but could instead invest in structures that would attract and coerce animals into moving their gametes more reliably. Unfortunately, even in the plant world, everything comes at a price. In exchange for their assistance, plants often provide their pollinator partners with nectar, a sugar-rich food source, or even oils and fragrances. To let their potential visitors know that they are open for business, they advertise their presence with brightly coloured flowers and strong smells.

Different plant species specialise in attracting specific types of pollinators, evolving traits that are best suited to their partners. Plants pollinated by birds may provide spots for them to perch while interacting with the flowers. Flowers pollinated by bats or moths might only open at night time. Some flowers even have these little hidden markings, called nectar guides, that are only visible to bees (who can see UV light). These act as arrows or landing strips, guiding the visitors to the nectar which is hidden away deep inside the flower. 

Of course, the animals aren’t visiting these flowers with the intention of moving pollen between plants, they are just there for the meal. When they stuff their faces into the flowers to get to the nectar, the pollen simply brushes up against them and sticks on for the ride. When they visit another flower, it might rub off on its stigma (the pollen receiver) and successfully transfer. (Sidenote: bees actually do intentionally collect pollen in special pouches called corbiculae, but they don’t mean to take it over to the next flower. Rather, they mean to bring it back to their hives and feed it to their babies!) 

Some plants get sneaky and skip the whole nectar-making thing altogether. Instead they entice and entrap their animal “partners” by posing as potential mates! This is called deceptive pollination and is common in many orchids pollinated by bees or wasps. Ready with their best pick-up lines, male insects fly over to these flowers that look and smell like females of their species. Then they tragically try to mate with these deceptions, only to find that the floral female is a fake, a facade. This strategy of catfishing your pollinator works because the male insects are hopeless romantics and are soon ready to be hurt again, chasing down another botanical beloved, and in the process, transferring pollen between the plants. 

Specialization and co-evolution has often been used to explain the ridiculous diversity of flower shapes, sizes, scents, and colours. The true tragedy is that with specialization, comes dependency. If a plant loses its main pollinator partner, it may have a harder time reproducing and eventually face population decline. With pollution, disease, and human activity putting many pollinators under stress, plants too may be threatened. Sometimes, the plant may be able to rely more on other pollinators that visit it, or subsist merely on self-pollination. But this isn’t always the case and if they can’t evolve, they die. As well, pollinators that can’t get to their plant partners may starve without that source of nutrition. They too are dependent on this relationship to survive and thrive. In this way, plants and their pollinator partners are inextricably linked, a tale as tragic as it is romantic. Without one, the other might not survive, and in one’s success, the other finds solace. Just as Cupid, the Roman god of love, fell for his human target Psyche, the pollinators, mediators of romance in the plant kingdom, are dependent on their floral partners.

Citations

1. Stephens, R. E., Gallagher, R. V., Dun, L., Cornwell, W. & Sauquet, H. Insect pollination for most of angiosperm evolutionary history. New Phytologist 240, 880–891 (2023).
2. Renzaglia, K., Lopez, R. & Schmitt, S. Scanning Electron Microscopy of Motile Male Gametes of Land Plants. BIO-PROTOCOL 7, (2017).
3. Cappellari, S. C., Schaefer, H. & Davis, C. C. Evolution: Pollen or Pollinators — Which Came First? Current Biology 23, R316–R318 (2013).
4. The Bee Movie (2007).