Flowers are the spectacular result of coevolution between plants and insects. When flowers evolved in plants around 130 million years ago, they took advantage of insects to help them pollinate each other, therefore promoting sexual reproduction and enhancing resilience to environmental changes and diseases.
The intricate dance between plants and their insect pollinators is one of nature’s most fascinating stories. Flowers evolved bright colors, enticing scents, and sweet nectar to attract insects, while insects, in turn, evolved specialized structures and behaviors to access these rewards and transfer pollen from flower to flower. This mutualistic relationship — plants get insects to transfer pollen between flowers and insects get rewarded with food— has driven an incredible diversification of both plants and insects, filling our world with an astounding array of forms and functions.
I was reminded of the complex biology of flower power during my recent trip to the the Canary Islands. It’s hard to visit here without noticing the distinctive cardón (short common name for cardonal-tabaibal or Euphorbia canariensis) that graces the hills of these volcanic islands. With its cactus-like look, this endemic plant, the symbol of Gran Canaria, appears perfectly adapted to arid conditions. It looks remarkably similar to cacti, such as the columnar cactuses you can see across the Americas.
But cardón ain’t no cactus. The Cactaceae family evolved 30 to 35 million years ago in the New World and are only found in Africa and its islands as intruders brought there by humans. No, cardón is a spurge, an Euphorbia, one of the most incredibly diverse and species-rich genus of flowering plants that counts about 2000 species. Botanists refer to it as one of the emblematic “big genera” of the plant world similar to Solanum, Ipomoea and others.
And cardón, like other Euphorbia species, produces no flowers. Well, at least not typical flowers as botanists define them. Their true flowers are highly reduced in size and buried in flower-like inflorescences called cyathia (sing. cyathium), which are perhaps even more sophisticated than true flowers. Essentially, Euphorbia reinvented flowers and built them from other plant parts.
Cyathia are a marvel of evolutionary innovation. Unlike true flowers, cyathia inflorescences mimic the appearance and function of a single flower. Each cyathium consists of a cup-like structure that houses several tiny, simplified flowers, including both male and female components, surrounded by nectar glands and often adorned with colorful bracts. This intricate design not only attracts insect pollinators but also ensures efficient pollination by concentrating reproductive structures in a compact form.
Check how cyathia mimic the petals of true flowers through bracts known as cyathophylls, which are in fact modified leaves that gives the inflorescence a remarkable flower-like appearance.
The cyathium’s sophisticated architecture highlights the remarkable adaptability of Euphorbia, allowing these plants to thrive in diverse environments while maintaining successful partnerships with their insect pollinators. Some authors have proposed that cyathia enabled the transition from wind to insect pollination, thus explaining the tremendous success of Euphorbia. However, other authors suggest that the capacity of certain Euphorbia species to repeatedly evolve Crassulacean Acid Metabolism (CAM) may have more to do with the genus’ species richness.
Back to cardón, in spring, it produces intricate reddish flowers, oops I mean cyathia, directly from its spiky stems. The cyathia tend to be arranged in groups of three perpendicularly along the ribs. Typically, the central cyathium in a group of three consists only of male (true) flowers, while those with both sexes have male flowers appearing after the female ones. As with other Euphorbia, the flowers sit on short stalks and are not very conspicuous.
These Euphorbia are remarkable at reinventing the wheel. They are prime examples of convergent evolution, not just with their false flowers and flower arrangement, but also with their cactus-like and succulent appearances. There are probably only a limited number of solutions for plants to survive in extremely arid environments, and cactus-like morphology is one of them. It’s as if you were to rewind the tape of life, play it again, and end up with the same story over and over.
According to James Horn and colleagues, there are about 850 species of Euphorbia with accentuated xeromorphic growth forms, meaning they have adaptive features typical of arid environments. This is an extreme case of parallel evolution as Horn et al. estimated 14 independent origins of xeromorphic growth forms in Euphorbia, which include traits such as the development of water-storage parenchyma, loss of leaves, photosynthetic stems, and gain of stem-borne stomata. I can’t wait to discover the molecular mechanisms behind convergent evolution towards xeromorphy within Euphorbia and between Euphorbia (Malpighiales, Euphorbiaceae) and their distantly related New World cacti (Caryophyllales, Cactaceae). With the first genome sequences of Euphorbia species finally available, we may soon start uncovering the molecular mechanisms of the many convergent adaptations these species have evolved.
So yes, we can imagine a world without colorful flowers. But that world will still be full of other types of flowers, just like the cyathia of cardón and the many other Euphorbia. The plant-insect partnership wouldn’t have it any other way.
Epilogue
I would like to thank my plant biologist colleagues who work on flowers and who inspired this post.
Acknowledgements
I drew inspiration from many sources, particularly the colleagues listed above. François Parcy’s book elegantly raised the question of how drab a world without flowers would be. I also thank Sebastian Schornack for feedback. The article was written with assistance from ChatGPT.
This article is available on a CC-BY license via Zenodo.
Cite as: Kamoun, S. (2024) Imagine a world without flowers. Zenodo. https://doi.org/10.5281/zenodo.12684896
