A small fraction of the huge diversity of flower heads within the daisy family
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Effeuillons la marguerite (let’s pluck the daisy)

Scientists from Kew’s Comparative Plant and Fungal Biology department discuss the fascinating structure and diversity of daisy flower heads, with the help of an old, ‘romantic’ game.

Blog team: 
Author: 
Oriane Hidalgo, Benjamin Coquillas & Luca Pegoraro

 

An innocent game becomes a source of scientific data

“He/she loves me… he/she loves me not”. Who hasn't spoken these words while picking the 'petals' of a daisy? This popular game of French origin (Figure 1), where the phrase stated with the last 'petal' supposedly indicates whether the object of affection returns their affection or not, has been hijacked and adapted by Kew scientists to generate data for research.


Figure 1: 'La Simplicité (Simplicity)', oil on canvas painting by Jean-Baptiste Greuze, 1759, Kimbell Art Museum, showing a lady playing “He loves me… he loves me not”.

Indeed, the method we have developed to characterise the morphological diversity of the flower heads of daisies, involves very carefully and patiently detaching each part of the flower, one by one. Of course, no incantation is used here when removing each 'petal', but instead – in a far more laborious way – each part is preserved and glued on a piece of paper to make a dissection sheet, showing all the component parts of each flower head (Figure 2).


Figure 2: Flower head of an oxeye daisy, Leucanthemum vulgare. L>R: lower & upper views of flower head, centre of the flower head, & the complete dissection of the flower head showing 69 individual flowers & involucral bracts.

Over the summer months, Kew scientists have 'effeuillé la marguerite' (been plucking the daisy) for representatives of the daisy family cultivated here in Kew’s living collections and from field collections. By using this novel approach, the huge complexity and diversity of parts which make up the flower head of different species is being revealed for the first time.

Daisies: more complex than they first appear

In fact, a daisy is not a flower but a cluster of many flowers (florets) arranged in an extremely condensed inflorescence that mimics a single flower. Such an arrangement is called a capitulum and it is found throughout the flowering plant family referred to either as ‘Asteraceae’ (based on the name of the nomenclatural type of the family, Aster), or the more traditional name ‘Compositae’ which refers to the composite nature of their flower head (both are equally acceptable names according to the Botanical Code of Nomenclature).

What can we see when we take a closer look at the capitulum of, for example, the oxeye daisy (Figure 2)? The outermost parts which are green, are called the bracts, which together are known as the involucre. They primarily function to protect the capitulum bud. Then, the first row of florets is made of ray florets with white petals fused in a tongue-like structure. They are specialised in pollinator attraction. Finally, at the centre of the capitulum, we find the disc florets, which are yellow. They resemble five point stars when fully opened. They assume most of the reproductive function of the flower. In total, this dissected oxeye daisy is actually made of 69 flowers (11 ray and 58 disc florets)!

Why are we less likely to find daisies with 10, 15 or 20 ray florets?

Let’s go back to the daisy plucking game; originally, the game included a range of increasing love intensities “Il/elle m’aime un peu, beaucoup, passionnément, à la folie, pas du tout!” (He/she loves me a little, a lot, passionately, madly, not at all). However, it is fairly uncommon to end with "not at all” (achieved with 5, 10, 15 or 20 ray florets) when plucking a daisy, and this has little to do with luck or love.

The explanation lies in the famous number sequence known as the Fibonacci series (as it was identified by Fibonacci, an Italian mathematician of the 12th century). The sequence comprises: 1, 2, 3, 5, 8, 13, 21, 34, 55 etc. and is built by adding the two preceding numbers together. It appears in many biological settings such as tree branching, leaf arrangements, uncurling ferns, and – fortunately for anxious lovers – in the number of daisy florets (e.g. Owens et al., 2016). Five and 55 are the only numbers in the Fibonacci sequence corresponding to “not at all”, however, the daisies used for the game usually have more than five and fewer than 55 ray florets.

Exploring the extraordinary diversity of daisy flower heads

Why are we studying daisy flower heads?

The daisy family (Asteraceae) comprises around 24,000 species distributed worldwide except in Antarctica, making it perhaps the most successful family of flowering plants in terms of species diversification (Funk, 2009). Asteraceae are highly visible in plant ecosystems and particularly well represented in threatened habitats such as grasslands and high altitude habitats. Furthermore, they are economically important, including many horticultural plants (e.g. chrysanthemum, gerbera) and crops (e.g. artichoke, lettuce, stevia, sunflower).

The capitulum is the most notable characteristic of the Asteraceae. Despite being a stable structure that is found consistently in the family, it shows an impressive diversity of forms throughout the family. Each part that constitutes the capitulum (the involucral bracts, ray florets and disc florets) dramatically varies in number, shape, size and colour across species. There are capitula with ray and disc florets (e.g. Achillea, Mutisia and Pallenis), some with exclusively disc florets (e.g. Cotula, with more than 300 florets in a 9 mm diameter head!) and others with only ray florets (e.g. Launaea). Sometimes, as in everlasting daisies, the involucral bracts are transformed into petal-like structures, which retain their colour for some time after the capitulum dries (e.g. Schoenia). 


Figure 3: A small fraction of the huge diversity of flower heads within the daisy family. A. Achillea alpina; B. Pallenis maritima; C. Mutisia decurrens; D. Cotula fallax; E. Launaea cervicornis and F. Schoenia cassiniana.

While it is widely admitted that the capitulum has greatly contributed to the success of Asteraceae, little is known about the biological and evolutionary significance of its diversification in the family. This is what we aim to explore in our project. Taking the first step, we are delineating the landscape of the morphological variation of capitulum across the family, so… let’s pluck the daisy!

– Oriane, Benjamin & Luca –

Oriane Hidalgo, Marie Skłodowska-Curie Fellow in Comparative Plant and Fungal Biology; summer student Benjamin Coquillas from the Ecole Normale Supérieure ENSEIRB MATMECA (Bordeaux, France); and Luca Pegoraro, PhD student in Comparative Plant and Fungal Biology.


Acknowledgements

We would like to thank Begoña Aguirre-Hudson, Peter Day, Laura Green and Sarah Phillips, for help with the equipment and protocols; Joanne Everson, John Sitch and Kit Strange for help with the living collections; Maarten Christenhusz for providing the material of Schoenia, and colleagues and collaborators working on Asteraceae for fruitful discussion.


References

Funk, V. A. (Ed.). (2009). Systematics, evolution, and biogeography of Compositae. International Association for Plant Taxonomy.

Owens, A., Cieslak, M., Hart, J., Classen-Bockhoff, R., & Prusinkiewicz, P. (2016). Modeling dense inflorescences. ACM Transactions on Graphics (TOG), 35(4), 136. Available online