What makes a flower attractive?

Spring plant walks are full of surprises! On a recent plant walk with a friend, we were passing by a shrub when the overwhelming sweet smell of honey and the sound of chaotic buzzing made us stop in our tracks. On closer inspection, we found that the tiny yellowish flowers of this Mediterranean Buckthorn (Rhamnus alaternus) were starting to blossom and a wide array of insects were busy pollinating the plant.

In the previous blog post "What links flowers, bees and electric fields?", we briefly saw the different parts of a plant's reproductive system. However, what attracts the insects necessary for pollination?

Floral scent is composed of volatile organic compounds. In addition to attracting and guiding pollinators, these compounds can also act as a defense mechanism and protect the plant against abiotic stress such as extreme temperatures or drought[1].

Earlier on the same plant walk, the bright colour and symmetry of several flowers of the Coronilla species had attracted our attention. Carotene pigments are usually responsible for orange, yellow and red colours. Anthocyanin pigments produce purple, red, magneta and blue colours. Pigments absorb and reflect different wave lengths of light to produce colours. It appears that different colours attract different pollinators.

Flowers present two main types of symmetry: zygomorphic (bilateral symmetry) and actinomorphic (radial symmetry).

Bilateral symmetry is characteristic of most flowers in the Lamiaceae family, also known as the "Mint" family, which includes many aromatic plants such as Rosemary (Salva rosmarinus) shown on the right, Thyme (Thymus sp.), Basil (Ocimum sp.), Lavander (Lavandula sp.) and Sage (Salvia officinalis). The size of the floral parts is not equal and so they are known as irregular flowers. Flowers in the Rosaceae (Rose) family, on the other hand, present regular flowers as the floral parts are all equal. Blackthorn or Sloe (Prunus spinosa) shown on the left, Plums (Prunus sp.), Apples (Malus sp.), Strawberries (Fragaria sp.) and Hawthorn (Craetaegus sp.) are some examples of plants in this family.

Radial symmetry allows a diverse number of pollinators to access from all directions, whereas bilateral symmetry appears to favour a more limited number of pollinators[2].

Here are two more examples showing the difference between bilateral and radial symmetry. Do you recognise the flowers of these two common food plants? (Answer at the bottom of the post)

Looking at flowers has been proven to bring a variety of mood-enhancing benefits. These include physiological calming, decreased blood pressure and heart rate, as well increasing feelings of compassion and energy[3]. So, why not step outside and discover some of spring's surprises for yourself?

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For more information on flowers and their uses, see the post "Three flowers to help repair damaged skin".

Réponses to the quiz: Vicia faba (Broad bean) + Fragaria sp. (Strawberry)

[1] Knudsen, Jette T.; Eriksson, Roger; Gershenzon, Jonathan; Ståhl, Bertil (March 2006). "Diversity and Distribution of Floral Scent". The Botanical Review. 72 (1): 1–120.

[2] Jiang Y, Moubayidin L. Floral symmetry: the geometry of plant reproduction. Emerg Top Life Sci. 2022 Sep 9;6(3):259-269.

[3] Urakami, J.; Huss, E.; Nagamine, M.; Czamanski-Cohen, J.; Zaccai, M. The Emotional Experience of Flowers: Zoomed In, Zoomed Out and Painted. Horticulturae 2022, 8, 668.