Friday 20 July 2018

Daisy rosettes indicate a survival advantage

Rosettes of daisy leaves are a common sight in lawns at this time of year
Rosettes of daisy leaves are a common sight in lawns at this time of year

Jim Hurley - Nature Trail

Daisies close their flowers at night and at dawn the white florets around the outside of the flower-head fold back to permit the yellow centre to turn and face the rising sun. The very common wild flower is, therefore, the 'day's eye', now spelled 'daisy' and pronounced 'day's-e' rather than 'day's-i'.

Daisies have a number of remarkable adaptations and the one that is the focus of this week's column is their rosettes. An adaptation is any process of change by which an organism or species becomes better suited to its environment. Via the process of evolution, nature selects adaptations that have survival value, like rosettes.

When a young Daisy begins to grow, its long leaves that are shaped like tablespoons do not grow upwards. Instead, they grow outwards, extending horizontally along the surface of the ground. They all grow out from a central point and overlap making a shape that looks like a rosette.

Daisies crowd together and it is a common sight to see a group of them growing together with all of the rosettes overlapping as shown in the image above.

New leaves grow out horizontally on top of older leaves as the rosette develops and becomes denser.

The advantage of this adaptation is, of course, that the Daisy's tightly-clustered leaves cover a saucer-shaped patch of soil, commandeering it for its exclusive personal use by covering and shading the ground thereby preventing seedling of other plants from germinating and growing.

The rosette adaptation conveys a survival advantage to Daisies, so nature selects the gene that causes it. As variation occurred and as genes mutated over the plants' long evolutionary history, nature fine-tuned the selection resulting in the Daisies we see today.

The rosette effect is achieved by the shortening of the length of stem between successive pairs of leaves causing the leaves to crowd on top of each other. The effect is so advantageous that lots of other unrelated plants have independently evolved the development of rosettes; dandelions, thistles, Ragwort, cabbages and lettuces are all good examples.

In desert succulents, the advantage of having rosettes is interpreted as a means of conserving water while in plants native to tall mountain ranges with sparse vegetation, the advantage appears to be sacrificial protection by the leaves of the roots from freezing during prolonged periods of snow and intense cold.

Our interpretations of nature, of millennia of evolution and of random natural selection never cease to amaze.

Wexford People

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