Symmetry

Most animals seem to be attuned to recognizing another individual’s symmetry as a means of judging its fitness as a potential mate. Among most human cultures, both men and women will usually choose the most bilaterally (equal halves) symmetrical face as the most beautiful. Thus, it’s no surprise that we humans prize the beauty of a highly symmetrical flower as well:  e.g., orchids, with their perfect bilateral symmetry, or dahlias, with their fascinating radial symmetry.

dahlia

dahlia

While plant breeders may have selected for the perfect radial symmetry of a dahlia, which we love to show off in our gardens, other forces were at work in the production of the spiral pattern of the seeds in a sunflower head, or disc flowers of the Black-eyed Susan, Purple Coneflower, and Sunflowers.

bumblebee-on-sunflower

Only one ring of yellow disc flowers are open for pollination at any one time.

Only one ring of yellow disc flowers are open for pollination at any one time.

purple coneflower

The arrangement of the emerging disc flowers is positioned so as to maximize their number in the available space.  When fertilized by pollinators, this will produce the maximum number of seeds per flower head — i.e., maximizing the fitness of that particular plant in its ability to pass on its genes.  To achieve this packing density, the disc flowers must lie at an angle of exactly 137.5 degrees from its neighbor, resulting in a series of spirals, rather than distinct horizontal or vertical rows. The same pattern is established in the growth of basal leaves of some plants (e.g., agave) and flower petals.

Chrysanthemum petals emerge in a spiral pattern, which reduces the overlap of petals in adjacent layers.

Chrysanthemum petals emerge in a spiral pattern, which reduces the overlap of petals in adjacent layers.

The spiral pattern is laid out with mathematical precision  — nature’s symmetrical design.  You can read more about the mathematical basis of the design in an earlier post.

What’s wrong with the coneflowers?

Have you seen flowers like this in your garden?

Instead of making flowers, the coneflowers are making tufts of green leaves.  Their flower-making potential has been damaged by a mite that infests the base of the flowerhead.

In some of the plants, the leaf tufts lie on top of the normal flowerhead, and in others, the flower head has been completely reprogrammed to produced just leaves.  It seems that the mite has completely modified the genetic program.

Fortunately ,the mite infestation has only occured in a small proportion of the flowers in the garden this year, but once the mite appears, you need to be careful to remove infested flowerheads and old vegetation from infected plants in order to reduce the mites’ impact next year.  See this article from the OSU extension program about the mite problem.