The Science of Botanical Art

Reduction and Fusion - Flowering Evolution 

By Dick Rauh 

Originally appeared in The Botanical Artist, Volume 13, Issue 1

 

As flowering plants have evolved there have been two major trends, reduction and fusion that characterize floral development. Contrary to an inherent sense that as things progress they become more complex, the thrust of flower evolution appears to reverse this pattern. If we look at the history of land plants, the very simple morphology of the earliest examples slowly gave rise to more complex structures as the need arose. Roots appear to replace the primitive rhizoids, the simple enations (outgrowths) of the Rhyniophytes become simple leaves that gradually evolve to more complex leaf structures, a single spore case at the apex of a stem multiplies, groups, forming cones until we reach the complicated forms of the immediate forebears of flowering plants. 

Recent research points to the water lilies as one of these immediate predecessors. Check out their flowers. The blooms have many undifferentiated units, spirally arranged sepals to petals to stamens surrounding a complicated gynoecium. It is as if the gradual progression of evolving land plants reached a crest of complexity and is slowly reversing the process. 

Economy begins to play a part, as plants discover fewer units are able to do the job of many, with much less expense of energy. Compare the more primitive flowers of a Star Magnolia with that of a Euphorbia or an Arum. In the former are many undifferentiated tepals, many laminar stamens, many separate pistils. These are reduced to a single stamen in the male flower, and a single pistil in the female in Euphorbias. You can’t get more reduced than that and still have a flower that performs it’s reproductive function. 

This didn’t happen overnight. Eons pass as the trend towards fewer and easily countable parts becomes apparent. This trend happening in the natural world rarely follows the neat progression we might expect. The path to reduction is often interrupted, or in some cases has evolved to a point and reverses to show characters that are considered more primitive. 

The rose family, fairly along on the evolutionary tree, shows the primitive character of many, many stamens. In the genus Rosa the gynoecium too has reverted to the apocarpous formulation of earlier plants. Whatever works. 

I often wonder if the ability of the early hybridists to increase the number of rose petals from the species countable five might not be the result of a genetic tendency toward multiples that is manifest in the stamens and pistils. A flower is the agent of pollination. If the pollinator requires less, why produce more? In those flowers still pollinated by wind the perianth is absent, the wind needing no flashy colors or delectable scent to do its work. 

The other pattern is fusion. Separate parts join together to form for example, a calyx cup, a corolla tube, a compound pistil, or an inferior ovary. If one thinks about this it really seems a logical sequence for a fragile and transient organ to follow. Flowers, with their ovaries are one of the last major steps on the evolutionary time-line and one that has proved to be a major advantage . In terms of speciation the angiosperms are hundreds of thousand times more successful than the gymnosperms whose seeds lie naked and unprotected on the scales of their cones. The flower’s fused, enclosing ovary is the first step in this history. 

More than one carpel fused into a unit is even safer, and we have endless variations on this theme. An ovary fused into the tissue of the receptacle provides even more protection, so we believe an inferior ovary is a later development on the evolutionary scale than the superior. 

Sepals that are fused into a cup are persistent in the mints and protect the precious fruit inside. The fused calyx of the mallows adds bracts to enhance the flower’s defense against insect predation, the sly bee that has found a way to the nectar without performing its vital pollination function. The corolla tube of the lobelia that provides added strength to withstand the probing of the hummingbirds beak is another example of this joining of the parts of one series together. This condition is called connation, and it exists because it provides the plant with meaningful gain. 

In an orchid the stamen is fused on to the style, forming a column, and the pollen itself is fused into little packets called pollinia, that prove to be an effective agent in this very successful genus. The filaments of many flowers are fused to the corolla tube rather than arising directly from the receptacle, often to facilitate the positioning of the anthers. In Salvia the filament fused to the corolla acts as a fulcrum for the pollen sacs on an elongated connective that rocks like a see-saw when the pollinator moves in. When the parts of one series are fused to another, as in this case, we call it adnation. 

Adnation, connation, compound ovaries, filaments that are epipetalous (attached to the petal), are all examples of fusion. Fusion and reduction, two trends that have brought to some genera advantages that have allowed them to flourish. There are enough examples of these developments to have made scientists regard them as important growth patterns.

  • Magnolia Maxime Flora, Watercolor, G.D. Ehret