Why do flowers grow like so?
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Mary, Mary, quite contrary, of nursery rhyme renown, knew 'how' her garden would grow. But trust scientists to ask about the 'why' behind "silver bells, cockle shells and pretty maids all in a row."

In fact, flowering plants pose a riddle with a complex answer that speaks to the nature of evolution, says a biology team led by computer scientist Przemyslaw Prusinkiewicz of Canada's University of Calgary. The larger question is: Why do plants and animals only come in certain shapes — at most four limbs for critters with a backbone or snails having a single shell that spirals outward — when the possibilities are endless? Why not an eight-legged horse or a pretzel-shaped shell?

A bit more seriously, "organisms display great diversity in shape and architecture, but the range of observed forms represents only a small fraction of what is theoretically possible," the team writes in the study published in the journal Science.

Natural selection — the crucial part of Charles Darwin's explanation of evolution that says in essence that survival-enhancing traits are passed on to future generations — seems to provide part of the answer. An eight-legged chicken may not be any better at outrunning predators than a two-legged one and will have to eat more to grow those extra legs. It's less fit, so sadly we see no super-duper drumstick deals at fried chicken outlets.

"However it is likely that developmental and genetic mechanisms also play a role" in these limited number of animal and plant shapes, the study says, pointing to evolutionary developmental biology or "evo-devo". Popularized by the University of Wisconsin's Sean Carroll, evo devo extends, and in some cases challenges, old-fashioned natural selection by showing how genes themselves, and the process by which creatures grow to maturity, are an added factor in the evolution of species.

Which brings us back to flowers. Such limited shapes can be seen in their growing tips of flowers, which only come in three flavors:

— Branches all ending in flowers (called panicles), for example in flowering Rowan trees.

— Branches lined with flowers on all sides (called racemes), such as Snapdragons

— Branches tipped with flowers that have flowers coming off one side (called cyemes), such as Forget-Me-Nots

"Why do we find these three main architectural types and not more or fewer?," the team asks in the study.

In the tropics, natural selection might favor panicles, the team argues, because the growing season lasts a long time and pollinators are always buzzing around. That means that waiting until the last possible moment of full maturity to pop open all your flowers, which after all are reproductive organs, presents better odds of reproducing. Away from the tropics, where cold snaps and less reliable growing seasons are a factor, the other designs might do better as the plants pop open their flowers in sequences, not all at once.

And in fact, a survey of a database of flower types finds that these trends broadly hold true, the study shows. But it doesn't explain why plant tips fall into just these three possible patterns when other arrangements could be possible that satisfy natural selection. And it doesn't explain why plants don't sport both raceme and cyeme patterns at once if they are both successful patterns.

Instead, the team proposes that the genes that control how plant tips grow, a process called inflorescence, limit the options for flower architecture. The team calls this constraint an "evolutionary wormhole" that prevents new shapes from appearing or established shapes popping up, willy-nilly, in families of plants. More formally, "movement along evolutionary wormholes that connect racemes and cymes appears limited, as individual (flower species families) seldom include both architectures," they conclude.

Looking at the humble mustard plant, Arabidopsis, the flower workhorse of the test-tube world, the University of Calgary biologists find that just two genes (one called LeaFY and the other Terminal FLower 1) control and limit how plant tips produce branches and flowers, for example. Reversing the amount of the two genes in the plants determines whether you produce racemes or cyemes (you have both options with the accommodating mustard plant). "Thus, evolution of inflorescences is constrained by the nature of the developmental genetic mechanism, as well as by the interaction between organism and environment."

So there you have an explanation for why "silver bells, and cockle shells and pretty maids all in a row" show up in Mary, Mary's garden. Flowers "cannot evolve freely, but are restricted to following paths of high fitness or evolutionary wormholes," the team figures. It's not quite as simple as a nursery rhyme, but it does provide a deeper answer to one of biology's big questions.

Each week, USA TODAY's Dan Vergano combs scholarly journals to present the Science Snapshot, a brief summary of some of the latest findings in scientific research. For past articles, visit this index page.

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A variety of flowers in bloom grace the Augusta National Golf Club during the 2007 Masters golf tournament in Augusta, Ga. Why do plants and animals only come in certain shapes when the possibilities are endless?
By Morry Gash, AP