An unusual arrangement of leaves in a 407-million-year-old fossilized plant is complicating scientists’ understanding of plant evolution.
Most land plants living today have spiral patterns involving the famous Fibonacci sequence of numbers. Because the spirals are so common, scientists have thought that the patterns must have evolved in some of the earliest land plants. But the leaves of the ancient plant, a member of one of the first plant groups known to have developed leaves, were arranged in spirals that can’t be described by Fibonacci numbers, researchers report in the June 16 Science.
The study “helps us to understand how [the] diversity of plants has been generated,” says botanist Barbara Ambrose, the director of laboratory research at the New York Botanical Garden in New York City, who was not involved in the research.
In the Fibonacci sequence, each number is the sum of the two previous ones: 1, 1, 2, 3, 5, 8, 13 and so on. Examples of spirals in plants that involve Fibonacci numbers can be seen in the arrangement of the leaves of some succulents, the bracts of a pinecone and the seeds of a sunflower, among many other plants (SN: 8/27/02).
In plants with spiraling patterns of leaves, all the leaves can be described by a set of curved lines that spiral clockwise out from the center as well as by a set of curved lines that spiral counterclockwise. If the numbers of clockwise and counterclockwise curves are both numbers found in the Fibonacci sequence, it’s known as Fibonacci spiraling.
Scientists aren’t sure why most modern plants adopt Fibonacci spiraling, but it might help maximize the amount of space between leaves or other plant parts (SN: 7/21/07). The patterns could also arise from the distribution of auxins, a type of plant growth hormone.
In the new study, Sandy Hetherington, a paleobotanist at the University of Edinburgh, and colleagues studied fossils of the extinct Asteroxylon mackiei, a member of a…
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