Leaves and sporangia developed in rare non-Fibonacci spirals in early leafy plants

Lateral plant organs, including leaves and reproductive structures, are arranged on stems in distinct patterns termed phyllotaxis. Most extant plants exhibit phyllotactic patterns that are mathematically described by the Fibonacci series. However, it remains unclear what lateral organ arrangements were present in early leafy plants. To investigate this, we quantified phyllotaxis in fossils of the Early Devonian lycopod Asteroxylon mackiei. We report diverse phyllotaxis in leaves, including whorls and spirals. Spirals were all n:(n+1) non-Fibonacci types. We also show that leaves and reproductive structures occurred in the same phyllotactic series, indicating developmental similarities between the organs. Our findings shed light on the long-standing debate about leaf origins and demonstrate the antiquity of non-Fibonacci spirals in plants.

Automated summary

In this study, the fossilized remains of early leafy plants were analyzed to investigate the arrangements of lateral plant organs. It was found that leaves exhibited diverse phyllotaxis, including spirals and spiral whorls. Additionally, the phyllotactic series of leaves and reproductive structures showed similarities, suggesting developmental connections between the organs. These findings provide insights into the debate on leaf origins and demonstrate the ancient occurrence of non-Fibonacci spirals in plants.