Robberecht, R. and P.S. Nobel. 1983. A Fibonacci sequence in rib number for a barrel cactus. Annals of Botany 51:153-155.

Introduction. Prominent longitudinal ribs are a characteristic morphological feature of many species of cacti. Ribs provide increased stem surface area for convective heat loss, thereby ameliorating high stem temperatures in hot desert climates, and facilitate stem swelling during periods of high water availability (Lewis and Nobel, 1977). The rib morphology of Ferocactus acanthodes (Lem.) Britton and Rose, a multi-ribbed barrel cactus common on rocky slopes in the Sonoran and Mojave deserts, was studied to determine whether the number of ribs is constant, random, or follows some discernible pattern during plant growth and development. Measurements of rib number in a F. acanthodes population were made on 100 randomly chosen plants at the Philip L. Boyd Deep Canyon Desert Research Center, a Sonoran desert site at 33° 39' N. 116° 23' W and 850 m elevation. The results showed that rib number was neither constant nor random, but rather increased with plant growth according to a mathematically defined sequence known as a Fibonacci sequence (Hoggatt, 1969): 1, 1, 2, 3, 5, 8, 13, 21... (the next number in the sequence is derived from the sum of the previous two numbers).

This research was supported by United States Department of Energy contract DE-AMO3-76SF00012.