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(American Journal of Botany. 2003;90:663-674.)
© 2003 Botanical Society of America, Inc.

Anatomy and Morphology

On the mechanical properties of the rare endemic cactus Stenocereus eruca and the related species S. gummosus1

Karl J. Niklas2,4, Francisco Molina-Freaner3, Clara Tinoco-Ojanguren3, Christopher J. Hogan Jr.2 and Dominick J. Paolillo Jr2

2Department of Plant Biology, Cornell University, Ithaca, New York 14853-5908 USA; 3Instituto de Ecologia UNAM, Apartado Postal 1354, Hermosillo, Sonora CP83000, Mexico

We examined the hypothesis that the procumbent growth habit of the rare, columnar cactus Stenocereus eruca is in part the result of a diminution of the mechanical properties of stem tissues by comparing the properties of S. eruca plants with those of the putatively closely related semi-erect shrub S. gummosus. Intact stems and surgically removed anatomically comparable regions of the stems of both species were tested in bending and tension to determine their Young's modulus and breaking stress. A computer program was used to evaluate the contribution of each region to the capacity of entire stems to resist bending forces. Our analyses indicate that the principal stiffening agent in the stems of both species is a peripheral tissue complex (= epidermis and collenchyma in the primary plant body) that has a significantly higher tensile breaking stress and greater extensibility for S. gummosus than that of S. eruca. Computer simulations indicate that the wood of either species contributes little to bending stiffness, except in very old portions of S. gummosus stems, because of its small volume and central location in the stem. These and other observations are interpreted to support the hypothesis that S. eruca evolved a procumbent growth habit as the result of manifold developmental alterations some of which reduced the capacity of tissues to support the weight of stems.

Key Words: biomechanics • Cactaceae • flexural rigidity • procumbent stem growth • stem bending • Stenocereus




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J. D. MAUSETH
Structure-Function Relationships in Highly Modified Shoots of Cactaceae
Ann. Bot., November 1, 2006; 98(5): 901 - 926.
[Abstract] [Full Text] [PDF]


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