Developmental anatomy of the stem of Welfia georgii, Iriartea gigantea, and other arborescent palms: implications for mechanical support

Changes in stem anatomy with radial position and height were studied for the arborescent palms Wefia georgii, Iriartea gigantea, Socratea durissima, Euterpe macrospadix, Prestoea decurrens, and Cryosophila albida. Vascular bundles are concentrated toward the stem periphery and peripheral bundles contain more fibers than central bundles. Expansion and cell wall thickening of fibers within vascular bundles continues throughout the life of a palm, even in the oldest tissue. Within individual vascular bundles, the fibers nearest the phloem expand first and fiber cell walls become heavily thickened. A front of expanding fibers moves outward from the phloem until all fibers within a vascular bundle are fully expanded and have thick cell walls. Peripheral vascular bundles differentiate first and inner bundles later. In the stem beneath the crown, vascular bundles and ground tissue cells show little or no size increase, but marked cell wall thickening during development for Welfia georgii. Beneath the crown, diameters of peripheral vascular bundles increase more than twofold for Iriartea gigantea, while diameters of central bundles do not increase. In Iriartea stems, ground tissue cells at the periphery elongate to accommodate expanding vascular bundles and cell walls become thickened to a lesser degree than in fibers; central ground tissue cells elongate markedly, but cell walls do not become thickened; and large lacunae form between central parenchyma cells. For Iriartea, Socratea, and Euterpe, sustained cell expansion results in limited, but significant increases in stem diameter. For all species, sustained cell wall thickening results in dramatic increases in stem stiffness and strength. THERE IS CONSIDERABLE variation among palm species in the capacity for sustained stem thickening during height growth (Schoute, 1912; Schatz et al., 1985; Rich, 1985, 1986; Rich et al., 1986). Some species, such as Wefia georgii, Prestoea decurrens, and Cryosophila albida have little or no capacity for sustained stem thickening. Other species, such as Iriartea gigantea, Socratea durissima, and Euterpe macrospadix have pronounced capacities for stem diameter increase during height growth. Stiffness and strength are concentrated toward the stem periphery and base of palms. Palms also have a major capacity to increase the stiffness and strength of stem tissue during height growth I Received for publication 21 April 1986; revision accepted 24 September 1986. Thanks go to Peter Ashton, William Bossert, Elizabeth Burkhardt, Thomas Givnish, Ghiselle Hemandez, Shawn Lum, Monika Mattmiiller, Thomas McMahon, Leda Mufioz, Lynn Phipps, Mauricio Quesada, P. B. Tomlinson, the late Martin Zimmermann, and the staff of the Organization for Tropical Studies. Research was supported by the Atkins Garden Fund of Harvard University, the Jesse Noyes Foundation through the Organization for Tropical Studies, a Fulbright Graduate Fellowship, and National Science Foundation Doctoral Dissertation Improvement Grant BSR84-13187. 2 Current address: Mailstop K495, HSE-12; UC Los Alamos National Laboratory; Los Alamos, NM 87545. (Rich, 1987). Herein, I characterize developmental anatomy in palm stems as it relates to mechanical support. I examine anatomical basis for sustained stem thickening and strengthening during height growth. I focus on the Costa Rican palm species Welfia georgii H. A. Wendl. ex Burret and Iriartea gigantea H. A. Wendl. ex Burret, and also include comparisons with Socratea durissima (Oerst.) H. A. Wendl., Euterpe macrospadix Oerst., Prestoea decurrens (H. A. Wendl.) H. E. Moore, and Cryosophila albida Bartlett. The outer portion of palm stems consists of a narrow cortex that is composed of unspecialized ground parenchyma and fibrous or fibrovascular strands. In some palms, especially small arecoid palms (e.g., Prestoea decurrens), the epidermis is permanent; however, in most palms, the epidermis is sloughed off. In palms that shed their epidermis, a superficial protective layer forms. Some palms have an etagenmeristem of the outer ground parenchyma, a zone of cell division without permanent initials (Philipp, 1923) that produces suberized or lignified secondary tissue; while other palms form the protective layer by suberization and sclerosis of the existing ground parenchyma (Weisse, 1897; Floresta, 1905; Philipp, 1923; Tomlinson, 1961). The inner portion of the