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Interxylary phloem: Diversity and functions

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Interxylary phloem is here defined as strands or bands of phloem embedded within the secondary xylem of a stem or root of a plant that has a single vascular cambium. In this definition, interxylary phloem differs from intraxylary phloem, bicollateral bundles, pith bundles, and successive cambia. The inclusive but variously applied terms included phloem and internal phloem must be rejected. Histological aspects of interxylary phloem are reviewed and original data are presented. Topics covered include duration of interxylary phloem; relationship in abundance between sieve tubes in external phloem and interxylary phloem; distinctions between interxylary and intraxylary phloem; presence of parenchyma, fibers, and crystals in the interxylary phloem strands; development of cambia within interxylary phloem strands; three-dimensionalization and longevity of phloem, systematic distribution of interxylary phloem; physiological significance; and habital correlations. No single physiological phenomenon seems to explain all instances of interxylary phloem occurrence, but rapidity and volume of photosynthate transport seem implicated in most instances.

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For providing material, thanks are due Dr. David Lorence for stems of Strychnos madagascariensis (National Tropical Botanical Garden), Dr. Peter H. Raven of the Missouri Botanic Garden for stems of various Onagraceae, and the University of California of California at Santa Barbara living greenhouse collections for material of Salvadora. John Bleck provided seeds of Orphium frutescens from which my specimens were cultivated. Mark Olson and Edward L. Schneider provided helpful suggestions. Use of laboratory facilities, including the SEM, at Santa Barbara Botanic Garden, is gratefully acknowledged.

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Correspondence to Sherwin Carlquist.

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Carlquist, S. Interxylary phloem: Diversity and functions. Brittonia 65, 477–495 (2013).

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