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Anatomy of the Palm Rhapis excelsa, X. Differentiation of Stem Conducting Tissue

1984; Arnold Arboretum of Harvard University; Volume: 65; Issue: 2 Linguagem: Inglês

10.5962/p.185926

2474-3283

P. B. Tomlinson, Jean-Luc Vincent,

Date Palm Research Studies

DESPITE THE APPARENT VASCULAR COMPLEXITY of the palm, its vascular development, structure, and function can be perceived quite readily.Compared with branched trees, it is architecturally simple (Hallé ef a/., 1978).Metaxylem and protoxylem are structurally and topographically very distinct.Vascular tissues are radially as well as tangentially separated (unlike those of dicotyledons), and due to the relatively massive meristematic region and the long time course of differentiation, successive events are widely separated in time as well as in space.Finally, primary structures are not obscured by later development of secondary vascular tissues.In dicotyledons recent advances in our understanding of vascular developmental processes have required very precise analyses of serial thin sections, as in the work of Larson (1982; and earlier papers cited therein) on Populus.In this paper we consider the sequence of initiation of xylem and phloem within the procambial template and the changes within these tissues as both radial and axial extension take place.Special attention is given to xylem differentiation in traces to a given leaf at various stages of its development, a topic being investigated physiologically by John Sperry at Harvard Forest.In the first paper in this series (Zimmermann & Tomlinson, 1965), a cinematographic method of analysis was used to describe the course of vascular bundles in the mature aerial stem of the small palm RAapis excelsa (Thunb.)Henry.In a topographic sense, traces serving a given leaf (leaf traces) usually branch to give an axial bundle that becomes a leaf trace at a higher level.Axial bundles in the region of leaf-trace departure are connected by short bridge bundles.In addition to establishing the principle of vascular continuity in the stem, that paper demonstrated the changes that take place in individual mature bundles throughout their length.It provided a basic, general framework (since termed the "Rhapis-principle''-see Zimmermann & Tomlinson, 1972; Tomlinson, 1983) for understanding the vasculature of monocotyledonous stems, which may be described as a regular pattern of outgoing leaf traces branching to generate topographically axial bundles and other derivatives.To add to the topographic analysis, in the fourth paper of the series (Zimmermann & Tomlinson, 1967), the sequence of initiation of strands within the developing crown was analyzed in terms of their inception as procambial strands.The principle of vascular development that has been shown to be generally applicable to monocotyledons was demonstrated.Procambial strands that connect to young leaf primordia are initiated within a cap of meristematic tissue.They are