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Electron microscopic study on the development of the bordered pit in coniferous tracheids

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The observations of the inner surface of Japanese red pine (Pinus densiflora Sieb. et Zucc.) earlywood tracheid wall in the various stages of the differentiation allowed the study of the development of the bordered pit. The process of the formation of the bordered pit membrane and the deposition of the cell wall layers in the pit border region were examined in detail by electron microscopy using the replication preparation technique. The microfibrillar structure of the margo and torus, which is characteristic of the bordered pit membrane of this species, was detected in the later stage of primary wall formation. The bordered pit membrane was shown to have the same microfibrillar structure as that of the matured one even at this stage of differentiation, although it was embedded in matrix substances. It appeared that the matrix substances were removed immediately after the warty layer had been formed in the tracheid wall.

The initiation of pit border development was observed as the deposition of circularly oriented microfibrils around the vicinity of the pit annulus in the later stage of the primary wall formation. The deposition of the S1-layer was indicated to contribute to the pit border formation until the diameter of the pit aperture reached its final size, simultaneously with the apposition of the layer of so-called B. T. or initial border thickening. So both the S1-layer and initial border thickening were assumed to be formed supplementing each other. In the period of the S1-layer formation, the transversely deposited microfibrils in the tracheid inner surface were detected to generally curve around the pit aperture and some of them extend to the outer surface of the developing pit border beyond the pit aperture. From these observations of replicas of the inner surface of the differentiating tracheid, a concept concerning pit border formation is proposed.

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The authors are indebted to members in Kyoto University Forest for providing the samples and to Dr. H. Saiki and other members of Wood Structure Laboratory in Department of Wood Science and Technology at Kyoto University for their assistance during this study. They also express their appreciation to Dr. W. A. Cote, Jr. (State University, College of Forestry, N. Y.) and Dr. W. Liese (Universität Hamburg) for their considerate advice.

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Imamura, Y., Harada, H. Electron microscopic study on the development of the bordered pit in coniferous tracheids. Wood Science and Technology 7, 189–205 (1973).

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