The anatomy of the leaf of red pine, Pinus resinosa. II. Vascular tissues

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Abstract
The anatomy and ultrastructure of the endodermis and enclosed vascular tissues of the midregion of the mature secondary needle-leaf of Pinus resinosa are described. Within the uniseriate endodermis are two vascular traces surrounded by transfusion tissue. The endodermal cells have differentially thickened walls which lack Casparian strips but are lignified. Plasmodesmata traversing pit regions form a symplastic interconnection between mesophyll, endodermal, and transfusion parenchyma cells. In the lateral bundle region plasmodesmata extend this symplastic pathway across the cell walls of subjacent transfusion parenchyma and richly protoplasmic albuminous cells to the metaphloem. Four distinct types of transfusion tracheids have been defined on the basis of cell shape and location. Transfusion tracheids in the lateral bundle regions form direct radial connections between metaxylem and endodermis.