Structure of Leaf/Branch Gap Parenchyma and Associated Vascular Tissues in Populus deltoides

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Abstract
Nodes of P. deltoides Bartr. ex Marsh. between leaf plastochron index 25 and 40 were examined by light and electron microscopy to determine the structure of both the birefringent (in polarized light) parenchyma (BP) ocuppying most of the leaf/branch gap and the spatially associated vascular tissues. The BP tissue is especially well developed next to the central leaf trace and axillary branch cylinder and is notably confluent with the rays of the laterally adjacent stem xylem. The BP cells have thick, lignified secondary walls with numerous pits traversed by abundant plasmodesmata. Virtually all BP cells except those near vessels have diffuse cytoplasm, relatively few organelles, and often large vacuoles. The stem and branch xylem consists of fibers, vessels, both vessel-associated and non-vessel-associated parenchyma cells, and isolation and contact ray cells. The leaf trace xylem is made up of vessels and vessel-associated parenchyma cells. Structurally intermediate cells occur where the stem xylem merges with the BP tissue. Most parenchymatous cells in direct contact with vessels have dense cytoplasm and numerous organelles. These may be BP cells, thin- or thick-walled vessel-associated parenchyma cells, or contact ray cells. Relatively numerous plasmodesmata connect the cytoplasmically dense cells with adjacent ray, non-vessel-associated parenchyma, and BP cells. Most living cells have plasmodesmata with constricted neck regions, but some BP and many ray cells also have plasmodesmata with unconstricted neck regions.

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