Paraveinal mesophyll, and its relationship to vein endings, in Solidago canadensis (Asteraceae)

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Abstract
Paraveinal mesophyll is described from leaves of a common goldenrod species, Solidago canadensis L. (tribe Astereae). This is the first report of paraveinal mesophyll from the Asteraceae. It is a uniseriate middle layer consisting of horizontally lobed cells that form a lacy meshwork between veins. It abuts the tightly cylindrical bundle sheath at the level of the xylem in all vascular bundles. Vein endings, however, differ from other vascular bundles in two ways: sieve tube members may extend to the vein tip, end at an intermediate point, or be absent, and lateral bundle sheath cells distal to the terminal sieve tube member swell greatly or protrude horizontally and interdigitate with adjacent paraveinal mesophyll cells. Cells of both paraveinal mesophyll and bundle sheath have fewer and smaller chloroplasts than other mesophyll cells; the chloroplasts mostly lie adjacent to intercellular spaces. During leaf development, the paraveinal mesophyll layer differentiates before other mesophyll layers. Solidago canadensis paraveinal mesophyll resembles the well-studied paraveinal mesophyll of Glycine max, except for differences in its anatomical relationship to minor veins and vein endings.