5.2.4  Unit pipe: generalised vein structure

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Amidst the great variety of plant leaf structures, pipelines carrying a transpiration stream have certain constant features. They consist typically of tightly packed xylem and phloem tissues surrounded by a parenchymatous or fibrous sheath. Both xylem and the phloem contain living parenchyma cells as well as their characteristic transporting conduits, vessels and/or tracheids, plus sieve tubes. There are no intercellular air-spaces, or only very small ones. The sheath acts both as a mechanical barrier that may confine pressure within the vein, and a permeability barrier that can control rates and places of entry and exit of materials. Exceptions are to be found at the ultimate ends of some dicotyledonous fine veins where tracheids or sieve elements may be unaccompanied by other cells, in the transverse veins of grasses which have no sheath, and in some special veins at leaf margins where the sheath is absent on the xylem side (see below).