A rapidly transpiring leaf can evaporate its own fresh weight of water in 10 to 20 min, though many plants such as cacti, mangroves and plants in deep shade have much smaller rates of water turnover. Leaf veins must carry this water to all parts of a leaf to replace evaporated water, and maintain cell hydration and turgor. When water supply fails to meet this demand, shoots wilt.
Vein distribution patterns differ markedly between broad-leaf species and grasses. Broad-leaf species generally have a highly branched network while grass species have parallel veins (Figure 3.20).
Veins 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 in the xylem tissue, plus sieve tubes in the phloem tissue. There are no intercellular air-spaces, or only very small ones.
The ring of cells forming the sheath around the xylem and phloem tissue 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 (Figure 3.21).