Strong support in favor of the smaller longitudinal veins as the sites of phloem loading is that aphids exclusively feed on small and intermediate veins of barley ( Hordeum vulgare). Structural and physiological evidence suggests that the small and intermediate longitudinal veins are responsible for phloem loading. The vein system consists of small, intermediate, and large longitudinal veins interconnected by transverse veins (also termed lateral veins or cross-veins), the smallest components of the venal network. In monocotyledons, the vein system extends from the leaf base and the longitudinal veins are mostly parallel to one another through the leaf blade. Structural and functional vein maturation proceeds in a narrow distinctly demarcated band. After completion of the major vein system, a second basipetal wave of vein differentiation begins. During this stage, the lower-order veins differentiate acropetally. The venal network develops initially from the leaf base. Leaves are compartmentalized into areoles, mesophyll areas enclosed by higher-order veins in which the highest-order vein ends blindly. From a mid-vein (lowest-order or first-order vein), vein orders of reducing complexity diverge. In dicotyledons, the vein system is mostly reticulate. Further expansion of the leaf coincides with differentiation of stomata and gas spaces, and maturation of the minor veins collectively transforms the developing leaf into a photoassimilate source. During these and subsequent developmental stages, when the major veins differentiate, the leaf is a net importer of reduced carbon and functions as a sink. Leaf primordia originate from a foliar buttress transforming into an axillar phyllopodium which, in turn, extends laterally by a complex interaction of several meristems. Thomas, in Encyclopedia of Applied Plant Sciences (Second Edition), 2017 Leaf Development
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