The Anatomy of the Leaf of Potato, Solanum tuberosum L. 'Russet Burbank'

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Abstract
In the compound leaf of potato, Solanum tuberosum ''Russet Burbank,'' phloem anastomoses occur throughout the petiole-rachis-midrib axis and are common in both the adaxial and abaxial phloem. Xylem anastomoses occur much less frequently. The frequency of anastomoses between the upper portions of the median bundle and the lateral bundles shows that the median bundle is not structurally disjunct from the lateral bundles in the midvein of the terminal leaflet. The width of the midvein fluctuates in relation to the divergence of the secondary veins in the terminal leaflet. An increase in width prior to the divergence of a secondary vein is accompanied by an increase in the number of phloem strands and tracheary elements. The first three orders of veins (midvein, secondary, and tertiary) are always major veins, i.e., associated with rib tissue. Quaternary veins may be either major or minor veins, i.e., embedded in mesophyll tissue. Most quinternary veins and all sexternary and septernary veins are minor veins. Abaxial phloem is found in all vein orders. Adaxial phloem is found in the midvein, all secondary and tertiary, many quaternary, some quinternary, and a few sexternary veins. Although both adaxial and abaxial phloem may be found in the next to highest vein order in any portion of a leaflet, the vein endings always lack adaxial phloem, regardless of their vein order. The two fimbrial veins are minor veins, contain phloem only intermittently, and converge with the midvein very near the tip of the terminal leaflet to form a hydathode.