Fig. 11.3-6. Transverse section of cactus stem (Calymmanthium substerile; from Peru, no common name). Many cacti have a thick cortex that grows outward as ribs; in this species, the ribs are thin (only a few millimeters) and project out from the main body several centimeters so in transverse section they look like thick leaves. The upper micrograph is a transverse section through the outermost part of one rib; the base of the rib and the center of the stem are to the right, out of the image. In a very low power view, this portion of a rib would look like just one arm of a star-shaped stem.
   The outermost cortex cells of cacti are aligned in rows, in a pattern that is similar to the palisade parenchyma of a leaf (so they are called palisade cortex; not easy to see in the upper micrograph, but is a little more obvious in the micrograph below). There is a set of cortical bundles that vascularizes the thick cortex, including the rib. The system of cortical bundles looks quite a bit like the veins of a leaf, and each bundle is collateral, having both xylem and phloem. At the low magnification of the upper micrograph, each cortical bundle is too small to be seen, but several are indicated by arrows.
            There has been research on this since the textbook was published; if you are interested, you can read these papers:
            Sajeva, M, and J. D. Mauseth. 1991. Leaf-like structure in the photosynthetic, succulent stems of cacti. Annals of Botany 68: 405-411.
            Mauseth, J. D., and M. Sajeva. 1992. Cortical bundles in the persistent, photosynthetic stems of cacti. Annals of Botany 70: 317-324.