Cortical Bundles in the Persistent, Photosynthetic Stems of Cacti

We examined 62 species in 45 genera of the cactus subfamily Cactoideae; all had collateral cortical bundles that permeated the broad, water-storing inner cortex and extended to the base of the outer, photosynthetic palisade cortex. Mean distance between cortical bundles was 0.75 mm, similar to the mean spacing (0.74 mm) of veins in leaves of Pereskia, a genus of relict leaf-bearing cacti. In 16 species, both young and extremely old stem cortex was available for study: in all of these, older bundles had larger amounts of phloem than did younger bundles, indicating that phloem had been produced for many years. In ten species, older bundles also had more xylem than younger bundles. In two genera (Rhipsalis and Selenicereus) there were caps of primary phloem fibres, and in a single species (Pilosocereus mortensenii) cortical bundle xylem contained libriform fibres. All cortical bundle tracheary elements were narrow (radius range, 0.91–8.2 μm; mode, 1.8–2.7 μm), similar to Pereskia leaf vein elements (radius range, 1.8–2.7 μm); this was much narrower than stem wood vessels (radius range, 10–42 um; mode, 23–28 μm). Longitudinal conduction of water and nutrients probably occurs predominantly in stem wood, with cortical bundles maintaining the broad, voluminous cortex, the outer part of which is the plant's photosynthetic tissue and the inner part of which stores water and starch. The cortex of the Cactordeae contains numerous leaflike characters; homeotic genes may be involved in its morphogenesis.