Cortical Bundles in the Persistent, Photosynthetic Stems of Cacti

We examined 62 species in 45 genera of the cactus subfamilyCactoideae; all had collateral cortical bundles that permeatedthe broad, water-storing inner cortex and extended to the baseof the outer, photosynthetic palisade cortex. Mean distancebetween cortical bundles was 0.75 mm, similar to the mean spacing(0.74 mm) of veins in leaves of Pereskia, a genus of relictleaf-bearing cacti. In 16 species, both young and extremelyold stem cortex was available for study: in all of these, olderbundles had larger amounts of phloem than did younger bundles,indicating that phloem had been produced for many years. Inten species, older bundles also had more xylem than youngerbundles. In two genera (Rhipsalis and Selenicereus) there werecaps of primary phloem fibres, and in a single species (Pilosocereusmortensenii) 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), similarto Pereskia leaf vein elements (radius range, 1.8–2.7µm); this was much narrower than stem wood vessels (radiusrange, 10–42 um; mode, 23–28 µm). Longitudinalconduction of water and nutrients probably occurs predominantlyin stem wood, with cortical bundles maintaining the broad, voluminouscortex, the outer part of which is the plant's photosynthetictissue 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. Cactaceae, cortical bundles, homeotic, xylem, phloem, evolution     

Leaf-like Structure in the Photosynthetic, Succulent Stems of Cacti

This research examined the hypothesis that as cacti evolve tothe leafless condition, the stem epidermis and cortex becomemore leaflike and more compatible with a photosynthetic role.All cacti in the relict genus Pereskia have non-succulent stemsand broad, thin leaves. All members of the derived subfamilyCactoideae are ‘leafless’, having an expanded cortexthat is the plant's only photosynthetic tissue. In Pereskia,leaves have a high stomatal density (mean: 50.7 stomata mm^–2in the lower epidermis, 38.1 mm^–2 in the upper epidermis),but stems have low stomatal densities (mean: 11.3 mm ₂, threeof the species have none). Stems of Cactoideae have a high stomataldensity (mean: 31.1 mm^–2, all species have stomata). Theouter cortex cells of stems of Cactoideae occur in columns,forming a palisade cortex similar to a leaf palisade parenchyma.In this palisade cortex, the fraction of tissue volume availablefor gas diffusion has a mean volume of 12.9%, which is identicalto that of Pereskia leaf palisade parenchyma. Pereskia stemcortex is much less aerenchymatous (mean: 5.3% of cortex volume).Cactoideae palisade cortex has a high internal surface density(0.0207 cm₂ cm^–2 which is higher than in Pereskia stemcortex (0.0150 cm₂ cm^–3) but not as high as Pereskia leafpalisade parenchyma (0.0396 cm₂ cm^–3). Pereskia stem cortexhas no cortical bundles, but Cactoideae cortexes have extensivenetworks of collateral vascular bundles that resemble leaf veins. Cactaceae, cactus, intercellular space, stomatal density, internal surface/volume, evolution