Leaf anatomy of ocotillo (Fouquieria splendens; Fouquieriaceae), especially vein endings and associated veinlet elements

Abstract

Leaves of ocotillo, a shrub of southwestern North America, lack xeromorphic features. After rain, a few leaves at each node expand and function for a short period, then abscise. This cycle may be repeated several times each year. Palisade layers occur interior to both epidermal surfaces, and the spongy mesophyll is reduced. Venation is camptodromous, with many vein endings. In the distal lamina half, sclerified bundle sheath cells ("veinlet elements") become increasingly common in minor veins and vein endings. Near the leaf tip, adjacent mesophyll cells also become sclerified, to such an extent that some areoles appear filled with these cells ("accessory veinlet elements"). Phloem is conspicuous because it stains intensely and occupies more volume than xylem in most bundles. In minor veins and vein endings, sieve tube members become increasingly more slender than associated phloem cells, and xylem frequently changes its position, becoming parallel with, or even abaxial to, the phloem. Phloem mostly ends before, less commonly with, the xylem.