Characteristics of the xylem

Growth ring boundaries are rather indistinct in most species (Figs. 1 and 2) except Capparis spinosa (Fig. 3). Ring boundaries are absent in Cadaba rotundifolia, Cleome brachyphylla and Cleome isomeris (Fig. 2). Capparis spinosa is ring-porous to semi-ring-porous (Fig. 3), diffuse-porous are Cadaba longifolia, Cleome arabica and Maerua crassifolia. Vessels are arranged in radial multiples of various length (Figs. 1-3, 6 and 7). Earlywood vessels with a diameter between 100-200 pm are characteristic of Capparis spinosa (Fig. 3). The diameter of all other species varies from 50-100 pm (Figs. 1 and 2). Vessels of all species have simple perforations and small intervessel pits in alternat ing or opposite position (Fig. 4). Vessels of most species are thick-walled (Fig. 5). Fibers containing large pits with slit-like apertures occur in the radial walls of all species. The distribution of axial parenchyma is paratracheal (Figs. 6 and 7). Rays are species-specific: uniseriate, homocellular with square and upright cells in Cadaba longifolia (Fig. 8), heterocellular with one to several square and upright marginal cells and 2-3-seriate in all other species (Figs. 9-11). Particular is the occurrence of included phloem in Cadaba longifolia, Cadaba rotundifolia (Fig. 12), Cleome brachyphylla (Fig. 13) and Maerua crassifolia. Large prismatic crystals are present in ray cells of Cadaba longifoilia, Cadaba rotundifolia and Maerua crassifolia (Fig. 14).

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Fig. 1. Diffuse-porous xylem with growth zones. Vessels are arranged in long, loose radial multiples. Stem of a 1 m-high shrub, Savannah, arid zone, Saudi Arabia. Cadaba longifolia, transverse section.

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