palisade tissue on the adaxial side and of arm-palisade tissue on the abaxial side. Veins not provided with bundle-sheaths. Clustered crystals occurring in the leaf and axis. Assimilatory tissue in the axis formed of chlorenchyma. Wood composite. Pith formed of thin-walled cells and forming occasionally an aqueous tissue.
Zygophyllum simplex L. Figs. 75, 76. Epidermal cells polygonal. Guard-cells elevated. Mesophyll composed of a subepidermal composite ring of palisade cells enclosing a massive aqueous tissue surrounding the central vascular bundles. Veins embedded. Peripheral veins numerous, strengthening the aqueous tissue and provided with sheaths of thick-walled cells, the outer ones containing chlorophyll and the inner ones containing clustered crystals. Veins traversing the aqueous tissue. T. S. of the axis kidney-shaped with angles projecting outwards at the grooved surface. Primary cortex forming an aqueous tissue. Wood composite. Pericycle distinctly isobilateral. Pith of thin-walled cells and forming an aqueous tissue. Clustered crystals numerous in the axis.
Fagonia cretica L. Figs. 77, 78. Epidermal cells with outer and inner walls convexly arched outwards and inwards. Outer walls superficially granulated. Guard-cells elevated. Mesophyll isobilateral. A group of palisade like cells faintly green and perhaps with a water-storing function, occurring between the lower epidermis and the vein of the mid-rib. Oxalate of lime not occurring in any form. Veins embedded and provided with bundle-sheaths of thin-walled cells. Assimilatory tissue in the axis formed of palisade cells. Wood composite. Pith formed of thin-walled cells.
Structure of the Axis.—The epidermis consists of tabular cells with outer and inner walls convexly arched outwards and inwards respectively. The lateral walls are usually straight. The outer walls are superficially granulated in Tribulus alatus, Fagonia cretica and Zygophyllum simplex. Large water-storing cells are intercalated amongst the ordinary epidermal cells in Seetzenia orientalis (fig. 71). Epidermal cells are thin- walled ; this can be accounted for by the fleshy character of the leaves.
The stomata occur on both the surfaces, though more numerous on the lower; they are surrounded by 3-6 ordinary epidermal cells. Guard-cells are elevated in species of Tribalus, Fagonia cretica Zygophyllum simplex (fig. 75), so that the front-cavity is on a level with the surface. In Seetzenia orientalis (fig. 72), the stomata are depressed and the guard-cells are either in the same plane or in a plane lower than that of the surrounding cells.
The elevated position of the stomata in species of Tribulus may be due to a protective covering of hairs and to the subepidermal
