aqueous tissue on the lower side, in F. cretlca to a comparatively extensive ventilating system and to the glandular' nature of the plant and in Zygophyllum simplex; to the abundance of aqueous tissue. The depressed position of the stomata in S. orientalis is perhaps clue to the absence of aqueous tissue in the mesophyll.
The mesophyll in species of Tribulus (fig. 68) is composed of a layer of long palisade cells on the upper side and of a subepidermal layer of polygonal aqueous cells and a layer of short palisade cells on the lower; the middle tissue is represented by the chlorenchymatous cells of the bundle-sheaths. In the cylinderical leaves of Z. Simplex, the assimilatory tissue forms a subepidermal composite ring of palisade cells and encloses a massive aqueous tissue of large thin-walled cells, which surrounds the central vascular bundles. In Scetzenia orientalis (fig. 71) there is a two-layered palisade tissue on the adaxial side and a two-layered arm-palisade tissue on the abaxial side; the middle tissue consists of thin-walled polygonal cells, some of which contain clustered crystals. The mesophyll in F. cretica (fig. 77) is composed of palisade tissue on both the sides with the middle tissue represented by the bundle-sheath cells. In F. cretica there occur rounded groups of faintly green palisade-like cells, with perhaps a water-storing function, between the lower epidermis and the vein of the mid-rib. Internal secretory organs do not occur in any of the members.
Oxalate of lime occurs in the form of clustered crystals in the neighbourhood of the veins in species of Tribulus, S. orientalis and Z. simplex in which the inner cells of the sheaths of the peripheral veins mostly contain clustered crystals (fig. 75).
The veins are embedded and are provided with green bundle- sheaths. The sheeth cells are thin-walled and polygonal in F. cretica they are cubical and thick-walled in the other members of the order The veins are numerous in species of Tribulus, F, cretica and Z. simplex. The abundance of veins corresponds with the abundance of watery contents in the mesophyll. The peripheral veins in Z. simplex form a supporting network of veins* for the massive aqueous tissue.
Hairy covering is present only on the leaf and axis, in species of Tribulus. It consists of simple unicellular hairs which are more numerous on the lower surface of the leaf (figs. 68, 69). External glands are not found on any of the members.
Structure of the Axis. — Epidermis in species of Tribulus consists of small thick-walled cells. The epidermal cells in S. orientalis are polygonal with outer and inner walls thickened and with lateral walls thin and undulated : there are large cells, perhaps with a water-
