The prostrate habit even affects the symmetry of the whole axis in F. cretica and Z. simplex where the axis is semi-terate and presents a kidney-shaped appearance in T. S., the grooved surface representing the side in contact with the substratum.
Soft bast forms a continuous ring and follows the outline of the wood cylinder. Oxalate of lime is found in the form of clustered crystals in the cortex and pith in the species of Tribulus and T. orientalis. Pith consists of thick-walled cells in species of Tribulus and F. cretica; in others it is composed of thin-walled cells' The pith in S. orientalis and Z. simplex may occasionally form an aqueous tissue.
General Review. — The plants are fleshy and characterised by the plasticity of the tissues as was seen in the different modifications introduced in the prostrate axis. Outer walls of the epidermal cells are not much thickened and are superficially granulated. There is abundance of aqueous tissue in the leaf and axis. Mesophyll is truly isobilateral and shows a distinct tendency to isobilateral symmetry. Hairs, when present, are unicellular. Guard-cells are elevated, so that the front cavity is on a level with the surface, with the exception of S. orientalis, where the stomata are depressed. The veins, are embedded and provided in some members with distinct bundle-sheaths. Oxalate of lime-occurs in the form of clustered crystals in the leaf and axis.
The assimilatory tissue is composed either of palisade cells or of chlorenchyma or is absent, in which latter case the primary cortex functions as an aqueous tissue. The pericycle is composed of groups of stone-cells and presents an isobilateral structure. Wood forms a composite hollow cylinder. Medullary rays occur only in F. cretica and are uniseriate. The size of the xylem bundles and the size and abundance of vessels vary on the upper and lower surface of the prostrate axis. Soft bast forms a continuous ring and follows the outline of the wood cylinder.
The pith is composed of thick-walled or thin-walled cells. In the latter case the pith may occasionally form an aqueous tissue.
GERANIACEAE.
Monsonia heliotropioides Boiss.— Plate X figs. 79, 80, 81. Leaf only. Epidermal cells polygonal with outer walls papillose especially so on the lower surface. Inner and outer walls equally thickened. Stomata occurring on both the surfaces and surrounded by ordinary epidermal cells. Guard-cells elevated. Mesophyll formed of greatly elongated palisade cells on the upper side and of arm-palisade tissue on the lower. Internal glands absent. Clothing hairs unicel-
