storing function, intercalated amongst the ordinary epidermal cells (fig. 73), Epidermal cells in F. cretica are tabular with outer walls thickened and cuticularised and with lateral walls thin and straight, Epidermis in Z. simplex is composed of polygonal cells with outer walls thickened and papillose. Outer walls are superficially granulated in all members except T. terrestris. The stomata are like those on the leaf.
The primary cortex in species of Tribulus is characterised by the presence of a layer of subepidermal collenchyma below which occurs cortical parenchyma with a water-storing function. The cortex in S. orientalis consists of chlorenchyma and forms an assimilatory tissue; it forms an aqueous tissue strengthened in the projecting angular portions by small stone-cell groups in Z. simplex. In F. cretica it consists on its outer side of an assimilatory tissue of palisade cells and of an inner portion of an aqueous tissue; it is strengthened by stone-cell groups which can be roughly arranged in three rings, the outermost groups being the largest and almost subepidermal in position and giving a ribbed appearance to the axis.
The pericycle is composed of rhomboidal groups of stone-cells in species of Tribulus, S. orientalis and F. cretica. The stone-cell tissue is extensive and is necessary to strengthen the massive cortical aqueous tissue. The pericycle in Z. simplex is composed of stone-cell groups and presents an isobilateral arrangement; the stone-cell tissue along the grooved portion is greatly reduced and consists of isolated stone-cells or of very small groups.
In prostrate axis of species of Tribulus and of S. orientalis the stone-cell tissue is more extensive along the upper portion of the axis. In spite of the prostrate habit of the axis of F. cretica the pericycle does not seem to be much affected except that the stone-cell groups are closer together on the upper side.
The structure of the wood presents an isobilateral appearance in all the members. Wood forms a composite hollow cylinder formed of closely placed xylem bundles. Interfascicular wood prosenchyma is little developed. Wood prosenchyma encloses the lower portion of the xylem bundles in T. terrestris, while in T. alatus it is replaced by wood parenchyma; this may form a specific difference. Medullary rags are absent except in F. cretica where uniseriate rays occur.
Along the upper surface which is more exposed to the sun and climatic factors and which consequently presents greater functional activity, the xylem bundles are larger with vessels larger and more numerous; on the lower side which is protected from the sun the xylem bundles are much smaller and the vessels smaller and less numerous.
