pression formed by the outer thickened epidermal walls (fig. 195) ; it may occasionally be found lying in a level with the surface (fig. 192).
The mesophyll (figs. 191, 194) is composed of short-celled palisade tissue on either side with an extensive tissue of vertically elongated large, thin-walled colourless cells in the middle. This middle tissue probably forms an aqueous tissue in both the species.
The veins are embedded and are enclosed in green bundle-sheaths. All the veins in S. oleoides have thin arcs of sclerenchyma on the upper and lower side, while only the larger veins in S. persica are protected on their lower side by arcs of stone-cells. There are numer- ous large groups of water-storage tracheids with pitted markings, occurring at intervals between the veins of both the species.
Internal secretory organs and oxalate of lime are not found.
Hairy covering is absent on the leaves of both the species. Short thick-walled unicellular clothing hairs are found on the axis of S. oleoides (fig. 196).
Structure of the Axis.— Epidermis consists of polygonal cells with outer walls greatly thickened and convexly arched outwards. Inner walls are thin. The stomata are like those on the leaf. Besides stomata, lenticels are found in S. persica. The primary cortex is composed on its outer side of chlorenchyma and on the inner side of a tissue of thin-walled colourless cells probably aqueous.
The pericycle is formed of radially elongated large groups of stone-cells separated by cortical aqueous cells. The pericycle presents a dumb-bell shaped appearance corresponding with that of the wood cylinder.
The structure of the wood is characterised by islands of soft bast (6gs. 193, 196). The wood cylinder is of uniform breadth in S. oleoides, while in S. persica it is much narrowed on opposite sides in one plane, thus giving a dumb-bell shaped appearance to the whole structure. Vessels are large and have rounded lumina, except those in the narrowed portion of the wood cylinder in S. persica, where the vessels are very small. Medullary rays are 3-5 seriate ; and in S. oleoides they are continuous with cortical parenchyma between the stone-cell groups of the pericycle (fig. 196). Wood parenchyma is pretty abundantly developed on the inner side of the wood cylinder and in the neighbourhood of the islands of soft bast.
Soft bast in S. ohoides forms a continuous ring and follows the dumb-bell shaped outlina of the wood-cylinder ; in S. persica soft bast occurs in groups on the outer side of the xylem bundles.
The pith is composed of thin-walled cells and is characterised by numerous sieve-sclereids (fig. 193 SV).
