PLANTS OF THE INDIAN DESERT. 7 



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). 



