The structure of the wood differs in the two species. In T. orientalis (fig. 46) the wood forms a composite solid central cylinder and is composed of numerous small 'vessels embedded in interfascicular wood prosenchyma. In T. dioica (fig. 45) the wood forms a composite hollow central cylinder enclosing a small pith tissue and consists of xylem bundles connected together by interfascicular wood prosenchyma. Medullary rays are 2-6 seriate in T. dioica. In T. orientalis they are uniseriate and few. The soft bast forms a continuous ring in T. orientalis, while in T. dioica it occurs in groups opposite the xylem bundles.
The pith of T. dioica is formed of thick-walled cells, those near the periphery being filled with granules; it does not occur in T. orientalis.
Internal secretory organs in both the species are represented by some of the cortical cells near the periphery with tanniniferous contents.
ELATINEAE.
Bergia ammanioides Roxb.— Pl. VI, figs. 47, 48, 49. A small number of cells with clustered crystals occurring near the veins. Secretory cells with tanniniferous contents found in the pith. Clothing hairs not found on the leaf. Hairs on the axis in the form of very thin-walled uniseriate trichomes. Glandular hairs absent on the leaf and axis. T. S. of the axis quadrangular. Assimilatory tissue in the axis formed of chloroqhyll containing parenchyma. Sclerenchymatous pericycle in the form of small stone-cells at the angles. Medullary rays absent.
Bergia odorata Edgew.— PI. VI, fig. 50 ; PI. VIII, figs. 51, 52. A layer of polygonal cells with clustered crystalsin the middle of the mesophyll. Numerous clustered crystals near the veins. Secretory cells with tanniniferous contents in the cortical parenchyma, soft bast and pith. Ordinary unicellular hairs and uniseriate trichomes found on the leaf and axis. Glandular shaggy hairs occurring on the leaf and axis. T. S. of the axis circular. Assimilatory tissue in the axis composed of short palisade cells. Sclerenchymatous pericycle forming a loose ring of small groups of stone-cells. Medullary rays ]-2 seriate.
Structure of the Leaf:—The epidermis is composed mostly of horizontally tabular cells with very large water-storing cells intercalated amongst them. The water-storing cells are sometimes divided by cross walls into unequal halves, the lower half being much larger than the upper one. The inner walls are thin and arched convexly inwards, so as to come into close contact with the assimilatory tissue below. The lateral walls are thin and straight ; the outer walls are flat and greatly thickened.
