Hairy covering consists of unicellular curved hairs with perhaps a function of absorbing water. Walls are thin and superficially knobbed. Glandular hairs are not found in the leaf and axis.
Structure of the Axis : —The epidermal cells are small with outer walls thickened and papillose. The primary cortex is formed of chlorenchyma.
The perioycle forms a more or less composite ring of stone- cells. The wood is composite. Vessels are large and few ; they are more abundant in the inner half of the wood. Interfascicular wood prosenchyma is extensive. Wood parenchyma is more abundantly developed than in Papilionaceae and enclose the inner ends of the xylem bundles. The medullary rays are unseriate.
The pith consists of small cells with thickened and lignified walls.
MIMOSEAE
Acacia Senegal, Willd. —Figs, 120, 121. Epidermal cells with outer walls thickened and papillose. Stomata occurring on both the surfaces. Mesophyll formed of palisade tissue on the upper side and of arm-palisade tissue on the lower. Tannin sacs present in the leaf and axis. Veins embedded. Clothing hairs unicellular. Cork subepider- mal. Pericycle forming a composite ring of stone-cells. Small groups of stone-cells in soft bast. Vessels large. Medullary rays uniseriate. Pith composed of cells with thick and lignified walls.
Structure of the Leaf: — The epidermal cells have outer walls thickened and papillose. Stomata occur on both the surfaces. The guard-ceils are situated in the plane of surrounding cells and the front cavity is placed in a depression formed by the papillose outer walls of the epidermal cells.
The mesophyll is composed of 3-4 layers of palisade cells on the adaxial side and of arm-palisade tissue on the abaxial side. The middle tissue consists of a layer of large parenchymatous cells with tanniniferous contents. Internal secretory organs are represented by tannin sacs occurring in the middle of the mesophyll, in cortex and pith.
The veins are embedded. The veins of the mid-rib are protect- ed on the lower side by strands of sclerenchyma.
The hairy covering consists of a few unicellular clothing hairs. Glandular hairs are not found.
Structure of the Axis: — The cortex is characterised by subepi- dermal cork. Some of the cells of the tanniniferous cortical parenchyma near the pericyclic stone-tissue contain solitary crystals of oxalate of lime.
