storage function intercalated amongst the ordinary epidermal cells in Hibiscus micranthus (fig. 57A). They are present on both the sides and have their inner walls convexly arched inwards so as to come into close contact with the assimilatory tissue.
The stomata are more numerous on the lower surface. The front cavity is placed in a depression formed by the outer thickened and papillose walls. The guard-cells are in the plane of the surrounding cells as in Abutilon fruticosum and Sida grewoioides (fig. 53), or they are elevated as in other species. The elevated position of the guard-cells can be accounted for by the occurrence of a dense covering of tufted hairs. The mesophyll is composed of palisade tissue on the adaxial side and of arm-palisade tissue on the abaxial side. The palisade tissue in Sida greioioides is formed of compact cylindrical groups. The mesophyll is characterised by the abundance of internal glands in Sida grewioides, Pavonia arabica and Gossypium herbaceum.
The internal secretory organs in Gossypium herbaceum are represented partly by some of the palisade cells and a layer of polygonal cells above the arm-palisade tissue with tanniniferous contents and partly by mucilaginous secretory cavities with a lining layer of cells, and situated in the middle of the mosophyll and in the arm-palisade tissue. In Pavonia arabica, there are mucilaginous secretory cavities situated in the middle of the mesophyll below the vascular bundles of the veins. In Sida greioioides (fig. 53), there are groups of loosely arranged palisade-like cells, faintly green in colour and placed between groups of palisade cells; there are also rounded or elliptical structures amongst the arm-palisade cells, also faintly green in colour. These structures in the mesophyll are either schizogenously formed internal secretory cavities or water-storage cells.
Oxalate lime occurs in the form of clustered crystals near the veins. In the axis clustered crystals occur in the cortex and pith of all species except Pavonia arabica. In Sida grewioides and Pavonia arabica numerous small-clustered crystals occur in the soft bast.
The veins are embedded except some of the larger veins in Gossypium herbaceum which are vertically transcurrent above and below partly by means of sclerenchyma and mostly by collenchyma. The veins are provided with bundle -sheaths of green thin-walled cells.
The hairy covering consists of densely placed tufted hairs. The rays are unicellular and are sunk directly in the epidermis, so that the hairs seem to be formed by a group of epidermal cells, fig. 55. The rays on the lower portion of the mid-rib are borne on a short multicellular stalk. The hairy covering is denser on the lower surface of the leaf.
