General Review:—Outer walls of the epidermal cells are thickened. Guard-cells are accompanied by ordinary 4-6 epidermal cells and are usually elevated above the plane of the surrounding cells. The mesophyll is either composed wholly of palisade tissue or is isobilateral, or is composed of palisade tissue on the adaxial side and of arm-palisade tissue or spongy tissue on the abaxial side. Internal secretory cells with tanniniferous contents are usually present in the leaf. The veins are embedded and are in some species provided with bundle-sheaths. Water-storing tracheids occur in the leaf and axis in some species. Mutlicellular capitate glandular hairs are of common occurrence. Peltate clothing hairs are present in Caclaba inclica. Primary cortex is characterised by an assimilatory tissue composed either of chlorenchyma or of palisade tissue. The pericycle is composed of groups of stone-cells. The structure of the wood is composite. The soft bast forms a continuous ring or occurs in groups. The pith is composed either of thin-walled or of thick-walled cells.
VIOLACEAE.
Viola Stocksii Boiss.—Epidermal cells of the leaf and axis with inner walls gelatinised and with outer walls thickened and muriculate. Palisade-like elongation of the epidermal cells characteristic of the axis. Stomata present on both the surfaces. Guard-cells elevated above the plane of the surrounding cells. Mesophyll formed of palisade tissue on the adaxial side and of arm-palisade tissue on the abaxial side. Internal secretory organs absent. Oxalate of lime in the form of large clustered crystals in the leaf. Veins embedded and not provided with bundle-sheaths. Hairy covering absent. Sclerenchymatous pericycle absent. Wood composite. Vessels small and arranged in numerous complete rows. Medullary rays uniseriate Collenchyma occurring at the inner margin of the wood. Pith composed of thin-walled cells.
Structure of the Leaf:—The epidermis is composed of tabular cells, with outer walls thickened, muriculate and convexly arched outwards, and with inner walls gelatinised. Lateral walls are wavy. The marginal epidermal cells have outer as well as inner^walls thickened. The gelatinised inner walls of the epidermis can absorb and retain water. The epidermis thus forms a kind of water-storing tissue. Stomata occur on both the surfaces. Guard-cells are elevated and are accompanied by subsidiary cells. The front cavity is on a level with the surface. The mesophyll is composed of palisade tissue on the adaxial side and of arm-palisade tissue on the abaxial side. Internal secretory organs do not occur either in the leaf or axis.
