The peculiar isobilateral structure of the wood of G. varians is due to the fact that portions of the axis with greater development of wood represent the plane affected by the prevailing wind and possessing greater functional activity ; the two opposite sides of the axis with poorly developed wood represent the plane less affected by the prevailing wind and have no necessity of developing an extensive water-conducting or strengthening tissue. The plane with well developed wood represents the direction of the prevailing wind. The wood parenchyma is not extensive and is found on the inner side of the wood ring.
The soft bast is very extensive in P. cynanchoides.
The pith consists of thin-walled cells. Some of the pith cells of S. brcvisligma contain spherical granular bodies.
General Review. — The epidermal cells have the outer walls greatly thickened. The guard-cells are situated in the same plane or quite below the plane of the surrounding colls. The stomata are accom- panied by subsidiary cells. The ventilating system is fairly extensive. Secretory cells with tanniniferous contents are found in some members. Oxalate of lime is found in the form of small clustered crystals in P. cynanchoides. Hairs, when present occur in the form of short uniseriate trichomes. Cortical parenchyma is extensive. The pericycle is either composed of groups of stone-cells or of bast fibres. The wood forms a composite hollow cylinder. Perforations of the vessels are simple. The wood prosenchyma is composed of cells with thick walls and with small lumina. The medullary rays are 1 — 3 seriate. Pith consists of thin-walled cells.
GENTIANACEAE
Enicostemma litorale BL— Fig. 201. Epidermal cells with outer walls very greatly thickened and with inner walls convexly arched inwards. Lateral walls thin and undulated. Stomata on both the surfaces elevated Mesophyll wholly of chlorenchyma. Internal glands absent. Oxalate of lime in the form of star-like clusters of acicular crystals. Leaves many-ribbed. Veins of ribs vertically transcurrent. Vascular bundles in leaf and axis bicol- lateral. Axis irregularly ribbed. Scleranchymatous pericycle absent. Cortex formed of chlorenchyma. Wood composite with an outer and inner ring of soft bast. Vessels in numerous complete rows. Medullary rays uniseriate. Pith of thin-walled cells.
Structure of the Leaf. — The epidermis consists of polygonal cells with outer walls very greatly thickened (fig. 201). The lateral walls are thin and undulated. The inner walh are. thin. The cuticle is
