The lateral walls are thin and undulated. The inner walls are thin in all members except in G. varians, where they are a little thickened.
The stomata are placed in depressions produced by the thickened outer epidermal walls in L. spartium. In G. varians, C procera on the lower surface and P. cynanchoides the guard-cells are elevated and the front cavity is situated on a level with the surface. The stomata % on the upper side in C. procera are placed in chimney-like depressions and the guard-cells are situated quite below the plane of the epidermal cells (fig. 197). Guard-cells are accompanied by subsidiary cells in all the members. The elevated position of the guard-cells is due to the fleshy character of the leaves. The depressed position of stomata in L. spartium and of those on the upper side in G. procera may be due to the poorly developed ventilating system and to the occurrence of compact palisade tissue on the upper side respectively.
The mesophyll is isobilateral in L. spartium, while in P. cynan- choides it consists wholly of arm-palisade tissue. It is composed of palisada tissue on the upper side and of arm-palisade tissue on the lower in G. varians and G. procera. In fleshy leaves the arm-palisade tissue is richly provided with a system of intercellular spaces. Chloro- phyll grains are spherical and fairly large and are found along the walls of assimilatory cells.
Internal secretory organs are represented by secretory cells with tanniniferous contents near the veins in P. cynanchoides. The veins are few, embedded and are not provided with bundle-sheaths.
The plants are entirely or almost leafless, or have fleshy leaves. This makes the development of a hairy covering quite unnecessary, except on young leaves and axes. Clothing hairs, present on young leaves and axes, may disappear, when they are fully developed. Short uniseriate trichomes, with verrucose walls and with a large terminal cell, occur though not in great number on both the surfaces of leaves of G. varians and L. spartium. C. procera has short, uniseriate, de- pressed trichomes which are more numerous on the lower surface (fig. 198).
Structure of the Axis. — The epidermis consists of small polygonal cells with outer walls thickened and convexiy arched outwards. It is two-layered in P. cynanchoides and is three-layered in L. spartium (fig. 200). The epidermal cells in S. brevistigma are longer than broad, thus making the epidermis quite compact and rigid. The stomata are situated in depressions produced by thickened outer epidermal walls. The guard-cells are in the plane of the surrounding cells in all members except S. brevistigma where they occur quite below the plane of surrounding cells, this producing a chimney-like pit above. The depressed position of guard-cells is necessary in the 640—2
