veins except the vertically transcurrent ones in G. niveus, where it forms arcs of palisade cells on the sides of the veins (fig. 298). In the leaf-blade of S. maritimus tubular palisade cells form groups below the epidermis, the adjacent groups being separated by a tissue of loosely arranged arm-palisade cells which are afterwards destroyed to form cavities alternating with the veins. In C. rotundas there is a tissue of thin-walled parenchymatous cells occurring below and between the veins and similarly forming cavities below the stomata. Cells above the zone of the palisade tissue in species of Cyperus and those on either side of the veins in S. maritimus form the aqueous articulation tissue with a two-fold function, that of collecting water and that of regulating the upwards curving movements of the leaf-halves. The abundance of the aqueous articulation tissue in species of Cyperus can be accounted for by the abundance of veins. In F. tenera leaf-blades are thin and the veins together with their girders of palisade cells and a few secretory cells occupy the whole space between the epidermis on either side.
In the leaf- sheaths of S. maritimus palisade cells form small groups on the sides of the isolated sub-epidermal girders. The veins, except those at the angular portions, are separated by a tissue of stel- late cells which, later on, are destroyed and cavities are formed. The occurence of these lysigenous cavities in abundance is the result of the water-supply being insufficient to maintain all the tissues in a fully developed leaf-blade. The thin-walled parenchymatous cells do not form any important tissue ; they, therefore, lose their turgidity, shrivel up and are destroyed to form cavities.
Internal glands are represented by numerous secretory cells with tanniniferous contents near the veins in the leaf-blades and leaf- sheaths and in neighbourhood of vascular bundles in the axis. A few secretory cells occur in the ground tissue of the axis.
Veins are numerous and are confined to the lower half of the mesophyll except in F. tenera and S. maritimus,' in which they are placed in the middle of the mesophyll. Larger veins usually alternate with smaller ones and when veins occur in two planes those in one plane alternate with those in the other (fig. 300). This gives rigidity to the leaf-blade and accommodates a larger number of veins in a small space of the lower half of mesophyll. In species of Gyperus and in
F. tenera smaller veins are enclosed in two rings, the outer one of stereome and the inner one of green sheath-cells. Larger veins are enclosed in a single outer ring of stereome and the inner ring is represented by two arcs of sheath-cells. Some of the larger veins in
G. nivens are vertically transcurrent above and below by stereome bundles (fig. 298). In F. tenera a single large vein occurs in the
