PLANTS OF THE INDIAN DESERT. 221
The vascular system consists of numerous peripheral bundles
seated on sub-epidermal stereome girders in all members except S.
quinquefarius and of a few inner bundles embeded in the ground
tissue. Peripheral bundles are of two sizes, larger ones alternating
with the smaller. Smaller bundles in species of Cyperus in F. tenera
and in S. maritimns are enclosed in an outer ring of stone-cells and
in an inner ring of green sheath-cells ; larger bundles are enclosed in
a single ring of stone-cells which is many layered on the inner side.
In G. arenarius, besides the peripheral bundles seated on the girders
there are other small bundles not protected by any sub-epidermal girders
and alternating with the former. Smaller bundles in S. quinquefarius
(fig. 310) occur in the assimilatory tissue and are not apposed to the
sub-epidermal girders ; the larger ones are placed in ground tissue
strands.
The ground tissue consists of thin- walled parenchymatous cells and is traversed in its outer portion by vascular bundles ; it is not differentiated in the centre into pith except in C. rotundus in which the central tissue is composed of a few larger thin-walled cells. The ground tissue in S. quinquefarius is represented by one-layered plates of parenchymatous cells enclosing large air-spaces. Vascular bundles occur in the traversing plates. The occurrence of sclerenchyma in the ground tissue of S. quinquefarius is the result of its being found on wet ground.
General Review. — Epidermal cells have outer walls greatly thick- ened and silicified. Inner walls are angular and lateral walls are straight. The upper epidermis is modified to form the aritculation tissue in species of Cyperus and in F. tenera. Stomata occur only on the lower surface in species of Cyperus and in F. tenera where the upper epidermis forms the articulation tissue ; they are found on both the surface in S. maritamus in which the epidermis on either side is not modified into the articulation tissue. The guard-cells are elevated and are accompanied by subsidiary cells. The front cavity is either placed in a depression formed by the outer thickened epidermal walls or it is elevated above the surface. Stomata on the leaf as well as those on the axis are characterised by obconical cavities around which are also developed obconical groups of elongated obli- quely placed cells. The photosynthetic tissue in the leaf and axis consists of tabular palisade cells. Veins are numerous and are arranged in a single row or in two rows larger veins alternating with the smaller. Smaller veins and vascular bundles in the axis in species of Cyperus and in F. tenera are enclosed in an outer ring of stereome and in an inner ring of sheath-cells ; larger veins are enclosed
