PLANTS OF THE INDIAN DESERT. 159
assimilatory tissue of palisade cells in both the species of Haloxylon and by the occurrence of an aqueous tissue in all the members. Collenchyma possesses chlorophyll and may function as an assimilatory tissue in S. foetida. Palisade cells form a continuous ring in H. salicornicum, while in H. recurvum the continuity of the palisade tissue is broken by groups of colourless palisade-like cells with perhaps a water-storing function.
The aqueous tissue is extensive in species of Haloxylon and it consists of a sub-epidermal layer of tabular aqueous cells and of a centrally placed tissue of polygonal cells surrounding the central vascular cylinder. The central aqueous tissue is bounded by a layer of cubical cells containing chlorophyll and resembling bundle-sheath cells ; and it is traversed by vascular bundles. The aqueous tissue in S. foetida is represented by a tissue of polygonal cells below collenchyma and it is not extensive.
In H. recurvum (fig. 288) there is cork developed just outside the ring of soft bast and it seems to be of pericyclic origin. It may have been developed to protect the poorly developed central vascular cylinder from desiccation. Pericycle consists of groups of stone-cells. Stone-cell groups in S. foetida are very small and isolated. Stone-cell groups in H. salicornicum are rhomboidal and present an isobilateral arrangement, the larger ones being developed in the plane most affected by the prevailing wind. In H. recurvum it presents a peculiar arrangement. It is composed of long broad and composite strands in the plane of gaps in the palisade tissue and of thin narrow strands or isolated stone-cells in other parts of the pericycle. The occurrence of stronger strands is perhaps necessary in the plane of gaps as the aqueous tissue in this plane is more extensive.
The vascular system is represented by peripheral bundles below the sheath-cells, by bundles traversing the aqueous tissue and by centrally placed bundles in species of Haloxylon ; in S. foetida it consists only of centrally placed bundles. Vascular bundles in the central cylinder are embedded in interfascicular wood prosenchyma of thick- walled cells in a concentric fashion, the largest ones being situated towards the pith. Vessels are small and have simple perforations. Medullary rays are not found.
In addition to a continuous peripheral thin ring of soft bast there are groups of soft bast on the outer side of the bundles, embedded in interfascicular wood prosenchyma.
Pith consists of thick-walled cells in.lH. recurvum; in other members it consists of thin-walled cells.
Oxalate of lime is found in the form of clustered crystals in some of the cells of the aqueous tissue of all the members. Besides
