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 assimila- 

 tory 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. 



i 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 collen- 

 chyma 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 affect- 

 ed 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 perfora- 

 tions. 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 



