Page:The Journal of Indian Botany.djvu/284

 Primary cortex is characterised by assimilatory tissue and collenchyma. The assimilatory tissue is formed of palisade cells in C. Burhia ; in others it is chlorenchymatous. The collanchyma is developed in the ribs of the ribbed axis. Eibs are strengthened by sclerenchyma in I. anabaptista, P. odorata and P. trilobus. Cork was not developed in any of the species examined. The endordemis, when differentiated, consists of tabular cells.

The pericycle is composed of a composite ring of bast fibres in species of Heylandia, Indigo/era, Psoralea and Phaseolus. It forms a composite ring of stone-cells in species of Crotal aria, Tephrosia, Alysicarpus and Bhyncosia. In species of Heylandia and Phaseolus groups of bast fibres are developed along three-fourth of the circum- ference of the axis, while along the remaining portion the pericycle is parenchymatous. This is curious and suggests that the axes are much inclined and that sclerenchyma is developed only on the upper side of the inclined axis. Small groups of bast fibres occur in soft bast of Alysicarpus vaginalis.'

The perforations of vessels are simple. Size of lumen, abundance and arrangement of the vessels vary in different genera and even in species. These differences may be useful in diagnosis of genera and species, if- due allowance is made for modifications introduced by con- ditions of the soil.

The vascular system, as a whole, is characterised by the poor development of vessels as regards size and abundance, by abundance of wood prosenchyma and by poorly developed wood parenchyma. These modifications are due to the arrest of the development of parenchyma owing to deficiency of water. A system of well developed water conducting tissue is not required in the axis of desert plants in which all structures are adapted to diminish transpiration. It should be observed that the size and abundance of vessels are usually inverse- ly proportional to each other.

Soft bast usually forms a continuous ring. It is characterised by the occurrence of tannin sacs in species of Indigo/era, Psoralea and Bhyncosia and by the presence of small groups of bast fibres in A. vaginalis.

Pith consists of thin-walled cells in species of Heylandia and Crotalaria, I. linifolia, I. argentia, species of Tephrosia, A. vaginalis, P. tribolus and B. arenaria ; and is formed of thick-walled and lignified cells in I. paucifolia, I. anabaptista, P. odorate and B. rhombifolia. Some of the pith cells in I. paucifolia and I. argentia hold tanninifer- ous contents. The pith composed of lignified cells adds to the rigidity afforded by the mechanical tissue ; when it is formed of thin-walled cells it may serve occasionally as an aqueous tissue.