PLANTS OF THE INDIAN DESERT. 297
(b) A loose ring of small groups of stone-cells e.g. Elatineae.
(c) Large closely placed groups of stone-cells e.g. Menisper-
maceae, Capparidaceae, Caryophyllaceae, Tamariscineae, Malvaceae, Sterculiaceae, Tiliaceae, Zygophyllaceae, Sim- arubaceae, Celastraceae, Cucurbitaceae, Ficoideae, Com- posltae, Salvador aceae, Asclepiadaceae, Boraginaceae, Convolvulaceae, Verbenaceae, Labiatae, Amarantaceae, Chenopodiaceae, and Polygonaceae.
(d) A composite and continuous ring of stone-cells, e.g. Sapin-
daceae, Papiliouaceae, Caesalpineae, Mimoseae, Cucurbi- taceae, Ficoideae, Covipositae, Convolvulaceae, Verbenaceae, Liliaceae, and Commelinaceae. (2) Isolated bast fibres or small groups of bast fibres, e.g. Cruciferae, Scrophulariaceae, Acanthaceae, Nyctaginaceae and Euphorbiaceae. (/) A loose ring of small groups of bast fibres, e.g. Lythraceae, Bubiaceae, Solanaceae, Scrophulariaceae, Nyctaginaceae and Euphorbiaceae. (g) Large closely placed groups of bast fibres, e.g. Cruciferae,
Sterculiaceae and Asclepiadaceae. (h) A composite and continuous ring of bast fibres, e.g., Polygalaceae, Bhamneae and Papilionaceae. In addition to the sclerenchymatous pericycle small groups of stone cells and of bast fibres occur in the soft bast of some species of Bhamneae and Malvaceae, respectively.
In Melhania Denhami the pericycle is represented by an outer composite and continuous ring of stone cells and of an inner ring of groups of bast fibres with parenchymatous pericycle intercalated between them. The composite and continuous ring of bast fibres in Polygala erioptera is stratified.
For employing characters of the pericycle for systematic purposes, it is necessary to follow the progressive development as well as the modifications which it undergoes. For differentiation of the pericycle may vary in branches of different thickness.
Conducting system. — The wood usually form3 a composite hollow cylinder. Vessels are less abundantly developed in desert plants than in ordinary mesophytes. This is quite natural, when all the tissues in desert plants are adapted to check transpiration, owing to the defi- ciency of water. The vessels are embedded in a ground tissue of wood prosenchyma which is usually formed of cells with thickened and lignified walls and with small lumina. The occurrence of ligni-
