296 THE JOURNAL OF INDIAN BOTANY.
Tiliaceae have the property of absorbing and retaining water, and may serve indirectly as a water-storing tissue. Secretions of mucilage thicken the water in the leaf and render it less easily transpired.
Deposits of calcium carbonate in the walls of clothing hairs of Cruci ferae and Boraginaccae may absorb moisture by their hygroscopic property and add to the water supply of the plant. The abundance of silica in the outer walls of the epidermal cells in Gyperaceae and Gramineae affords effective protection against the attacks of animals and adds to the rigidity of the plant which is necessary against the strong winds of the desert.
Sclerotic Modifications of the Pericycle. — The pericycle is com- posed either of a composite and continuous ring of stone-cells or of bast fibres, or of large closely placed groups of stone-cells or bast fibres, or of a loose ring of stone cells or bast fibres- In a few cases it consists of a few isolated stone-cells or bast fibres.
A sclerenchymatous pericycle is not developed in a few orders as will be mentioned below. The abundant development of scleren- chyma in the pericycle is due to the arrest of parenchyma, owing to the deficiency of water. This can be seen from the fact that parenchyma is abundantly developed and that a sclerenchymatous pericycle is altogether absent in fleshy plants of Violaceae, Portulaceae, Burseraceae, Bosaceae and Gentianaceae. A sclerenchymatous pericy- cle serves as a supporting tissue against the strong winds of the desert.
In some species of Gompositae, Verbenaceae, Amarantaceae and Chenopodiaceae the pericycle presents an isobilateral symmetry which is caused by unequal development of pericyclic sclerenchyma in the two planes, larger groups of stone cells or of bast fibres being developed in the plane which corresponds with the direction of the prevailing wind. In inclined axes of some species of Zygophyllaceae, Papilion- aceae and Convolvidaceae the tissue of stone-cells or of bast fibres is abundantly developed on one side which seems to be the upper side of the axis, while on the other side the pericycle consists of a few isolated small groups of stone cells or of bast fibres. The abundant development of a supporting tissue on the upper side is necessary to protect the axis from the stretching strains on the upper side and consequently from bending.
The different types of the pericycle that occur in the axis of the desert plants are as follows : —
(a) Isolated stone-cells or small groups of stone-cells, e.g. Elatineae, Boraginaceae, and Convolvidaceae,
