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, 



