294 THE JOURNAL OF INDIAN BOTANY.
by a dense covering of hairs and by secretions from the internal glands which thicken the watery contents in the plant tissue, so as to be less easily transpired.
Secretions. — The most common form of secretions is tannin which shows itself by its yellowish colour and which occurs both in the leaf and axis, or only in the leaf or axis. Another secretion of usual occurrence is oxalate of lime in various forms of crystals and occurring abundantly both in the leaf and axis. Silica is abundantly deposited in outer epidermal walls of species of Cyperaceae and Gra- mineae. Deposits of carbonate of lime are found in the walls of clothing hairs of some species of Boraginaceae and in cystolith-like structures in the basal epidermal cells of clothing hairs of some species of Gucurbitaceae, Boraginaceae and in the enlarged epidermal cells of the leaf of some species of Acanthaceae. Mucilaginous secretory organs occur in a few orders as will be mentioned below. The epi- dermal cells of the leaves of Zizyphus contain cellulose slime. Balsam canals occur in the phloem of vascular bundles in Burseraceae. Con- tents of some of the other secretory organs in the leaf and axis cannot be determined with certainty as I had to deal with herbarium material.
The different types of secretory organs that occur in the desert plants are as follows : —
(a) Oxalate of lime. —
(1) Clustered crystals e.g., Violaceae, Polygalaceae, Portula-
ceae, Elatin'xe, Malvaceae, Tiliaceae, Zygophyllaceae, Rhainneae, Sapindaceae, Moringaceae, Rosaceae, Lyth- raceae, Onagraceae. Ficoideae, Asclepiadaceae, Borag- inaceae, Convolvulaceae, Amarantaceae, Chenopodia- ceae and Polygonaceae.
(2) Solitary crystals, e.g., Sierculiaceae, Tiliaceae, Simarub-
aceae, Burseraceae, Boraginaceae, Convolmdaceae and Euphorbiaceae.
(3) Acicular raphides e.g., Flcoideae, Rubiaceae, Compositae,
Nyctaginaceae and Commelinaceae.
(4) Styloids e.g., Ficoideae.
(5) Clusters of acicular crystals, e.g., Menispermaceae and
Gentianaceae.
(6) Crystal sand, e.g., Rubiaceae, Solanaceae, Acanthaceae
and Nyctaginaceae-
(b) Calcium carbonate deposits, e»g., walls of hairs of Cruet'
ferae and Boraginaceae.
