PLANTS OF THE INDIAN DESERT. 295
(c) Silica, e.g., Cyperaceae and Gramineae.
(d) Myrosin cells, e.g., Moringaceae.
(e) Balsam canals, e.g., Burseraceae. (/) Cellulose slime, e.g., Bhamneae.
(g) Cystolith — like structures with calcium carbonate deposits, e.g., Cucurbitaceae, Borginaceae and Acanthaceae.
(h) Secretory cells with mucilaginous membranes e.g., Tilia- ceae, Papilionaceae, Rosaceae and Convolvulaceae.
(i) Secretory cavities with a lining epithelium, e.g., Malvaceae, Sterculiaceae, Tiliaceae, Bhamneae, and Bubiaceae.
(j) Tannin.
(1) In the epidermal cells, e.g., Sterculiaceae and Bosaceae.
(2) In the mesophyll or in the different tissues of the axis,
e.g., Capparidaceae, Caryophyllaceae, Portulaceae, Malvaceae, Sterculiaceae, Gelastraceae, Bhamneae, Sapindacexe, Papilionaceae, Mimoseae, Cucurbitaceae, Bubiaceae, Gompositae, Asclepiadaceae, Boraginaceae, Convolvulaceae, Nyctaginaceae, Polygonaceae, Euphor- biaceae, Commelinaceae, and Cyperaceae.
Crystals of different forms may occur in the same plant either in the same tissue or in different tissues. They usually occur in the lumina of the cells except in species of Nyctaginaceae in which crystal sand is deposited in the outer walls of epidermal cells. The employ- ment of different types of crystals for systematic purposes is not desirable, as solitary or clustered crystals may occur at different stages of development of the plant tissues.
Crystals of oxalate of lime occur usually near the veins and it appears that secretions of oxalate of lime gives acridity to the tissues of the plants and prevents the plants from being easily devoured by animals.
Tannin is abundantly found in different tissues of the plant usually near the veins ; and it seems to protect the plants in various ways. It gives acridity to the juices of the plant and prevents it from being easily devoured by animals. Tannin sacs, as they usually occur near the vascular bundles of the veins, pour their secretions into the water, which is consequently less easily transpired. Besides its general property of protecting the plant- tissues against desiccation, it may be used up in lignification of cells, as lignification is a prominent feature in the desert plants.
Cells with mucilaginous' membranes and secretory cavities which may also be mucilaginous in Malvaceae, Sterculiaceae and
862—39
