Page:The Journal of Indian Botany.djvu/800

292 THE JOURNAL OF INDIAN BOTANY. (2) With a uniseriate stalk and with a head divided by a

single vertical wall, e.g., Verbenaceae.

(3) With a uniseriate stalk and with a head divided by

horizontal and vertical walls, e.g., Cucurbitaceae and Acantkaceae.

(4) With a uniseriate stalk and with a head irregularly

divided, e.g., some Compositae and Solanaceae.

(5) With a biseriate stalk and with a head divided by

vertical walls, e.g., some Qompositae. (e) Shaggy hairs. —

(1) With a multicellular stalk and with a multicellular head,

e.g., Capparidaceae and Elatineae.

(2) With multicellular stalk with a biseriate head, e.g., some

Compositae.

(/) Depressed spherical salt-glands. —

Placed in pits and divided by a single vertical and horizontal wall with two epidermal subsidiary cells at the base, e.g., Tamariscineae.

It is very difficult to employ glandular hairs for systematic pur- poses. Glandular hairs in the first place do not belong to any con- stant form in a species. The head, especially, varies in structure a great deal ; it may be unicellular, uniseriate or biseriate on the same leaf. Besides, the nature of secretion cannot be determined with certainty, especially in herbarium specimens.

External glands are, of course, valuable to the desert plants. They pour their secretions on the surface of the leaf and assist the clothing hairs in protecting the plant structures against intense heat and light, reflected from the sand. In the case of salt-secreting glands in Tamer iscineae, the pits in which they are situated are filled with secretions of hygroscopic salts which absorb moisture from the atmosphere and supply it to the plants in dry season.

Water Storage Tissues. — Contrivances for water-storage are characteristic of desert plants. Water-storing cells occur in the epidermis, in the middle of the mesophyll in the palisade tissue, in cortex, in pith and in the form of water-storing tracheids.

Large water storing cells bulging inwards and outwards, are intercalated amongst ordinary epidermal cells in some of the species of Capparidaceae, Elatineae, Malvaceae, Zygohyllaceae, Moringaceae, Papilionaceae, Ficoideae, Boraginaceac, and Polygonaceae. Water- storing epidermal cells have the inner walls very thin and convexly arched inwards, so that they come into close contact with the assi- milatory tissue, water being thus quickly supplied to it.