Page:The Journal of Indian Botany.djvu/796

288 THE JOUENAL OP INDIAN BOTANY. apposed to the epidermis above and with sclerenchyma below in Commelinaceae,

(d) or the veins are vertically transcurrent only above by col-

ourless parenchyma in Sterculiaceae and by collenchyma in Bosaceae,

[e) or the veins are apposed to the epidermis above and below

with sclerenchyma above and below respectively in some Tiliaceae and some Gramineae, (/) or the veins are embedded in the flat leaves of other species. In the centric leaves of Chenopodiaceae a ring of chlorenchy- matous cells, resembling sheath-cells, occurs immediately outside the peripheral veins. The more centrally placed veins traverse the aqueous tissue.

As regards the function of the sheath, it may merely form a protective envelop to the veins resembling endodermis ; but as it usually contains chlorophyll it may assist the photosynthefcic tissue as well as it may form an efferent tissue. The veins in Gyperaceae are enclosed in a ring of stone-celis outside the sheath ; the ring of stone-cells seems to form a protective envelope.

Water-storing tracheids occur at intervals between the veins in some Capparidaceae, Tamaricineae, some Tiliaceae, Simarubaceae, Bhamneae and Salvador aceae. The occurrence of water-storing tra- cheids in these orders is brought about by their being found usually in the driest portions of the desert. The occurrence of veins in two planes in Caryophyllaceae and Cy per aceae affords room for the deve- lopment of a larger number of veins and at the same time gives rigidity to the whole blade.

Hairy covering. — The hairy covering consists of clothing and glandular hairs. The clothing hairs may be : —

{a) unicellular, either simple or specialised, (b) uniseriate, either simple or specialised, (c) peltate, {d) stellate, (e) tufted, (/) of the candelabra type and (g) shaggy.

(a) Simple unicellular hairs may be papillose, elliptical or dome- shaped with walls smooth, muriculate, verrucose, rugose or calcified. They occur in some Elatineae, some Malvaceae, Zygophyllaceae, Sima- rubaceae, Sapindaceae, Moiingaceae, Mimoseae, Lythraceae, Ficoideae, some Bubiaceae, Salvador aceae, Boraginaceae some Scrophulari aceae, some Acanthaceae, some Verbenaceae, some Euphorbiaceae and some Gramineae. Special forms of hairs are either hooked as in some Poly- galaceae or two-armed as in Craciferae. Simple unicellular stiff or woolly hairs of greater length and with walls smooth or muriculate