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 aque- 

 ous tissue. 



As regards the function of the sheath, it may merely form a pro- 

 tective 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 en- 

 closed 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 



