PLANTS OF THE INDIAN DESEKT. 287
an extensive aqueous tissue. In the flat leaf-blades with the assimilatory tissue either bifacial or isolateral, there is sometimes a tissue in the middle of the mesophyll and formed of polygonal cells.
(a) either colourless and aqueous in some Cruciferae, Salva-
doraceae and some Boraginaceae, {b) or assimilatory in some Cruciferae, Asclepiadaceae, and
Convolvulaceae, (c) or glandular in some Boraginaceae and Polygonaceae. Some of the palisade cells in Zygophyllum, Geraniaceae and Rhamneae are faintly green in colour and may form water storing cells. In many species there occurs a sheath of almost cubical efferent cells, usually green or sometimes colourless, round the veins. The sheath as it usually contains chlorophyll, may assist the assimilatory tissue in photosynthesis, as well as it may form a protective and efferent tissue.
Veins : — The mid-vein is generally vertically transcurrent above and below by means of sclerenchyma, collenchyma or colourless parenchyma, it is usually prominent below and is either grooved or prominent above. In many species, as will be mentioned below, the veins of the ribs being usually vertically transcurrent above and below by sclerenchyma or collenchyma. The smaller veins are either embed- ded or are vertically transcurrent above and below or apposed to the epidermis on one side and vertically transcurrent by sclerenchyma or collenchyma on the other. The veins in the majority of cases are enclosed in sheaths of largo parenchymatous cells, either containing chlorophyll, or being glandular, or colourless or with crystals. In flattened leaf-blades. —
(a) either all veins are vertically transcurrent above and below (i) by sclerenchyma in Tiliaceae, some Papilionaceae, some Boraginaceae, some Cyperaceae and some Gram- ineae (ii) by collenchyma in some Malvaceae and some Papilionaceae (iii) by aqueous cells in some Cyperaceae (iv) and by colourless parenchyma in Celastraceae, {b) or only the larger veins are vertically transcurrent above and below by colourless parenchyma and collenchyma respectively in Elatineae and some Convolvulaceae,
(c) or only the veins of the ribs of the leaves which are many- ribbed are vertically transcurrent above and below (i) by collenchyma in Sapindaceac and Gentianaceae (ii) by colourless parenchyma in Labiatae (iii) by aqueous tissue above and by collenchyma below in Geraniaceae (iv) and
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