PLANTS OF THE INDIAN DESEKT. 287 



an extensive aqueous tissue. In the flat leaf-blades with the assimi- 

 latory 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 effer- 

 ent 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 paren- 

 chyma, it is usually prominent below and is either grooved or promi- 

 nent 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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