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A TEXTBOOK OF THEORETICAL BOTANY 



Between the upper and lower epidermis lies the tissue called collectively 

 the mesophyll. It is not normally homogeneous but is differentiated 

 into two layers. The upper layer is formed of cylindrical cells lying closely 



W." -aS' 



, Subsidiary cell 







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^ 



I •:. 



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Fig. 985. — Seduiu spathijoliiiui. Epidermal section of 

 the leaf from the lov\er surface showing stomata 

 with subsidiary cells. 



together, with their long axes perpendicular to the leaf surface. This is called 

 the palisade layer, from its fancied resemblance to a row of fence slats 

 (Fig. 986). Its cells are rich in chloroplasts which form a practically con- 

 tinuous layer on the long walls. Between the cells are narrow intercellular 

 passages. 



The lower portion of the mesophyll consists of very irregularly shaped 

 cells which are only in contact at their protuberances and among which, there- 

 fore, there is a continuous svstem of large intercellular spaces. This is given 

 the apt name of the lacunar or spongy mesophyll. Its cells also contain 

 chloroplasts, but not usually so many as in the palisade layer. The dis- 

 tinction of these two layers is clearest in Dicotyledons and is often lost in 

 Monocotyledons, where the whole mesophyll may be spongy. 



At the level where the two mesophylls are in contact lie the vascular 

 bundles or veins of the leaf (Fig. 987), varying in size between the massive 

 midrib, which may show some secondary thickening, down to single tracheids 

 at the extremities of the smallest veins. Thev are normallv orientated with 



