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A TEXTBOOK OF THEORETICAL BOTANY 



horse-shoe form or into an almost complete ring, instead of the centrally 

 placed bundles of the truly centric leaf. 



The lacunar structure of the mesophyll, though most marked in the spongy 

 tissue, extends to the palisade also, between all the cells of which run clefts 

 connecting with the spaces in the spongy layer. The cells are normally rounded 

 in cross section, and a view perpendicular to the leaf epidermis shows clearly 

 the extent of the intercellular space system in the palisade, which is not so 



Fig. 1007. — Eryugiuin niaritinnnii. Transverse section of 

 leaf showing equifacial structure. 



apparent when viewed in the usual transverse leaf section. The size and 

 closeness of the palisade cells varies with external conditions. In leaves 

 growing in sunshine they are long, large, and close together, and in leaves in 

 heavy shade they tend to become short, thin, and loosely arranged. The 

 same is true of the spongy tissue, and the total volume of intercellular space 

 may be twice as great in a shade leaf as in a sun leaf of the same species. 

 The average volume of the internal space is about 20 per cent, of the leaf 

 volume. The internal area of cell surface exposed in the mesophyll is difficult 

 to estimate with any accuracy, but investigations by Turrill established that 



