CHAPTER IX 



The Structure of the Leaf 



The blade of a dorsiventral foliage leaf exhibits the following 

 general structure in transverse section. Beneath the colourless 

 epidermis (Fig. 51, Ep) of the upper side are one or more laj'ers 

 of vertically elongated cells constituting the palisade tissue [Pa), 

 which is especially concerned with carbon dio.xide assimilation ; 

 its cells are deep green owing to the numerous chloroplasts. 

 Between the palisade layer and the lower epidermis lies the 

 loose " spongy " tissue (Sp.), which is composed of irregular cells 

 separated by many and often conspicuous intercellular spaces 

 [In.). This tissue contains fewer chloroplasts and communicates 

 with the external atmosphere by way of the stomata, its chief 

 function being to facihtate gaseous exchange. Pahsade and 

 spongy tissues together constitute the thin-walled ground-tissue, 

 or mesophyll, of the leaf. Here and there the section will pass 

 through veins, some cut transversely, others obliquely or longi- 

 tudinally ; the veins include the vascular tissue with xjdem 

 towards the upper and phloem towards the lower side, and each 

 is surrounded b3' a well-defined layer of cells, the hmdle-sheath 

 [Sh). 



Good material for a detailed study of the structure of an 

 ordinary leaf is furnished by the Fuchsia. The features of the 

 epidermis have been fuUy described in the previous chapter and 

 require no further mention . The palisade cells are four to six times 

 as long as broad and form a single layer (Fig. 51, Pa). They are 

 attached on the one hand to the upper epidermis, and on the other 

 to the rounded ceUs constituting the uppermost layer of the 

 spongy parenchyma. Narrow intercellular spaces, extending the 

 whole depth of the palisade laj'er, occur at intervals between 

 the cells, but these spaces are only apparent here and there 



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