THE AXGIOSPERMAE : LEAVES 1019 



The Mesophyll. 



Typically the leaf mesophyll consists of one or more layers of prismatic 

 cells arranged anticlinally, with narrow spaces between them, constituting 

 the palisade layer, while below this lies the lacunar or spongy tissue, 

 made up of irregularly shaped cells, between which is a connected system of 

 large intercellular spaces, which almost completely surround the cells and 

 contain an internal atmosphere which is kept in communication with the 

 exterior air through the stomata. 



The elongation of the palisade cells has been compared with tiie elongation 

 of epidermal cells into hairs, and it has been suggested that both may be 

 due to the operation of similar factors, such as strong light or rapid transpira- 

 tion associated with exposure, which check the growth of the leaf in area 

 and promote the lateral extension of the cells. There may be some truth in 

 this, but it is obviously not a complete explanation and further information 

 is needed. 



The fundamental structure of the mesophyll is found in all normal 

 dorsiventral leaves, both dicotyledonous and monocotyledonous, though in 

 the latter the palisade layer is usually less developed than in the former. 

 Leaves of the grass type, however, are exceptional. They have parallel veins 

 which lie close together, with ribs of sclerenchyma above and below each 

 vascular bundle. This cuts up the mesophyll into a number of independent 

 strips, in each of which there are, at the top and the bottom respectively, 

 two belts of tissue containing chlorophyll, while the central portion is some- 

 times lacunar in structure, but more often composed of thin-walled, colourless 

 cells which store water. 



Dicotyledons may also have a palisade on both sides of the mesophyll. 

 This is formed, for example, in plants with a normally vertical leaf position 

 and in others which grow under such conditions {e.g., on sand) that the under 

 sides of the leaves receive strong lighting (Figs. 1007 and 1008). Such leaves 

 are called equifacial, or sometimes isobilateral. A further modification of leaf 

 structure is that called centric. The extreme type of centric leaf is cylindrical, 

 with all the mesophyll tissue arranged radially round one or more centrally 

 placed veins (Figs. 1009 and 10 10). A less extreme type is found in some 

 species, especially of Grasses and Sedges, in which the leaf as a whole is 

 bilateral, but the veins are surrounded by radiating palisade cells, examples 

 being Papyrus and Portulaca. The truly centric leaf is really a special case of 

 equifacial development, but there is another closely similar type of centric leaf 

 which is derived in quite a different way, by the suppression of the upper 

 surface during development, and the overgrowth of the lower surface, which 

 thus surrounds the whole leaf. They are really extreme cases of the rolled or 

 folded leaf. Such leaves are called unifacial (see Fig. ion). They may be 

 cylindrical, as in Jiincus and Scirpiis, or they may be flattened in the vertical 

 plane, as in Kochea falcata, but they are always recognizable by the fact that 

 in cross section they show a line of vascular bundles, usually cur\-ed into a 



