STRUCTURE OF MESOPHYLL 309 



or xeromorphic plants), thoroughly well protected against 

 water loss, that can survive in these habitats. 



Structure of the Mesophyll. — The commonest type 

 of foliage leaf is the dorsiventral leaf, in which the upper 

 and lower sides are distinctly differentiated. In the 

 first place the stomata are usually present in much 

 greater numbers on, or are even confined to, the lower 

 surface of dorsiventral leaves. Secondly, the mesophyll 

 is differentiated into two strata — the palisade and the 

 spongy tissue. The palisade tissue lies immediately 

 below the upper epidermis.' It consists of from one to 

 several layers of cylindrical or prism-shaped cells with 

 their long axes perpendicular to the surface of the leaf, 

 and with narrow intercellular spaces between them.^ 

 The cytoplasm lining the side walls of the palisade cells 

 is packed with chloroplasts (Figs. 11, 50), the palisade 

 tissue being the main photosynthetic tissue of the leaf. 

 The spongy tissue occupies the space between the palisade 

 and the lower epidermis, and consists of rounded or 

 irregularly shaped cells with large intercellular spaces 

 between them. Under each stoma there is usually a 

 particularly large intercellular space. The spongy cells 

 contain chloroplasts, but not nearly so many as the 

 palisade cells. The spongy tissue is the main transpiring 

 tissue of the leaf. 



The more a plant is exposed to sun and dry air the 

 greater the development of palisade tissue, and in some 

 plants which Uve in dry sunny climates the whole of the 



" Sometimes separated from it by a layer of hypoderm {vtt6, below, 

 6ipiJ,a, skin), which often acts as a supplementary water tissue, and 

 is often thick walled and helps to strengthen the mechanical structure 

 of the leaf. 



" The palisade cells are generally in close lateral contact, so that 

 on a cross-section of the leaf no intercellular spaces may be visible. 

 But if a " fiat " section parallel to the leaf surface be made through 

 this tissue, the narrow intercellular spaces can always be seen between 

 the cells. 



