PS THE LEAF 73 
more numerous in the latter. Many mesophytic leaves have 
no stomata on the upper surface ; the rapid withering of the 
sunflower is partly due to the stomata on both surfaces, for 
transpiration is thus necessarily increased. The greater part 
of the section consists of cells with thin walls and abundant 
chlorophyll ; the two rows nearest the epidermis consist of 
elongated cells, and form what is called the palisade paren- 
chyma, for the cells are packed closely together without any 
Spaces, whereas the parenchyma of the under surface has 
intercellular spaces; consequently the under side of the leaf is 
paler than the upper, for there is less chlorophyll, owing to 
the fact that the cells are not so closely packed. The paren- 
chyma nearest the under surface of the leaf has intercellular 
Spaces, with which the stomata communicate ; this is called 
spongy parenchyma to distinguish it from the palisade tissue of 
_ the upper surface. The veins, or fibro-vascular bundles, form 
a network in the parenchyma; the wood of each bundle lies 
towards the upper surface, the bast towards the lower. 
_ This leaf by its structure is well adapted for assimilation, 
and allows of considerable transpiration. The thinness of 
the mesophytic leaf is a constant feature, for if the leaf were 
not thin some of the cells would be inaccessible by light, and 
would therefore not be able to assimilate. The thinness of 
the cuticle is another important characteristic, for on that 
largely depends the degree to which transpiration is possible. 
Structure of Vv ith the leaf of the Sunflower may be com- 
Xerophytic pared that of a Xerophyte, such as that of a grass 
oe growing on sandy seashores, therefore in a dry 
| situation. When the air is moist the blade of the leaf is 
Spread out, with its upper surface flat, or nearly so; but when 
the air is dry, owing either to a scorching sun or high wind, 
the leaves fold or roll up, so that the lower surfaces are alone 
exposed. The rolling up is so complete that at first sight one 
hardly realizes it is the leaf-blade which is so infolded—the 
appearance is rather that of a stem or stalk. 
The examination of a transverse section of the leaf-blade 
Shows that this infolding is due to some of the upper 
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