102 MESOPHYLL. [CH. VII 



the cellular tissue in which the chlorophyll bodies are con- 

 tained. The cellular tissue or parenchyma of the leaf is 

 known as mesophyll, and is one of the points in which the 

 two sides of the leaf differ from each other. On the upper 

 side of the leaf there is, beneath the epidermis, a charac- 

 teristic layer of cells known as palisade tissue, because in 

 a section they look like planks placed side by side to make 

 a paling. The figure (fig. 46) shows this aspect ; it also 

 shows that the palisade tissue is the part of the leaf 

 which contains the greatest amount of chlorophyll. The 

 lower half of the leaf is seen in the same figure to be 

 made up of cells of irregular form, so arranged as to leave 

 large spaces among them. Those who have lived in a 

 chalk country must have seen walls built of flints in which 

 the large irregular spaces are filled with mortar. If the 

 mortar is imagined to be air, and each flint a cell, an idea 

 is obtained of the structure of the lower layer of the leaf, 

 which from its loose texture is called spongy tissue. It is 

 the presence of the air in the spongy layer which gives 

 the lighter colour to the lower side of many leaves. This 

 may be proved by a simple experiment. A leaf of the 

 lesser celandine, or an arum leaf, is placed in water and a 

 strong inhalation is applied to the cut stalk held between 

 the lips. In this way air is sucked out of the leaf, 

 and water finds its way through the epidermis and takes 

 the place of the air which has been removed. The 

 moment at which it enters is clearly perceptible by the 

 change in colour, the lower surface turning dark green 

 as the water fills up the air spaces in the spongy tissue. 

 If the epidermis is stripped from the lower surface of a 



