THE LEAF 



217 



seen in a microscope. The upper and the under surface are 

 formed by flat cells with thick walls. This is the outer skin or 

 epidermis (a)* Between the two skins there is a layer, more or less 

 thick, of soft and green tissue, the upper part of which consists 

 of oblong cells, arranged at right angles to the surface, and 

 placed so evenly parallel to each other that they have been com- 

 pared to the pales of a fence. They are called palisade tissue (6). 



d s 



Fig. 198.— Vertical section of a leaf (320 times enlarged). 



a. Epidermis, b. Palisade tissue, c Spongy tissue. 



d. A vein or rib. S. Stoma. 



Below the palisade tissue is another of quite a different form, 

 consisting of cells that are not so closely packed, but have large 

 air-spaces between them, like a sponge. They form the spongy 

 tissue {€). The illustration also shows a bundle of other cells 

 in the middle (cl). These constitute a vein running through, 

 and supporting, the blade of the leaf. In the cells of the 

 palisade and spongy tissues we can see a number of small, green 

 spots. These denote the chlorophyll-granules that give the 

 leaves their green colour. The palisade tissue contains a much 

 greater quantity of them, and this is the reason why leaves are 

 generally dark-green on the upper, and light-green on the under 



