THE SPRUCE FIR 249 



that the diagonals of the square are vertical and 

 horizontal (see Fig. 101). The epidermis has a thick 

 outer wall, which is cuticularised. The stomata are 

 arranged in lines along each of the four flat surfaces 

 of the leaf. The guard-cells are sunk below the 

 surface, and are almost covered in above by two very 

 thick-walled epidermal cells, so that scarcely anything 



FIG. 101. Transverse section of a leaf of Picea excelsa. s, 

 stomata ; r, resin canal. In the middle observe the 

 central cylinder with a single vascular bundle, in which 

 the xylem is directed upwards. Magnified 82. (R. S.) 



can be seen of the stoma itself in a surface view of 

 the leaf. In a transverse section (see Fig. 102) there 

 appear to be two pairs of guard-cells, one above the 

 other, but only the lower cells belong to the stoma, 

 and they alone take part in the opening and closing 

 of the pore. Beneath each stoma is a large inter- 

 cellular air-cavity. 



