39 



Tlie upper surface of the thallus is interrupted by 

 cup-like depressions called Gemmm cups. With the 

 aid of a lens the upper surface is seen to be divided 

 into small, diamond-shaped areas. The limits of the 

 areas are dark-green, the areas themselves being 

 greyish. In the middle of each area is a dot-like 

 opening surrounded by foiu' narrow cells containing 

 no chlorophyll and curved in the form of a 

 crescent. 



GENEEAL ANATOMY. 



In surface view the cells of the epidermis appear 

 to be polygonal. The under surface shows no divi- 

 sion into areas. 



A cross- section of the thallus beginning at the 

 dorsal (upper) surface, shows the following 

 structure : — 



1. Epidermis. 



This is very distinctly differentiated, and is inter- 

 rupted by large stomata of a peculiar form. The cells 

 are generally oblong or polygonal, and contain 

 numerous small chlorophyll grains. 



The Stomata. Each stomata stands in the 

 centre of a rhombic plate which overarches a 

 large air-cavity, and which is formed by the 

 upper cells becoming detached from those 

 underneath. The stoma is formed by the 

 simple separation from one another of four 

 or more epidermal cells which afterwards 



