I20 GENERAL BIOLOGY 



layer of cells that is protective to the moist internal tissues. 



This surface layer is epidermis. Examined with a lens, it 



will be seen to be marked out in minute polygonal areas all 



over the upper surface 

 of the thallus and on 

 close inspection a minute 

 pore will be seen in the 

 middle of each of these 

 areas. 



An examination of a 

 cross section of the thal- 

 lus, fig. 70 a, b, shows 

 clearly the relation of 

 parts. The thallus is a 

 large aggregate of cells, 

 and the cells are highly 

 differentiated. They 

 form two principal kinds 

 of tissue: 



Epidermis, of flat 

 transparent cells that 

 cover the whole exterior, 

 some of which are modi- 

 fied slightly in shape to 

 form the borders of the 

 breathing pores, and 

 others are modified very 

 greatly, being extended 



into long filaments, to form the rhizoids (fig. 70 b, e). 

 Parenchyma, of softer cells, richer in protoplasm, filling 



the whole interior and differentiated into two principal 



sorts (fig. 70 d, u and v) : 



i. Assimilatory parenchyma of smaller, thin walled cells, 



containing abundant .chlorophyl, and situated in an upper 



FIG. 70. Conocephalus. a, cross section of 

 the thallus, showing rhizoids and scales 

 beneath; b, a bit of the same more en- 

 larged; p, breathing pore in the upper 

 epidermis; r, rhizoids; s, scale; t, one of 

 the areas about a pore (empty) ; v, common 

 parenchyma; c, a breathing pore, as seen 

 from the surface; d, details of a single 

 area; p, pore; u, assimilatory parenchyma; 

 v, common parenchyma, e, a single entire 

 rhizoid cell. 



