408 BRYOPHYTES (MOSS PLANTS) 
under surface, cells are differentiated into thread-like structures 
called rhizoids, which attach the plant to the substratum. In 
the notches about the margin are cells which function like the 
meristematic cells of the higher plants, and thus have to do with 
the addition of new cells whereby the growth of the plant is 
maintained. The cells of the upper region of the thallus are 
differentiated into an epidermis, which affords protection against 
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A, section through a thick portion of a thallus, showing the following features: 
the upper epidermis and the chlorenchyma tissue (ch!) just beneath divided 
into chambers by partitions (0); the layers of cells (p) between the chloren- 
chyma and lower epidermis, giving thickness and rigidity to the thallus; and 
the lower epidermis with rhizoids (h) and scale-like plates of cells (b). B, sec- 
tion near the margin of the thallus and more highly megnified, showing the 
following features: upper epidermis (0); a chamber of chlorenchyma tissue 
(chl) bounded by the partitions (s) and into which the chimney-like air pore 
(sp) opens; the lower epidermis (u); and two layers of supporting tissue (p). 
From J. M. Coulter, originally after Goebel. 
evaporation, and into tissues which utilize the air and sunlight 
in manufacturing carbohydrates. Highly magnified sections 
through a thallus are shown in Figure 864. In the epidermis are 
many chimney-shaped pores which permit the air to reach the 
filaments of food-making cells in the chambers beneath. 
On the thalli shown in Figure 363 are also shown some small 
cups and some erect stalks with expanded tops. These struc- 
