THE DIFFERENTIATION OF THE PLANT-BODY 19 
be seen. The common bog moss (Sphagnum) shows its 
stem to have on the outside several layers of large empty 
cells whose walls are marked with spiral thickenings. 
Fig, 20.—TRANSVERSE SECTION OF Ftc. 21.—SEcTion or STEM oF Moss, 
Stem or Sphagnum. SHOWING CENTRAL STRAND OF 
THIN-WALLED CELLS SURROUNDED 
BY CoRTEX AND EpIpERMis. THE 
WaLLs OF THE OUTER CELLS OF 
THE CORTEX ARE CONSIDERABLY 
THICKENED. (After Sachs.) 
Inside these a further protective layer of small cells with 
uniformly thick walls is met with (fig. 20). In the smaller 
mosses the outer layers of the cortex are thickened (fig. 21). 
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Fic, 22.—TRANSVERSE SECTION OF THE BLADE OF A LEAF, SHOWING THE 
OvuTER WALLS OF THE EPIDERMAL CELLS THICKENED AND CUTICU- 
LARISED. x 100. 
In the higher terrestrial plants we have evidence of 
great specialisation for protective purposes, a special 
tegumentary system being developed, which varies ‘in 
