THE DIFFERENTIATION OF THE PLANT-BODY 23 
whose walls are marked with spiral thickenings. Inside 
Fia. 20,—TRANSVERSE SECTION OF Fig, 21.—Sxction or Stem oF Moss, 
Stem oF Sphagnum, SHOWING CENTRAL STRAND oF 
THIN-WALLED CELLS SURROUNDED 
BY CorTEX AND EpmpErMis. Tur 
WALLS OF THE OUTER CELLS oF 
THE CORTEX ARE CONSIDERABLY 
THICKENED. (After Sachs.) 
these a further protective layer of small cells with uni- 
formly thick walls is met with (fig. 20). In the smaller 
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Fra. 22.—Transverse Srction of THE BLADE oT A LEAF, SHOWING THE 
OvuTER WALLS oF THE EPIDERMAL CELLS THICKENED AND CUTICULARISED. 
x 100. 
mosses the outer layers of the cortex are thickened 
(fig. 21). 
In the higher terrestrial plants we have evidence of 
