THE DIFFEKENTIATION OF THE PLANT-BODY 23 

 whose walls are marked with spiral thickenings. Inside 



Fro. 20. TRANSVERSE SECTION OF 

 STEM OB 1 Sphagnum. 



FIG. 21. SECTION OF STEM OF Moss, 

 SHOWING CENTRAL STRAND OF 

 THIN-WALLED CELLS SURROUNDED 

 BY CORTEX AND EPIDERMIS. THE 

 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 



Fia. 22. TRANSVERSE SECTION OF THE BLADE OF A LEAF, SHOWING THE 

 OUTER 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 



