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 

 STEM OF 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.) 



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). 



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

 OUTER 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 



