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. TBANSVEBSE SECTION OF 

 STEM OF Sphagnum, 



FIG. 21. SECTION OF STEM OF Moss, 

 SHOWING CENTBAL STBAND OF 

 THIN-WALLED CELLS SUBBOTJNDED 

 BY COBTEX AND EPIDEBMIS. THE 

 WALLS OF THE OUTEB CELLS OF 

 THE COBTEX ABE CONSIDEBABLY 

 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. TBANSVEBSE SECTION OF THE BLADE OF A LEAF, SHOWING THF. 

 OUTEB WALLS OF THE EPIDEBMAL CELLS THICKENED AND CUTICU- 

 LABISED. 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 



