118 



THE BIOLOGY OF A PLANT. 



Fro. 49.— Section showing the epidermis (ep) and the underlying sclerotic paren- 

 chyma (.s.p) of the rhizome of Pteris aquiUna. Canals, sometimes branching, are 

 everywhere seen. These served to keep the once-living cells in material con- 

 nection. 



Fig. 50.— Cross-section of sclerotic prosenchyma of the rhizome of Pteris aquilina. 

 The enormously thickened walls consist of three layers, are perforated by canals, 

 and are lignified or turned into wood. 



