THE STEUCTUEE OP THE STEM 



363 



outer zones, on the contrary, contain cells which are living, and 

 which are charged with water. The secondary wood can thus 

 be divided into the alburnum or sapwood, and the duramen or 

 heart-wood. 



The increase in thickness of the stem thus brought about 

 sets up great and gradually increasing tension in the cortex, 

 leading to early rupture of the epidermis. To replace the latter 

 such stems show a continual formation of layers of phellogen, 

 each giving rise to a cork layer externally, and generally to 



Fig. 7i6. 



,pe 



Fig. 746. Section of bark of Quercus semlifim'a. Alter Kny. pe. Periderm 

 layers, arising at different depths in tlie cortex. 



phelloderm or secondary cortex internally. This takes place 

 usually at a very early date immediately below the epidermis. 

 The periderm so formed being impervious to moisture, all the 

 cells . external to it dry up and perish. Successive layers of 

 phellogen arising continually deeper and deeper in the cortex 

 cause the external dry tissue to be very complex in structure. 

 The phellogen layers often extend as far internally as the 

 secondary bast. The mass of external tissue thus formed is 

 known as the harh. It includes all the layers of periderm, with 



