340 THE WOODY STEM 



Bark. The original phellogen does not, as a rule, 

 remain active indefinitely. In most cases it stops 

 dividing after some years, and a new phellogen is formed 

 in a deeper layer of the cortex, producing a second layer 

 of cork, separated from the first by some layers of cortical 

 cells. This second layer is succeeded by a third, after 

 another interval, at a still deeper level, and so on till 

 the new phellogen come to be formed in the parenchyma 

 cells of the secondary phloem. The later formed 

 phellogens cut deeper and deeper into the secondary 

 phloem, the older parts of which, together with the 

 pericycle and cortex (the whole mass being called the 

 outer bark), are thus killed, being cut off from the living 

 and growing tissues of the stem on each side of the 

 cambium. The zone of functional living phloem (inner 

 bark) thus remains approximately constant in width, 

 being continually reduced on the outside by the forma- 

 tion of deeper and deeper layers of cork, and continually 

 added to on the inside by the cambium. This is a 

 parallel phenomenon to the formation of heartwood 

 from the older secondary xylem. In an old tree-trunk 

 only a comparatively narrow cylinder on each side of 

 the cambium is alive and functional in conduction. 

 The great mass of the tissue of the trunk is dead (heart- 

 wood and outer bark), though it may be useful to the 

 tree the heartwood giving extra support, the outer 

 bark efficient protection. 



The bark of trees, as is well known, varies very much 

 in appearance. Some, like the plane tree and the 

 " paper birch," have smooth bark, which scales off in 

 uniform flat thin layers. This is because the phellogens, 

 and consequently the layers of cork which they produce, 

 are uniform and parallel. Others, such as the common 

 oak, the elm and the pine, are rugged, the surface being 



