35] 



CHAPTER II. THE TISSUES. 



153 



in the Beech, during the whole life of the tree ; the outer cork-cells 

 split off as the trunk of the tree increases in thickness, while the 

 phellogen, growing and extending in a tangential direction, gives 

 rise to new layers of cork. When, as in most cases, new layers of 

 phellogen arise after a few years in the deeper -tissues, leading to 

 the development of corresponding layers of periderm, an external 

 investment of a more or less complicated structure comes to be 

 formed. In consequence of the impermeability to water of these 

 secondary layers of periderm, all the tissues lying externally to them 

 become dried up. These dried-tip tissues, which may belong to 

 different tissue- 

 systems and include 

 the most various forms 

 of cells, constitute 

 what is known as 

 Bark. When the 

 primary periderm is 

 superficial, the new 

 secondary layers of 

 periderm are only arcs 

 of the circumference, 

 and as their margins- 

 are in contact with 

 the periderm which 

 has been previously 

 formed (Fig. 119), a 

 scaly bark is formed, 

 that is, isolated 

 patches of tissue are 

 transformed into bark. 

 This bark is stretched and torn by the increasing size of the 

 trunk, and the scales of it may be shed, as is the case in the Plane, 

 or they may adhere one upon the other, as in the Pines and Larches, 

 or remain connected by the bast-fibres in long strips as in Robinia. 

 When, on the other hand, the primary periderm has been formed in 

 the deeper layers of the cortex, the secondary periderm often forms 

 complete concentric rings ; thus hollow cylinders of the cortex are 

 transformed into bark (ringed bark). The longitudinal rupture of 

 this kind of bark is effected by the bast-fibres enclosed in it (e.g. 

 Vine, Clematis, and Thuja). 



There are frequently in the periderm of both stems and roots 



Fig; 119. Formation of Scaly Bark in a Larch, as 

 seen in a piece of the outer portion of the stem cut both 

 transversely and longitudinally (nat. size) ; r the 

 secondary cortex ; fe plates of cork ; b the scales of bark 

 cut off by the cork. 



