362 YOUNGKEN— ON THE MYRICACEAE 



fibre elements takes place subsequently (7 p. 100). The external layer 

 of the cortex gives rise to the cork cambium. 



Secondary Structure 



Aerial Stem of M. cerifera, L. 



(Plate 87, Fig. 19) 



The cork consists of several layers of brick-shaped cells, whose walls 

 are for the greater part suberized. The contents of these cells stain 

 variously with safranin and methyl-green. 



Separating the cork from the cortex, one notes a phellogen layer of 

 tangentially elongated cells. The cortex is usually narrow and shows 

 comparatively small angular intercellular air spaces. Its cells are 

 tangentially elongated. Some of them contain gummy lignin and tannin, 

 while many are filled with starch grains, or crystals of calcium oxalate 

 which may be in the form of monocUnic prisms or rosette aggregates. 

 Occasionally these crystals may also be seen in the air-spaces. Stone 

 cells of irregular form and size, arranged singly or in small groups, are 

 scattered throughout this region. The outer 3 or 4 layers of exocortex 

 are composed of collenchymatic cells and show no air-spaces. A con- 

 tinuous zone of sclerenchyme fibres and cells is present in the pericycle. 

 The sclerenchyme cells are of different sizes and shapes and show varying 

 degrees of lignification. Frequently cells containing crystals of calcium 

 oxalate are found adhering to them. Some of the stone cells contain 

 gummy lignin. The phloem region is traversed by numerous primary 

 and secondary medullary-rays which generally broaden out toward the 

 the cortex, arching in many phloem masses. The primary medullary- 

 rays are usually 1-3 rows of cells in width in this region. The secondary 

 medullary-rays of the phloem vary from 1-2 rows of cells in breadth. 

 The sieve tubes show obHque sieve plates. Their walls show numerous 

 transverse pits. Tannin and crystals are quite abundant in the phloem 

 parenchyma. Both stone cells and narrow bast fibres are sparsely 

 scattered either singly or in small groups amongst the other phloem 

 elements. The bast fibres are accompanied by crystal fibres. Each cell 

 of the crystal fibres contains a monoclinic prism of calcium oxalate. The 

 cambium zone is distinct and somewhat wavy. The xylem region is 

 traversed by numerous primary and secondary medullary-rays which 

 separate this zone into many narrow wood wedges. The primary medul- 

 lary-rays of this region are mostly 1-2, rarely 1-3 rows of cells wide, the 

 secondary, mostly 1 cell, rarely 2 rows wide. The walls of the medullary- . 

 ray cells are lignified. The tracheae are pitted and appear circular to 



