STEM STRUCTURE 171 



As shown in Plates 57 and 58, the tissues usually- 

 present in barks, in the order of their arrangement, 

 beginning with the outermost, are as follows: 



1. Cork, 



2. Phellogen and phelloderm, 



3. Cortical parenchyma, 



4. Stone cells, 



5. Medullary rays, 



6. Phloem elements. 



It is convenient to divide the bark into outer, 

 middle and inner layers or portions. In such a division, 

 the outer bark includes the cork, phellogen and phel- 

 loderm structures. The middle bark is the region 

 between the phelloderm and the outer ends of the 

 medullary rays, and thus includes most of the cortical 

 parenchyma and stone cells. The inner bark layer 

 is traversed by the medullary rays and includes these 

 structures, together with the phloem elements. 



CHARACTER OP BARK STRUCTURES 



Cork. — The periderm or corky layer of barks is 

 usually very thick and consists chiefly of dead cells. 

 The individual cells are generally dark colored and, 

 in transverse section, appear as rectangular or polyg- 

 onal cells with thick or thin walls. The walls of cork 

 cells are strongly suberized; and the intercellular 

 spaces are so small that, except for the openings 

 caused by the formation of lenticels, the cork forms 

 an impervious covering around the stem tissues. 



Phellogen and Phelloderm. — In many barks these 

 layers may be so reduced as almost to escape notice. 

 The phellogen consists of one or more layers of color- 



