THE BARK 



293 



403. Cambium tissue a-b, in larch, May 20. Lower 

 quarter, cells of old xylem. Upper quarter, 

 cells of old phloem. Diameter increase just 

 about to begin. Medullary rays are shown. 

 Magnification 500 times. 



497. Bark. — In most 



woody plants, that part 



of the stem which is 



outside the cambium 



ring is called bark. At 



first it contains the 



epidermis or outer layer 



of cells, the phloem 



and the cortex lying 



between the epidermis 



and the phloem. The 



gradual growth of the 



stem causes the outer 



dead layers of bark 



to crack more or less 



irregularly and finally 



to split off. Examples of this can be seen on the trunks 



of any large trees. Before the tree is many years old, 



the cortical cells of the bark become much crushed and 



are lost to view. The epidermis is shed rather early in 



the life of the tree. 



498. Usually 



very early in the 



life of the stem a 



corky layer of bark 



is produced. This 



is the product of an 



active layer of cells 



called phellogen. 



This layer is first 



found at those 



places where the 



' **v. stomates were 



v located. The epider- 



464. Section of baaswood stem, five years old, • • r- + -11 



The cone-shaped growths of phloem are plainly seen. mls « nrt>L CTOW U< U 



