96 STRUCTURAL BOTANY 



g. Periderm 



We know that the young stem is covered by the 

 epidermis; the old stem, however, is coated by a 

 greyish bark, much thicker and harder than the 

 epidermis; this bark splits lengthways here and 

 there; as the stem grows thicker, the cracks widen 

 and expose the softer tissues below, until these too 

 become coated over by new bark. 



If we examine the bark of the Wallflower micro- 

 scopically, we find that its outer layers consist of the 

 epidermis and the whole of the cortex in a dried and 

 withered condition (see Fig. 35, e and co). The inner 

 part of the bark is formed of closely-packed cells, 

 which are arranged in regular radial rows, and which 

 have their walls corky or suberiscd. These walls 

 have undergone a similar change to the cuticularisa- 

 tion of the outer epidermal cell-wall, and so the corky 

 tissue has become almost impermeable to water or 

 watery vapour, just as . is the case with the ordinary 

 corks used in bottles, which are made of this tissue, 

 obtained from the Cork Oak (Quercus suler). The 

 cortex, which lies outside this corky zone, is thus 

 completely cut off from all supplies of water and 

 food from within, and consequently withers and dies. 

 Now we will see how the bark is formed. 



The pericycle, which encloses the whole vascular 

 cylinder, is at first one cell thick. Soon after 

 secondary thickening has begun, however, its cells 

 divide, so that it becomes three or four cells in thick- 

 ness. These divisions take place rather irregularly. 



