THE STEM. Ill 



cells will be sloughed off.* (b) In a majority of cases the 

 generating layer of the periderm is formed in the cortex, 

 either immediately under the epidermis (fig. 128) or in one 

 of the deeper layers (fig. 127). (c) In other instances the 

 generating layer is formed in the pericycle. If the pericycle 

 is more than one layer of cells thick, it may be formed in 

 the innermost or in any one of the external parts. In this 

 case, therefore, there will be killed all the tissues of the 

 cortex and any of the stelar tissues which lie outside the 

 portion of the pericycle from which the generating layer is 

 formed. 



138. Perennials. — Plants which live for a single year have 

 usually but a small amount of periderm formed, or sometimes 

 none at all. In those, however, which are perennial, peri- 

 derm is formed not only during the first year's growth, but 

 the activity of the generating layer is resumed at the begin- 

 ning of succeeding seasons, so that annual additions are 

 made to it. In the cork oak, for example, there is an extraor- 

 dinary development of cork, which becomes so thick and 

 is so resistant to the passage of water that it serves for the 

 manufacture of stoppers. In the bottle-cork mechanical 

 tissues occur, not in zones, but in isolated patches, forming 

 the gritty masses in poor corks. 



139. Secondary periderm. — The dead tissues which accu- 

 mulate from year to year upon the outside of perennial stems 

 constitute a large part of what is known as the bark. In the 

 bark of most trees one or more generating layers form in 

 addition to the first, giving rise thus to secondary periderm 

 (fig. 129). The secondary periderm may be either concentric 

 with the first, in which case the outer parts of the bark will 

 be made of concentric layers which separate readily from 

 each other ; or the new generating layer may intersect the 



* The epidermis sometimes continues to grow for many years, while a 

 secondary cortex is formed under it. In this case no sloughing off occurs. 



