92 OUTLINES OF PLANT LIFE. 



therefore perish sooner or later. How much of the stem will 

 thus be killed depends upon the position of the layer of cells 

 which produces the cork. 



Annual shoots have usually but a small amount of periderm 

 formed, or sometimes none at all. 

 In perennials, periderm is formed not 

 only during the first year's growth, 

 but the activity of the cork cambium 

 is resumed at the beginning of suc- 

 ceeding seasons, so that annual addi- 

 tions are made to it. 



113. Bark. — The dead tissues 



which accumulate from year to year 



upon the outside of perennial stems 



"■"ecrio-r^of /oui'TrTt constitute a large part of what is 



Sm'TlpSmiT?.! known as the bark. The inner part 



^fh ^o//row° o'f aTonr; of the bark bclongs to the stele. (See 



S^^iW^-Xue"; 1II7-) In the bark of most trees 



Haberiandt. ^^^ ^^ morc cork-forming layers 



originate in addition to the first, giving rise thus to sheets 



of cork either concentric with the first, or intersecting it 



(%■ 83). 



In the first case the dead outer parts may peel off in con- 

 centric sheets, as in the birch. In the second the dead parts 

 break away in the form of scales or flakes, as in the hickory, 

 sycamore, or apple. In many trees the dead outer portions 

 are only gradually worn away by the action of the weather, 

 becoming seamed or deeply furrowed lengthwise. 



114. Secondary wood and bast. — The position of the in- 

 ternal growing layer, the stelar cambium, is not subject to the 

 same variations as the external one. 



In the many dicotyledons whose stems increase in 

 diameter, the strands of wood and bast are in a single circle 

 parallel to the surface, the bast bundles in each pair being on 



