THE GROWTH OP ROOTS AND STEMS. 



181 



Beech, there may be a considerable formation of bark owing 

 to the dying off of the older cork-layers. But in most cases 

 this first cork-cambium dies, sooner or later in those plants 

 where it has a superficial origin, early in those where it is 

 deep-seated. It is replaced by a new cork-cambium developed 

 in the deeper tissue. This produces a new cork-layer, and as 

 a result all the outlying tissues (the original cork, etc.) die 

 and are added to the bark. If the succession of secondary 

 cork-cambiums is rapid, it often happens that the cork comes 

 to lie close to the bast, and in some cases the new cork may 

 even arise in the bast, e.g. Vine and Clematis. 



In some trees the bark comes away in sheets, and is spoken 

 of as ring -bark. This may be due either to the first cork- 

 cambium being persistent or to the successive cork-cambiums 

 appearing in the form of regular rings. But in most trees 

 the bark is given off in scales (scale-bark) ; this is due to 

 the fact that the secondary cork-cambiums do not arise as 

 regular rings or layers, but in the form of curved strips. 



215. Lenticels. In the young green shoot the epidermis 

 has stomates allowing for the free entry and exit of gases. 

 When the cork-tissue 

 is developed we usually 

 find certain structures 

 known as lenticels, 

 which usually form 

 small oval patches on 

 the surface of the shoot. 

 In Birch the lenticels 

 are long, transverse, 

 and very conspicuous 

 on the white bark. 

 Sections show that at 



these points the COrk- mgi 59 ._Section through a Lenticel. 



Cells are not in Close Phellogen = Cork-cambium. 



contact, but have sepa- 

 rated from, each other and form a loose powdery mass through 

 which gases can readily pass (Fig. 59). 



These lenticels are as a rule developed immediately under- 

 neath the stomates. Where a thick mass of cork is developed 

 they form long passages or canals, filled with the powdery 



EPIDERMIS 



CORK 



LOOSE CELLS 



PHELL.OGEN 



CORTEX 



