PHELLOGEN AND CORK 81 



All these started life as seedling plants, thinner than a 

 lead-pencil. 



3. Now consider bark more closely. On some 

 trees, as species of FICUS (Banyan, Peepul, etc.), the 

 bark is smooth, and being thin, quite a small cut will 

 reach the cambium and young wood layers. But in 

 most trees the bark is thick, and cracked on the 

 outside, and if one cuts it with a knife there is no 

 water or sap : it is quite dry and dead. The drying 

 up of the outer parts is due to the formation of a 

 special waterproof substance called cork. This is formed 

 by a layer called the phellogen which grows like the 

 cambium but produces cork on its outside. Cork, being 

 almost impervious to air and water, prevents loss of sap 

 by evaporation from the surface, and it also cuts off the 

 outer tissues from the sap, so that they dry up and die. 



If the phellogen occurs only just underneath the 

 epidermis and forms but little cork on its outer 

 side, the bark may remain smooth, as in FICUS. But 

 when, as more usually happens the phellogen occurs 

 deeper in, nearer the wood, there is much more 

 bark tissue outside it. And since this bark, being 

 dead, cannot grow or expand as the tree increases in 

 thickness, the pressure of the growing wood from within 

 causes it to split open in longitudinal cracks. 



This is the cause of the familiar rough appearance 

 of bark ; and different kinds of trees differ in the 

 roughness of their bark according to the difference in 

 the depth of the phellogen below the surface and the 

 amount of bark which is formed. 



In some trees, e.g. MILLINGTONIA (the Indian 

 cork-tree) and in the ordinary Spanish Cork-Oak, the 

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