GROWTH. 129 



picted on the outside of the old is placed on its face to 

 wards the centre. 



201 . When we examine the liber of a stem, we shall 

 find it composed of a number of very fine concentric 

 layers, generally separable from each other, and varying 

 in number according to the age of the plant ; these layers 

 are the several depositions of new matter which have 

 been generated during the growth of the plant. Mal- 

 pighi found, that in one Chesnut tree, two librous layers 

 had been formed during the year, but he discovered 

 afterwards in a branch two years old, six layers. Mirbel 

 observes, that in the Elm and Lime tree, four layers are 

 generated every year. Treviranus thinks, that in a 

 great number of cases, each zone of wood is represented 

 by two layers of liber ; but as a general rule no definite 

 conclusions have been arrived at upon the subject. 



202. The Epidermis, which covers the stems of young 

 plants, remains sometimes only for a very short time in 

 a whole condition, but cracks or peels off ; in other cases 

 it will remain for three or five years, when, unable to 

 bear any longer the extension of the bark beneath, it 

 begins to show fissures. 



203 . In different plants the various layers composing 

 the bark increase in a greater or less ratio with regard 

 to each other ; thus, according to Mohl, in Banksia and 

 Hakea oleifolia, the enlargement of bark is most de- 

 pendent upon increase of the stratum parenchyma- 

 torum ; in Acer campestre upon that of the stratum 

 suberosum and librosum ; and in Quercus suber upon 

 that of the stratum suberosum. 



204. The developement of Cork (the stratum suber- 

 sum) in the Quercus suber or Cork Oak, generally com- 

 mences in the third or fourth year of the growth of the 

 plant ; and it is said, always at the lenticular glands, 

 from which it spreads itself over the rest of the plant, 

 but dies soon after its development. In the Birch a 

 corky generation begins at the eighth year, but unlike 

 that of the Cork-tree, it keeps fresh for several years, 



