Vegetable Physiology for the Year \^*6Q, S5 



layer remain, as in general, quite undeveloped, and here, 

 especially at the base of the trees, the bark is often more than 

 twice as thick as the ligneous body. From these few in- 

 stances we already see that the increase of the bark in thick- 

 ness, even in cultivated plants nearly related to each other, 

 may consist of the predominating development of quite dif- 

 ferent cortical layers. 



The bark of the birch is well known from its peculiar 

 structure and its various colours. The young annual branches 

 of this tree also possess an epidermis, which is covered with 

 fine hairs : under this is situated a small layer of tabular cells, 

 which represents the cork layer, and directly covers the cellu- 

 lar envelope. This cellular layer appears at the surface as 

 soon as the epidermis falls off (in the second or third year) ; 

 the single cells then become brown, and new layers of cells 

 are deposited on the interior surface of this cellular mass. 

 This mass now forms the well-known birch bark, which con- 

 sists of thin white lamellae, which we can peel off one after the 

 other. Mohl proposes to give this cellular mass the name of 

 Periderma, while the external layer is known by the name 

 of Epiilermis. 



If we examine the bark from the stem of an old birch we 

 find that it consists of a great number of brown layers, which, 

 as in the leaves of a book, lie one over the other, and are very 

 easily stript off. They are clothed on both surfaces with a 

 white covering, which consists of very thin-sided colourless 

 diametrically deposited cells, which are rather less compressed 

 than those of the brown layer, where the cells are very thick- 

 sided and filled with a brown substance. From the eighth 

 to the tenth year there is for the first time alternately deve- 

 loped in the birch along with every layer of the brown cork 

 tissue, at the same time a white layer, which consists of larger 

 and more delicate cells; until this period the formation of 

 new layers only takes place on the one surface of the Peri- 

 derma, The white and the brown substance of the bark of 

 the birch seem to be more distinct masses than those in the 

 cork, where the borders of each layer may also be distinguished 

 by their different colours. (See the anatomical difference of 

 these layers in the figure which Link has given in his Icon, 

 AnaL Bot. Tab. vi. fig. 13.) 



Very remarkable is the difference between the cork sub- 

 stance of the cork-oak and the brown whitish layers of the 

 birch bark, since these remain for a long time attached to the 

 stem without cracking, and gradually peel offj while the cork 

 substance splits and falls off. The inner layers of the birch 



