86 OAK. [CH. VI 



The cork-cells being formed by successive division 

 of the phellogen cells acquire the same regular pattern- 

 like arrangement that has been described for the cambium. 

 It is shown in figs. 39, 40, where the cork is seen in trans- 

 verse and longitudinal section. 



The suberisation of the walls of the cork-cells is not 

 the only change that occurs ; an equally striking feature 

 is the disappearance of the protoplasmic contents : 

 so that cork, like pith, comes to be a mass of air-con- 

 taining cells. The fact that the cell walls are extremely 

 impervious to water, added to the fact that the cells 

 contain air, gives the floating power of cork. The 

 impermeability to water also gives the quality which 

 allows the periderm of the Cork Oak to be made into 

 " corks " for bottles. 



The phelloderm need not be described in detail: it 

 consists of collenchyma in whose cells chlorophyll-bodies 

 are found. 



As the oak-tree becomes older there is a more complex 

 formation of cork, which leads to the rough scaly look 

 observable on the trunk. Into this formation I shall 

 not enter. 



Secondary Phloem. 



A transverse section of the bark of a 4 or 5 year 

 branch of the oak shows, under a simple lens or low power 

 of the microscope, a stratified appearance. The concentric 

 lines which produce this appearance are due to the same 

 general cause which accounts for the annual rings in 

 the xylem, namely, that the products of cambial activity 



