158 



BOTANY' 



PART I 



contents (phlobaphene) consisting mainly of tannins and products 

 derived from them. The cork tissue frequently shows an alternation 

 of narrow thick-walled with thin-walled and larger cells. These 

 layers mark annual growths. Even thin corky walls, being imperme- 

 able to water, prevent the loss of moisture by transpiration. Thicker 

 layers of cork afford mechanical protection and hinder the invasion 

 of the plant by parasites. 



The phelloderm formed to the inner side of the phellogen in many 

 cases, such as the Currant, attains a considerable thickness. Its 

 cells retain their living protoplasm, and usually contain chloroplasts. 

 They ultimately become rounded off, so that intercellular spaces are 



Fic:. 166. — Transverse section of the outer part of a one-year-old twig of Pyrus communif: made in 

 autumn. It shows tlie commencement of the formation of the periderm, p, cork ; })(/, 

 pliellogen ; pd, phelloderm ; col, coUenchyma. The cork-cells have their outer walls thickened 

 and have brown dead contents, (x 500.) 



formed between them, and serve to increase the thickness of the living 

 cortex. The term periderm includes both cork and phelloderm. 



The cork of the Cork-oak [Quercus Suher) is formed of broad layers of soft large 

 cells, alternating with narrow layers of thinner cells, which mark the limit of the 

 annual growth. This may be seen in bottle-corks. The first, spontaneously de- 

 veloped cork of the Cork-oak is stripped otf, whereupon a new phellogen is formed 

 in the deeper-lying tissue. The cork thus produced is removed every six or eight 

 years, and furnishes the cork of economic value. 



In many cases the phellogen takes its origin in the epidermis (Fig. 166 2^9)- 

 This is the case in the Willow, in most Pomaceae, and in a number of other woody 

 plants. The epidermal cells become divided into outer and inner cells, the latter 

 of which assume the function of a 2)liellogen. More frequently the phellogen 

 develops irom the layer of cells next adjoining the epidermis, as, for example, in 

 the Elder {Sambucics nigra), where it takes its origin from the outermo.st layer of 



