MORPHOLOGY 



147 



The phellogen ( 124 ) is as a rule a cambium with a persisting initial 

 layer from which cork-cells are produced to the outside and phello- 

 derm to the inside. 



The production of cork -cells is usually greater than that of phelloderm. If the 

 phelloderm forms only cork-cells the initial cells are often used up in the process, 

 and are then replaced from the adjoining ground tissue (p. 128). This is found 

 among Monocotyledons and also in some Dicotyledons, e.g. Valcriuna ojficinalis. 



The young cork-cells usually have a tabular shape, fit closely 

 together without intercellular spaces, and possess suberised, secondary, 

 thickening layers. The cork cells usually possess brown walls. 



~cof 



Fin. 157. Transverse section of the outer part of :i one-year-ol'l twi^ of 1'ynis communis made in 

 autumn. It shows the commencement of the formation of the periderm. />, cork ; pg, 

 phellogen ; pd, phelloderm ; ml, collenchyma. The cork-cells have their outer walls thickened 

 and have brown dead contents, (x 500.) 



They may be thin (Fig. 155 /) or thick, frequently thickened on one 

 side (Fig. 157), and occasionally to such an extent that they are 

 known as stone cork. Thin-walled cork-cells usually contain air ; 

 those with thickened walls frequently have reddish - brown cell 

 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. 



