THE EPIDERMAL TISSUE. 



chlorophyll. In this manner the green cortical tissue of some dicotyledonous plants 

 becomes thickened by the layers of tissue proceeding from the phellogen, which Sanio 

 terms the suberous cortical layer (Phelloderm). This occurs, for example, in two year 

 or older branches of Salix purpurea and S. alba, the beech, &c. In such cases the phel- 

 logen lies between the periderm and the phelloderm, the outer of its daughter-cells 

 producing cork- cells, the inner phello- 

 derm (Fig. 79). The layers of periderm 

 which first undergo conversion into cork 

 sometimes bear a very close resemblance 

 to true epidermis, as, for instance, in first 

 years' branches (August) of Pinus syh'estris, 

 where, while the epidermis still remains, 

 the cork-cambium is formed in the cor- 

 tical parenchyma, and at first presents 

 the appearance as if a second epidermis 

 were formed with cells greatly thickened 

 on the outside. 



As the epidermis is at first replaced by 

 the periderm, so the periderm is afterwards 

 replaced by the formation of bark when the 

 increase in thickness continues long and vi- 

 gorous. In larger woody plants, as oaks and 

 poplars, the surface of one-year-old boughs is 

 covered with epidermis, that of several-year- 

 old boughs with periderm, that of the older 

 branches and of the stem with bark^ The 

 formation of bark depends on the repeated 

 production of new lamellae of phellogen in 

 the succulent cortical tissues of Conifers and 

 Dicotyledons which continue to grow from 

 within outwards. Layers of cells which 

 can extend themselves through the most 

 different tissues of the cortex, become 

 changed into cork-cambium, which be- 

 comes torpid after the production of 

 thicker or thinner lamellae of cork, /. e. 

 cork cut out, so to speak, from the 



Fu;. 79. — roriuation of cork in a one-year old branch o 

 Ribes nigrinn; part of a transverse section; e epidermis, h 

 hair, b bast-cells, pr cortical parenchyma distorted by the in- 

 crease in thickness of the branch ; A' the total product of the 

 phellogen f; /t the cork-cells arranged radially in rows formed 

 from c in centrifugal order, pd phelloderm (parenchyma con- 

 taining chlorophyll formed from c in centripetal direction) 

 (Xsso). 



/. e. ceases to be active. These lamellae of 

 cortex, scaly or annular pieces of the surface ; 

 everything which lies outside them becomes dried up ; and since this process is con- 

 stantly repeated on the outside of the stem, and the new lamellae of cork continu- 

 ally intrench further on the growing cortical tissue, a layer, constantly increasing in 

 thickness, of dried up masses of tissue becomes separated from the living part of the 

 cortex, and this is the Bark. The process is very clear in the bark of the oriental plane 

 which detaches itself in large scales ; and almost as clear in old stems of Pinus syl'vestris. 

 Since the bark does not follow the increase in thickness of the stem, it splits in longi- 

 tudinal crevices from the surface inwards, as in the oak, if the direction of weakest 

 cohesion requires it ; in other cases it peels off in the form of horizontal rings from 

 the stem (ring-bark), as in the cherry. 



The Lenticeh are a peculiarity of cork-forming Dicotyledons ; they appear before 



^ A considerable increase of thickness is not always combined with the formation of periderm, 

 as, e. g. in the sunflower and other annual stems. In Viscum, the epidermis always remains 

 capable of development, and its thick cuticular layers render the protection of periderm super- 

 fluous ; the formation of cork is also not a necessary consequence of vigorous increase of thickness ; 

 the copper-beech and the cork-oak, for example, form only periderm. 



