144 DERMAL SYSTEM 



while the intervening phelloid-layers are, on the contrary, thin-walled 

 and in addition inclined to split along a predetermined plane : in this 

 case it is hygroscopic deformation of the corky layers and dead cortical 

 tissues that leads to disruption within the phelloid layer or along one 

 of its margins. In the case of the active type the relations are reversed, 

 the corky cells being thin-walled and the phelloid layers thick-walled 

 and lignified : here it is the phelloid with or, more often, without the 

 co-operation of other tissues of the bark which brings about separation 

 within a zone of cork-cells. This complex subject, however, urgently 

 requires further investigation, especially as regards the mechanics 

 of the actual process of separation. Passive absciss-phelloids have 

 been found by Von Hbhnel in Boswellia papyri/era, Philaddphus 

 coronarius, Myrtus communis, spp. of Callistemon and Fuchsia, etc. In 

 Boswellia papyri/era each corky zone consists of some ten to fifteen 

 layers of thick-walled flattened elements. The phelloid-cells, on the 

 contrary, are arranged in single layers, and have thin outer and lateral 

 walls ; their inner walls are very peculiar, being greatly thickened, 

 strongly lignified and, at the same time, silicified, and provided with 

 inwardly projecting flanges, which run principally in the longitudinal 

 direction. The lateral walls break very easily, so that the separation 

 of a layer of bark always exposes the inner wall of a phelloid layer : 

 owing to their resemblance to the thickened outer walls of an epidermis, 

 these walls are particularly well fitted to form the outer boundary of 

 the thick subjacent layers of cork. 



Active absciss-phelloids are described by Von Hohnel as occurring 

 in Picca cxcclsa, Araucaria excelsa, Pinus sylvestris, Taxus baccata, and 

 Salix europaea. They almost invariably consist of several layers of 

 very thick-walled cells, which alternate with strips of thin-walled cork. 



The age at which our native trees begin to form bark varies greatly 

 according to the species. Scale-bark appears on the trunk of Pinus 

 sylvestris and P. nigricans between the 8th and 1 Oth seasons (v. Mohl) ; 

 the corresponding age for our native species of Oak is 25 to 36 (Hartig), 

 for the Alder 15 to 20, for the Lime 10 to 12, and for the genus Salix 

 8 to 10, or even less. Bark formation is long postponed until the 

 50 th season, or even beyond that in Abies pectinata, Car pinus, the 

 Cork-Oaks, etc. The trunk of the Beech (Fagus sylvatica) never forms 

 anything but superficial periderm. Where the formation of bark is 

 tardy or altogether absent, the mechanical strength of the periderm is 

 enhanced by the production of large quantities of sclerenchyma ; such 

 periderm is termed " stone bark " by Hartig. 



