BARK. 



149 



418. In some plants, notably the birch, papery layers exfo- 

 liate from time to time, while in some other plants, e. g., the 

 shag-bark hickory, large strips of irregular form and thickness 

 are 'detached. Owing to the mode of their formation, such sepa- 

 rated pieces may contain very heterogeneous elements. Of them 

 Sachs says: 1 " Not un- 



frequently the formation 

 of cork penetrates much 

 deeper [than the peri- 

 derm] : lamellae of cork 

 arise deep within the stem 

 as it increases in thick- 

 ness ; parts of the funda- 

 mental tissue and of the 

 fibro-vascular bundles, or 

 of the tissue which after- 

 wards proceeds from them, 

 become, as it were, cut 

 out by lamellae of cork. 

 Since everything which 

 lies outside such a struc- 

 ture dies and dries up, a 

 peripheral layer of dried 

 tissue collects, which is 

 very various in its form 

 and origin. This struc- 

 ture, abundant in Conif- 

 erae and in mam 7 dicoty- 

 ledonous trees, is the bark, the most complicated epidermal 

 structure in the vegetable kingdom." 



419. Injuries of the stem. The stem, especialty in the case 

 of plants living many years, is particularly liable to injuries, the 

 most frequent of which are of course the wounds left by the fall- 

 ing of the lower limbs. It is proper to treat here of the natural 

 repair of such injuries. 



420. When any part of a plant suffers serious mechanical 

 injury by which the deeper tissues are exposed, the surface of 



1 Text-book, 2d Eng. ed., 1882, p. 95. 



FIG. 117. Formation of cork in a branch of Ribes nigrnm, one year old ; part of a 

 transverse section; e, epidermis; h, hair; b, bast-cells; pr, cortical parenchyma dis- 

 torted by the increase in the thickness of the branch ; K, total product of the phellogen e ; 

 k, the cork-cells radially in rows, formed from c in centrifugal order; pd, phelloderm 

 (parenchyma containing chlorophyll formed centripetally from c). (Sachs.) 



