BARK. 



149 



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

 liate from time to time, while in some otlier 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:' "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- 

 tui'e, abundant in Conif- 

 erse and in many dicotj'- 



ledonous trees, is the hark, the most complicated epidermal 

 structure in the vegetable kingdom." 



419. iDJnries of the stem. The stem, especiall3' in the case 

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

 most frequent of which are of course the wounds left b}' the fall- 

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

 repair of sucli 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 Eibes nigrum, one year old; part of a 

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

 torted by the increase in the thicltness of the branch ; K, total product of the phellogen c ; 

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

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



