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 u in- 

 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 



*/ o 



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- 

 ene and in main' dicoty- 

 ledonous trees, is the bco-k, the most complicated epidermal 

 structure in the vegetable kingdom." 



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

 of plants living many years, is particular!}* 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. eel., 1882, p. 95. 



FIG. 117. Formation of cork in a branch of Ribes nigrum, 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 c ; 

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

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



