lo8 CELLULAR TISSUE. 



SECTION II. 

 CORK. 



Sect. 24. Cork^ is formed in the mature plant as a tissue having the fundamental 

 physical properties of cuticularised epidermis, the place of which it supplies when 

 the latter is thrown off in the normal course of development (comp. Chap. XV), or 

 where the living parenchyma is laid bare by wounding, or where it is to be isolated 

 from the effects of injuries which have penetrated to the interior. Rarely, as in 

 many bud-scales, cork appears simultaneously as a strengthening of persistent 

 .epidermis. 



All investigated phanerogamic land-plants are capable of forming cork. In the 

 Cryptogams it is only found in isolated cases, namely on the surface of the Rhizome 

 of the Ophioglossese ^ 



Cork- formation always originates in the epidermis or in living parenchymatous 

 cells, and indeed in the latter without distinction, to whatever member or region they 

 may belong. Wound-surfaces, of whatever sort, are closed and healed by it ; and 

 diseased or dead parts are isolated from those still living. In the normal course of 

 development (disregarding the surfaces of separation of members thrown oif, which 

 are here to be placed in the category of wound-surfaces) it appears especially on the 

 surface of such parts as the stems and roots of most Dicotyledons, Gymnosperms, and 

 a few Monocotyledons, which have a long-continued and abundant growth in thick- 

 ness, not continuously followed by the successive peripheral layers of tissue (Chap.XV)j 

 it is less generally present on the surface of long-lived, firm, but not continuously 

 thickening stems and roots of Monocotyledons. Most of the latter retain their 

 epidermis, but it is replaced by cork in the- stem and the roots of Pandaneae, 

 epiphytic Aroidese (Philodendron, Monstera, Anthurium, Tornelia '), the roots and 

 rhizomes of Dracaenese, Strelitzia, Dioscorea, Zingiberacese. Finally, in rare cases, 

 formation of cork occurs normally on the surface of leaves *, as on the scales of the 

 winter buds of many dicotyledonous and coniferous trees, Aesculus Hippocastanum'^, 

 Ulmus montana, Populus, Carpinus, Corylus, Abies excelsa". 



The cork-forniation begins in a single layer of cells parallel to the surface which 

 is to be shut off, by the occurrence of divisions also parallel to that surface. This 

 layer of cells, which relatively to the cork-formation may be called the iniiial 

 layer, is the .epidermis itself in certain cases of normal development of Dicotyle- 

 dons, to be more accurately treated of below (Chap. XV) ; in all other cases_ it is a 



' Sanio, in Pringsheim's Jahrb. II. p. 39. Further literature in Chapter XV. [See also F. von 

 Hohnel, 'Ueber den Kork und verkorkte Gewehe iiberhaupt,' Kaiserl. Akad. d. Wiss. in Wien. 

 Nov. 1877.] 



' Rassow, Vergl. Unters. p. 121. ■- Von Tieghem, Str. d. Aroid^es, /. c. 



' [Compare Bachmann, iiber Korkwucherungen auf Blattem, Pringsheim's Jahrb. XII. p. 191.] 



' Hanstein, Botan. Zeitg. 1868, p. 721. 



« Areschoug, Om den inre byggnaden i de tradartade vaxternes Knoppfjall. Lunds Univ. 

 Arsskrift, torn. VII (1870). 



