90 



MORPHOLOGY OF TISSUES. 



(c) Cork, and Epidermal Formations produced by it'^ (Periderm, Lenticels, Bark). 

 When succulent organs of the higher plants, no longer in the bud-condition, are injured, 

 the wound generally becomes closed up by cork -tissue ; i. e. new cells arise near the 

 wounded surface by repeated division of those which are yet sound, and these, forming 

 a firm skin, separate the inner living tissue from the outer injured layers of cells. The 

 walls of this tissue offer the strongest resistance to the most various agencies ; shnilar to 

 the cuticular layers of the epidermis in their physical behaviour, flexible and elastic, 

 permeable only with difficulty by air and water, they for the most part soon lose their 

 contents and become filled with air. They are arranged in rows lying at right angles to 

 the surface, of parallelopipedal form, and form a close tissue without intercellular 

 spaces. These are the general distinguishing features of cork-tissue. It is formed not 

 merely on wounded surfaces, but arises in much greater mass where succulent organs 

 require an effectual protection {e.g. potato-tubers), or where the epidermis is unable to 

 keep up with the increase of circumference when growth in thickness continues for a 

 long period. In these cases, which occur but seldom in Monocotyledons {e. g. stem of 

 Dracaena), but very generally in several-year-old stems and roots of Conifers and Dico- 

 tyledons, the cork-tissue is formed even before the destruction of the epidermis ; and 

 when this splits under the action of the weather and falls off, the new envelope formed 

 by the cork is already present. The cork-tissue is the result of repeated bipartition of 

 the cells by partition-walls, rarely in the epidermis-cells themselves, more often in the 

 subjacent tissue. These partition-walls lie parallel to the surface of the organ ; here and 

 there, where the increase of the circumference necessitates it, divisions also take place 

 in a vertical direction, by which the number of the rows of cells is increased. From 

 the two newly formed cells of each radial row (/. e. perpendicular to the surface of the 

 organ) one remains thin-walled and rich in protoplasm, and in a condition capable of divi- 

 sion ; the other becomes transformed into a permanent cork-cell. Thus arises usually 

 parallel to the surface of the organ a layer of cells capable of division, which continues to 

 form new cork-cells, the Cork-cambium or layer of Phellogen. In general this is the inner- 

 most layer of the whole cork-tissue, so that the production of cork advances outwardly, 

 and new layers of cork are constantly formed out of the phellogen on the inner surface of 

 those already in existence. But, according to Sanio, it also happens at the commencement 

 of the formation of cork that the formation of permanent cells proceeds centripetally, or 

 an alternation of centripetal and centrifugal cell-formation takes place in t\iQ young cork- 

 tissue. But sooner or later the centrifugal formation of cork always takes place with 

 phellogen lying on the inner side, which follows from the circumstance that the tissues 

 lying on the outside of completely suberised layers of cells sooner or later die. Usually the 

 formation of cork begins first at single places of the periphery of lignified branches ; but 

 gradually the phellogen forms a connected layer, from which new layers of cork are con- 

 tinually pushed forwards outwardly. When in this manner a continuous layer of cork 

 arises, steadily increasing from the inside, it is termed Periderm. The development and 

 configuration of the cork-cells may change periodically during the formation of periderm ; 

 alternate layers of narrow thick-walled and broad thin-walled cork-cells are formed ; the 

 periderm then appears stratified, like wood showing annual rings (as in the periderm of 

 Quercus Suber, Betula alba, &c,). In some cases the phellogen of the periderm gives rise 

 not only to cork-cells, by which the periderm increases in thickness, but parenchyma- 

 cells are also formed containing chlorophyll ; this always however happens in such a man- 

 ner that only daughter-cells of the phellogen lying on the inner side (facing the substance 

 of the wood) undergo this metamorphosis into permanent parenchyma-cells containing 



' H. von Mohl, Vermischte Schriften bot. Inhalts, pp. 221-233. Tubingen 1S45. — J. Hanstein, 

 Untersuch. iiber den Bau u. die Entwickelung der Baumrinde. Berlin 1853. — Sanio, Jahrl). fiir wiss. 

 Bot. II, p. 39. — Merklin, Melanges biol. du Bulletin de TAcad. Imp. dcs sciences de St. Petersbourg, 

 vol. IV, Feb. 26, 1864. 



