lo8 MJRPHOLOGY OF TISSUES, 



with bark '. The formation of bark depends on the repeated production of new layers 

 of phellogen in the succulent cortical tissues of Conifers and Dicotyledons which con-i 

 tinue to grow centrifugally. Layers of cells which can extend through the niost; 

 different tissues of the cortex are changed into cork-cambium, which ceases to bej 

 active after the production of thicker or thinner layers of cork. These layers of 

 cork cut out, so to speak, from the cortex, scaly or annular pieces of the surface ;| 

 everything which lies outside them becomes dried up; and since this process is con- i 

 stantly repeated on the outside of the stem, and the new layers of cork continually I 

 intrench further on the growing cortical tissue, a mass of dried up portions of tissue, 

 constantly increasing in thickness, becomes separated from the living part of the 

 cortex ; and this is the Bark. The process is very clear in the bark of the oriental 

 plane which detaches itself in large scales, and almost as clear in old stems of the Scotch 

 fir. Since the bark does not follow the increase in thickness of the stem, it splits in 

 longitudinal crevices from the surface inwards, as in the oak, according to the direction 

 of weakest cohesion ; in other cases it peels off in the form of horizontal annular bands 

 from the stem (ring-bark), as in the cherry. 



Lenticeh are a peculiarity of cork-forming Dicotyledons. They appear before the 

 formation of periderm in branches during their first year, as long as the cortex is still 

 covered with uninjured epidermis, and are visible as roundish bodies. At the end of 

 the first or in the following summer, the epidermis splits above the lenticel in the 

 direction of its length ; the lenticel becomes changed into a more or less projecting 

 wart, which is often divided by a central furrow into two lip-like ridges ; its surface is 

 generally brown, its substance to a certain depth dry, brittle, and cork-like. With the 

 further increase in thickness of the branch, the lenticels become extended in a direc- 

 tion transverse to the branch, and present the appearance of transverse streaks ; 

 when afterwards cork or bark is formed, the splitting of the cortex commences with 

 these, and they become indistinguishable (as in the silver poplar, apple, and birch) ; by 

 the scaling off of the bark they are of course removed. According to Unger, the 

 lenticels arise only at those portions of the cortex where stomata occur in the epidermis ; 

 according to Mohl the inner cortical parenchyma projects in a wart-like manner 

 through the outer, and forms a cork-tissue, which, on the formation of periderm, 

 coalesces with the cork of that tissue ; as occurs, for example, in young potato-tubers. 

 The formation of cork on the lenticel continues for a number of years, until the cortex 

 which afterwards grows from within dies off on the outside, periderm or bark- 

 formations becoming interposed between the lenticels and the living part of the cortex. 

 In many trees, as Cratoegus, Pyrus, Salix, Populus, where the formation of periderm 

 begins from single spots, and becomes further extended, the lenticels are, according to 

 Mohl, the points of departure ^ 



Sect. i6. The Fibro- vascular Bundles l — The tissue of the higher 

 Cryptogams and of Phanerogams is traversed by filiform or string-Hke masses of 



* A considerable increase of thickness is not always associated with the formation of periderm, 

 as, for example, in the sunflower and other annual stems. In Viscum the epidermis always remains 

 capable of development, and its thick cuticular layers render the protection of periderm super- 

 fluous. The formation of bark is also not a necessary consequence of vigorous increase of thickness; 

 the copper-beech and the cork-oak, for example, form nothing but periderm. 



2 [For a detailed investigation of the development of lenticels, see E. Stahl, Entwickelungs- 

 geschichte und Anatomic der Lenticellen, Bot. Zeitg., 1873,] 



3 H. von Mohl, Vermischte Schriften, pp. 108, 129, 195, 268, 272, 285. — Ditto, Bot. Zeitg. 

 ^855, p. 873. — Schacht, Lehrb. der Anat. u. Phys. der Gewachse, pp. 216, 307-354. — Nageli, 

 Beitrage zur wiss. Bot. Leipzig 1858, Heft i.— Sanio, Bot. Zeitg. 1863, no. 12 et sey.— Nageli, Das 

 Dickenwachsthum des Stammes u. die Anordnung der Gefasstrange bei den Sapindaceen. Miinchen 

 1864. — Caractere et formation du liege dans les dicotyledons, in Rauwenhoff 's Archives Neerlandaises, 

 vol. V. 1870, [For the recent literature see Vines in Quart. Jour. Micr. Sci. 1876, pp. 388-3(^8.] 



