io8 
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- 
tinue to grow centrifugally. Layers of cells which can extend through the most 
different tissues of the cortex are changed into cork-cambium, which ceases to be 
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- 
stantly repeated on the outside of the stem, and the new layers of cork continually 
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. 
Lenticels 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 tlie 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 Cratcegus, Pyrns, 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 ^ — The tissue of the higher 
Cryptogams and of Phanerogams is traversed by filiform or string-like 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 Anatomie der Lenticellen, Bot. Zeitg., 1873,] 
2 H, von Mohl, Vermischte Schriften, pp. 108, 129, 195, 268, 272, 2S5. — Ditto, Bot. Zeitg. 
^855, p. 873. — Schacht, Lehrb. der Anat. u. Phys. der Gewächse, pp. 216, 307-354. — Nägeli, 
Beiträge zur wiss. Bot. Leipzig 1858, Heft i. — Sanio, Bot. Zeitg. 1863, no. 12 et seq. — Nägeli, Das 
Dickenwachsthum des Stammes u. die Anordnung der Gefässtränge bei den Sapindaceen. München 
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. Sei. 1876, pp. 388-398.] 
