io6 
MORPHOLOGF OF TISSUES. 
epidermis lying above the centre is divided by several bipartitions into four, six {Mar- 
chantia, Fegatella), or several {Reboiullia) cells, which are arranged radially about a point 
where their walls unite. Here the cells separate from one another, and the cleft (Fig. 
89, 5 and C, po) is surrounded by four, six, or more guard-cells {si). Each of these cells 
is finally divided by walls parallel to the surface into from 4 to 8 cells lying one above 
another, and the stoma becomes a canal surrounded by 4, 8, or more rows of cells. 
(c) Cork, and Epidermal Formations formed from it^ (Periderm, Lenticels, Bark\ 
When succulent organs of the higher plants, no longer in the bud-condition, are injured, 
the \vound 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 outermost injured layers of cells. 
The walls of this tissue resist the most various agents; similar to the cuticular layers 
of the epidermis in their physical properties, 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 sur- 
face, of parallelopipedal form, and constitute a close tissue without intercellular spaces. 
These are the general distinguishing features of cork-tissue. It not merely forms on 
wounded surfaces, but arises in much greater mass where succulent organs require an 
eflrectual protection, as on 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 
DraccEno^i, but generally in stems and roots of Conifers and Dicotyledons when 
several years old, the cork-tissue is formed even before the destruction of the epider- 
mis; 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 itself, more 
often in the subjacent tissue. These partition-walls lie parallel to the surface of the 
organ ; where the increase of the circumference necessitates it, vertical divisions also 
' H. von Mohl, Vermischte Schriften bot. Inhalts. Tübingen 1845, PP' ^^i, 233.^ — Hanstein, 
Untersuch, über den Bau u. die Entwickelung der Baumrinde. Berlin 1853. — Sanio, in Jahrb. für 
wiss. Bot., vol. II. p. 39. — Merklin, Melanges biol. du Bulletin de l'Acad. Imp. des sciences de 
St. Petersbourg, vol. IV. Feb. 26, 1864. 
