io6 



MORPHOLOGy OF TISSUES. 



epidermis lying above the centre is divided by several bipartitions into four, six (Mar- 

 chantia, Fegatella), or several {Rehouillia) cells, which are arranged radially about a point 

 where their walls unite. Here the cells separate from one another, and the cleft (Fig, 

 89, B 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. 



V\0.^q.—Marcha7ttiapolymorpha. Part of a young- fructification ; A vertical section, o epidernns, .S" partition- 

 wall between the air-cavities with their chlorophyll-cells chl\ g large parenchymatous cell ; sp stoma ; B and C young 

 stoHiata seen from above ; po cleft ; si guard-cells ( x S5o)- 



(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 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 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 

 effectual 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 

 DraccEna), 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 hot. Inhalts. Tiibingen 1845, pp. 221, 233. — Hanstein, 

 Untersuch. iiber den Bau u. die Entwickelung der Baumrinde. Berlin 1853. — Sanio, in Jahrb. fiir 

 wiss. Bot., vol. II. p. 39. — Merklin, Melanges biol. du Bulletin de TAcad. Imp. des sciences de 

 St. Petersbourg, vol. IV. Feb. 26, 1S64. 



