CORK. 109 



layer of parenchyma lying immediately below this or deeper. In the healing of wounds 

 it is as a rule that layer of parenchyma which lies immediately within that injured by 

 the wound : still there occur exceptions to this rule, as a more deeply-seated layer 

 may become the initial layer of the cork. The forms respectively of the initial cells, 

 and of the layer of cork which results from their division, depend therefore upon that 

 of the surface to be shut off. In wounds it may have all possible shapes ; in normal 

 cork formation it resembles the normal surface of the member. 



By the divisions parallel to the surface the single layer of initial cells is converted 

 into a multi-seriate meristematic zone. Of the layers of this zone the outer, i.e. those 

 opposite the normal surface, or the wound surface, assume at once the properties of 

 cork-cells and accordingly lose the power of division. But a single layer of cells 

 bordering the cork-cells internally retains as a rule the properties of meristem, and 

 therewith the power of continuous division : this is the cork-forming ox phellogenetic 

 meristem, or the phellogenetic layer. 



On parts still increasing in girth, the layer of phellogen follows this growth, and 

 by divisions perpendicular to the surface can increase the number of its cells, and of 

 the layers of cork produced from them. 



From what has been said it follows that the cells of a mass of cork are arranged 

 in rows perpendicular to the surface, each of which corresponds to one initial cell. 

 As the cork-covered surface increases in circumference each series may successively 

 be doubled. The serial arrangement of the elements of the cork perpendicular to 

 the surface is always very regularly preserved. The walls parallel to the surface 

 usually correspond pretty closely in neighbouring series one to another, so that 

 besides the arrangement in perpendicular series there is also a no less regular 

 arrangement in layers parallel to the surface. 



The succession of the divisions parallel to the surface has been carefully studied 

 by Sanio in the normal cork formation in the cortex of ligneous plants, especially 

 Dicotyledons, and will for these cases be more minutely described in Chap. XV ^. In 

 other cases there exist no detailed investigations of the succession of divisions. But 

 one will hardly be wrong in assuming as the rule for the great majority of these 

 cases the simplest of the types of succession distinguished by Sanio, that termed by 

 him centripetal. In this the initial cell divides in the direction of the surface into two 

 nearly equal daughter cells, of which the outer becomes directly a cork-cell, the inner 

 a meristematic cell. In all the successive divisions which follow this, the same 

 process is repeated, the outer cell always becomes directly a cork-cell, the inner 

 remains meristematic. For the other types of succession also, which will be described 

 in Chap. XV, the same general result holds, that at least to every mass of cork which 

 grows to any considerable extent, new cork-cells are added from the meristem, which 

 is to be found on its inner surface. 



The average number of the strata of cells produced in one cork-layer is in the 

 majority of cases small, the layer thus appears as a thin skin varying in thickness 

 according to the special case from two to twenty cells. When this persists for a 

 long time it retains nearly an equal thickness, since the outer layers die oif and peel 

 away. As this proceeds they are renewed from behind by the zone of meristem. 



* Compare also there the figures relating to cork formation. 



