CORK. Ill 



styphelioides a tangential annular thickening, which is uneven and wavy, and runs 

 round the middle of the wall. 



Corresponding to thq usual conditions of structure of membranes, the thicken- 

 ing mass is in the cases of stronger thickening internally superposed upon a delicate 

 homogeneous limiting layer (' primary membrane '). 



The membranes, which in the young meristematic stage are cellulose walls, 

 appear in the mature cork-cell always siiberised to a varying extent, i. e. com- 

 posed of cork subsla?ice. Of the properties of this body we know that the su- 

 berised membrane shows, in its behaviour under reagents, and its coarser physical 

 properties, specially its slight permeability for water, completely similar phenomena 

 to the cuticle and the cuticularised membranes. Its chemical composition, and the 

 statement that it contains nitrogen, which was found on the analysis of large masses 

 of cork, are doubtful ^ When examined microscopically the high refractive power of 

 the suberised membrane is striking ; its outlines appear dark and sharply marked in 

 the illuminated field, from the time when the suberisation begins : thicker membranes 

 (e. g. of the stems of the Birch or Beech) appear brightly luminous ; in thinner 

 ones one often observes as the microscope is focussed varying phenomena of 

 colour ^ 



According to the stage of the suberisation we may distinguish totally or partially 

 suberised membranes, or layers of them. The former behave under reagents like 

 cuticle (p. 74), cellulose cannot be proved present in them; the latter still contain 

 cellulose even when old. 



Often the wall is suberised all round and throughout its whole thickness: periderm 

 of Fagus, Salix, and many thin-walled cork-cells on the surface of roots and tubers. 

 On the other hand, the totally suberised mass often forms an outer lamella of the wall 

 going all round, and it is lined internally by a layer of a different material, — partially 

 or often perhaps not at all suberised, — which, after treatment with solution of potash, 

 shows a clear cellulose coloration with iodine and sulphuric acid, or Schultze's solution. 

 This lining is very slightly developed, only as an extremely delicate skin in the 

 thin-walled cork-cells of Quercus suber and Betula. It is in this case only visible 

 after not too strong warming with potash : continued boiling with this reagent 

 destroys or alters it again : in Q. suber its destruction is accompanied by peculiar 

 phenomena, which still remain to be carefully investigated. In many other cork- 

 cells the cellulose-containing layer is to be seen, immediately within that which is 

 totally suberised, as a less refractive mass applied to this internally, forming 

 sometimes a thin layer equally thick all round, e. g. branches of Nerium, root of 

 Rheum rhaponticum ; sometimes a stronger thickening mass, equally thick all round 

 (flat cork-cells of Boswellia papyrifera), or it is thicker on one side (Zanthoxylon 

 fraxineum, Populus fastigiata, Platanus occidentalis). Without being acted upon by 

 potash, it does not turn blue : in Platanus it turns blue after warming for a short 

 time with this reagent; in the other cases named even after the reagent has acted 

 for a few minutes without the temperature being raised. Most of the cork-cells 



* On the chemical relations of cork, its products of decomposition, &c., compare the epitomes 

 and citations of the literature in Hofmeister, Die Lehre von der Pflanzenzelle, p. 252 ; Wiesner, 

 RohstoPfe, p, 479; Gmelin-Kraut, Handb. d. Chemie, VII, i. p. 593; Husemann, Pflanzenstoffe, 

 p. 1016. ^ Compare Sanio, I.e. p. 57. 



