74 MORPHOLOGY OP THE CELL. 



to the fact that their cavity answering to the intercellular space 

 of a stoma is often filled with water instead of air, these have 

 been called water-pores. At certain times liquid water passes 

 through these pores, collecting at the opening and sometimes 

 leaving there, upon evaporation, slight incrustations of calcic 

 carbonate. Water-pores assume different forms and vary much 

 in size. Good examples are afforded by many Aroideae, by the 

 teeth of the leaves in some species of Fuchsia, the leaf-margins 

 in Tropaeolum, etc. 1 



Small rifts of nearly the same shape can be found in certain 

 grasses ; but in these the aperture comes from a mechanical rup- 

 ture, 2 and the underling structure is very simple.' 



CORK. 



243. This protective tissue is formed beneath and replaces 

 epidermis in the older superficial parts of plants ; it also con- 

 stitutes the films by which wounds are healed. Only the inner 

 layers of cork-tissue possess cellular activity, those which lie 

 outside of them having perished : the former contain protoplasm 

 and are capable of cell-division ; the latter contain air, and 

 occasionally small clusters of crystals. The inner, active, and 

 growing layers are known as cork meristem, cork cambium, or 

 Phellogen ; the outer, produced from this and no longer living, 

 make up the bulk of the outer bark, and are ordinarily called 

 cork. Although the older cork-tissues must be further described 

 in Chapter III., under "Bark," their elements may be conven- 

 iently treated of now in connection with the cells which produce 

 them. 



244. Origin. Cork may arise from several different sources, 

 the principal of which are the following: (1) from division of 

 cells in the epidermis (e. g., species of Pyrus, Salix, Viburnum, 

 etc.) ; (2) more commonly from underlying parenchyma, in a few 

 cases even from that which occurs in the inner bark (the bast 

 parenchyma), as in Vitis and Spiraea; (3) from parenchyma at 

 injured surfaces, as in the healing of wounds. 



245. It is normally produced upon the stems and roots of 

 flowering plants, especially dicotyledons. Its cells are generally 



1 For a full account of water-pores, see de Bary's Anatomic, p. 54, and 

 Jahrb. konigl. botan. Garten, Berlin, 1883. 

 a De Bary : Anatomie, p. 57. 

 Gardiner : Proceedings Camb. Phil. Soc., 1883. 



