74 MORPHOLOGY OP THE CELL. 



to the fact that their cavit} T 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 Aroidere, by the 

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

 in Tropseolum, 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 underlying structure is very simple. 3 



CORK. 



243. This protective tissue is formed beneath and replaces 

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

 stitutes the films bv which wounds are healed. Onlv the inner 



' */ 



Ia3 r ers 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 ciystals. The inner, active, and 

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

 P hello y en ; 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 (<?. (/., 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 Spiram ; (3) from parenclryma at 

 injured surfaces, as in the healing of wounds. 



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

 flowering plants, especial ly dicotyledons. Its cells are generally 



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

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



2 De Bary : Anatomie, p. 57. 



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



