210 



PART II. THE INTIMATE STRUCTURE OF PLANTS. 



[35. 



and more deeply seated, penetrating at length into the bast-tissue 

 of the stele. 



The periderm, or secondary tegumentary tissue, the tissue formed 

 externally from the phellogen, consists of parenchymatous cells 

 more or less cubical in form, though sometimes somewhat elongated 

 tangentially (Fig. 155) ; the cell-walls may be thin or considerably 

 thickened ; generally speaking, the walls are completely suberised 

 (see p. 106), whence the tissue is often termed Cork; the cells 

 gradually lose their protoplasmic contents, and become filled with 

 air ; moreover, no intercellular spaces are formed in the tissue. 

 In view of its structure, it is clear that the periderm is a tissue 



which offers an 

 obstacle to the 

 passage of water; 

 hence all the tis- 

 sues, in a stem or 

 root, lying extern- 

 ally to the peri- 

 derm can receive 

 no supplies of 

 water, and must 

 dry up, and are 

 eventually exfoli- 

 ated. The more 

 deeply seated the 

 phellogen, the 

 greater is the 

 amount of primary 

 tissue thrown off ; 

 thus, when the 

 phellogen arises in 

 the inner layers of 



FIG. 155. Periderm of one-year's shoot of Ailanthus 

 glandulosa (trans, sect. ; x 350) : e the dead epidermis ; fc cork ; 

 the inner shaded layers are merismatic, the innermost being 

 the phellogen, those external to it being young periderm 

 cells ; v primary cortex. 



a heterogenous pericycle (see p. 167), as in Berberis, Lonicera, 

 etc., where the outer portion of the pericycle is fibrous, the epi- 

 dermis, the primary cortex, and the outer portion of the pericycle 

 are exfoliated. 



The cells of the periderm are not always completely suberised. 

 In some cases (roots and stems of Onagraceae, Hypericaceae, some 

 Rosaceea, etc.) some of layers of the periderm consist of cells with a 

 suberised zone like that of the cells of the endoderrnis (see p. 166), 

 though these cells usually become completely suberised eventually. 



