152 



PART II. ANATOMY AND HISTOLOGY. 



35 



which offers an obstacle to the passage of water ; hence all the 

 tissues, in a stem or root, lying externally to the periderm 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 a heterogenous pericycle (see p. 119), 

 as in Berberis, Lonicera, etc., where the outer portion of the 

 pericycle is fibrous, the epidermis, 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 Onagracese, Hypericacese, some 



Rosacese, etc.) some 

 layers of the peri- 

 derm consist of cells 

 with a suberised zone 

 like that of the cells 

 of the endodermis 

 (see p. 116), though 

 these cells usually 

 become completely 

 suberised eventually. 

 In other cases (e.g. 

 stem of Poteriiim, 

 Alchemilla, Agri- 

 monia, Epilobium) 

 the periderm con- 

 sists mainly of cells 

 with cellulose-walls, 



FIG. 118.-Periderm of one-year's shoot of Ailanthus V + PAT1 w hir}i m- 



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



the inner shaded layers are merismatic, the innermost being tercellular Spaces are 



the phellogen, those external to it being young periderm f nrTnp ^ together 

 cells; r primary cortex. 



with occasional com- 

 pact layers of cells with a suberised zone. 



When the' primary periderm is of superficial origin, it forms for 

 many successive years the external investment of the branch ; it 

 may attain considerable thickness, as in the Cork-Oak, and at the 

 same time exhibit an alternation of dense and loose layers (e.g. the 

 Birch, in which the layers may be peeled off in thin white sheets) ; 

 sometimes (as in Acer campestre and the Cork-Elm) it forms wing- 

 like projections from the angles of the branches. In a few trees, as 

 the Silver Fir, the primary periderm persists for some years, or, as 



