PERIDERM. 



165 



formation of periderm occurs before the commencement of the growth in thickness, 

 or simultaneously with it, or later. By periderm is understood a layer of tissue 

 consisting of cork-cells, which in normal cases surrounds the whole of the shoot- 

 axis or root as a continuous envelope. In most cases it is of inconsiderable 

 thickness ; occasionally, however, as in the Cork-Oak, it forms a layer (bottle-cork) 

 several centimeters thick. We have in the peel of the potato a very instructive 

 example of the formation of simple periderm. In some cases, as in the Elm, the cork- 

 periderm grows out in the form of isolated longitudinal ridges. Thicker periderm 

 may consist of alternate layers of thick-walled and thin-walled cork-cells; and the 

 individual thin layers of cork not rarely become stripped off from one another, as on 



V~.h 



Fig. 170.— Commencing development 

 of periderm in the shoot-axis of ^ttojia 

 cheirimolia (transverse section), e epi- 

 dermis ; cc cells dividing to form cork ; r 

 cortical parenchyma containing chloro- 

 phyll. 



^ftÖ;V 



Fig. 171.— Cork formation in a first year's branch oK Rihes ntgrmn 

 part of transverse section, c epidermis ; h hair; b bast-cells ; pr cor 

 tical parenchyma, distorted by the growth in thickness of the branch 

 A' the total product of the phellogcn (f) ; k the radial rows of cork-cells, 

 developed from c in centrifugal order ; /rfphelloderm— parenchyma con^ 

 taining chlorophyll, and arising centripetally from c (X 550). 



the older branches and younger stems of the Birch, and especially in many species 

 of Melaleuca. The periderm is a strengthened substitute for the epidermis. The 

 suberised cell-walls fit together on all sides without intercellular spaces, and possess 

 the properties of the cuticle and the cuticularised outer cell-walls of the epidermis. 

 The protection which they afford to the inner tissues against the evaporation of their 

 fluids is increased by the cork-cells dying and losing their sap immediately after be- 

 coming fully developed ; while granular or amorphous matters not rarely fill up the 

 cavities in part or entirely (Birch). The origin of the periderm is due to a formation 

 of tissue similar to the cambium. This cork-cambium or Phellogen either (and more 

 rarely) originates in the epidermis itself, or in the layer of primary cortical tissue 

 immediately beneath it; or in a layer lying somewhat deeper. The cells referred to 



