SECONDARY STRUCTURE OF THE STEM 107 



phellogen or cork cambium, are sloughed away. Their place is 

 gradually taken by the bark which consists in many gymnosperms 

 and dicotyledons of dead or dying secondary phloem and of areas 

 of i^eriderm. The intra-cambial primary tissues, i.e., primary 

 xylem and pith, are completely buried within the cylinder of 

 secondary xylem and, as a result of stretching and compression, 

 may be crushed or even destroj'ed. 



The early phases in development of secondary vascular tis- 

 sues in woody stems are usually accompanied by the formation 

 of a periderm or corky tissue beneath the epidermis. Func- 

 tionally, the periderm acts as a protective layer, replacing in 

 this respect the epidermis which is eventually killed and sloughed 

 away. Structurally, the term periderm is applied to the phello- 

 gen or cork cambium and its two derivative tissues, viz. : cork or 

 phellem, and phclloderm. The first-formed phellogen in the stem 

 appears to arise as a result of the regressive differentiation of 

 epidermal, cortical or pericyclic parenchyma cells. Its initiation 

 is indicated by the tangential division of certain cells. In some 

 species, these first tangential divisions appear in the epidermis. 

 Most commonly, perhaps, the phellogen originates in the outer- 

 most cells of the cortex. There is evidence that in some stems, 

 the cortical phellogen first makes its appearance beneath the sto- 

 mata, at which point lenticels are produced. Continued spread 

 of the phellogen from these structures may result in the forma- 

 tion of a cylinder of cork cambium. As a result of the repeated 

 tangential division of the phellogen cells, the derivative tissues 

 exhibit alignment of the cells in radial rows. Cells differentiat- 

 ing towards the outside of the phellogen lose their protoplasts, 

 acquire suherin in their unpitted walls, and finally mature as cork 

 cells. The phelloderm tissue, which is usually much less in extent 

 than the cork, originates from the inner derivatives of the phello- 

 gen. Phelloderm eells are described as being parenchyma-like in 

 retaining their protoplasts and in having simple pits in their 

 cellulose walls. The functional life of the first-formed phellogen 

 in woody stems is short and new layers of cork cambium arise 

 successively from deeper regions of the cortex and pericycle 

 until, finally, the living eells of the secondary phloem participate 

 in periderm formation. The ultimate result is the production 



