CORTICAL SYSTEM. EPIDERMIS. 517 



in the stem, never alter in condition after the first season of 

 growth, and turn outwards to terminate at the surface of the stem 

 above and below, anastomosing with their successors. 



In the Dicotyledons the bundles in a young shoot somewhat 

 resemble those of Monocotyledons, but they stand in a regular ring 

 round the pith. On the inside they present spiral and annular 

 vessels ; next, a mass of prosenchyma with dotted ducts, which 

 passes gradually into the cambium-layer; the latter is bounded 

 externally by liber, among the bundles of which are v asa propria. 

 These bundles are indefinite in their growth, producing new layers 

 of permanent tissue, liber-cells on one side of the cambium and 

 wood-cells on the other. Hence the division of the bundle into 

 phloem or bast tissues, and xylem or wood, separated by the cam- 

 bium when present. The phloem consists of thin-walled, often 

 latticed cells, sieve tubes, and of large thick-walled liber-cells. The 

 xylem consists of thick-walled cells and pitted vessels, surrounded 

 by woody parenchymatous cells. Sometimes the cells do not be- 

 come woody. Great variations occur in the degree in which the 

 several elements are present, even at different parts of the same 

 bundle. The lower extremities elongate indefinitely in the 

 root ; the upper extremities anastomose and become continuous 

 with their successors ; and above all, the cambium-region is ' an 

 indefinite focus of development, forming a new layer of woody 

 substance inside, and a new layer of liber outside during every 

 season of growth. 



Protecting Sheath. This term is applied by Caspary to a single 

 layer of cells without intercellular spaces surrounding each fibro- 

 vascular bundle, or surrounding the entire ring of bundles. By 

 Van Tieghem this layer is called the endoderm. Its cells are usually 

 more or less lignified. 



The Cortical System. Epidermis. In young stems and in her- 

 baceous organs generally this system is termed the epidermal 

 system ; as stems grow older, this gives place to the bark. The 

 cortical system may therefore be defined as comprising all the 

 tissues outside the cambium ring when present. 



The simplest form in which the cortical system exists is that 

 of a simple layer of flat cells firmly united by their sides, forming 

 a continuous coat over the surface of a plant, called the epidermis. 

 The constituent cells of the epidermis do not divide tangentially, 

 but always parallel to the surface. These cells, moreover, are 

 entirely devoid of chlorophyll or granular matter, and are derived 

 from the dermatogen cells. The epidermis is usually caducous, 

 being succeeded by the formation of corky periderm cells. Where 

 the stem remains green the periderm is not formed, but the epi- 



