STRUCTURE OF STEMS; WOOD 



291 



Epidermis 



(1) An outer covering or skin, called the epidermis, as 

 in the leaf. 



(2) Next inward, a layer of soft tissue, called the 

 cortex. This is generally green. 



(3) A wood layer. 



(4) A central portion, called the pith. When trees 

 are hollow, it is 



usually the pith 

 and the wood 

 nearest to it that 

 have decayed. 



From the pith 

 to the cortex 

 there are rays, 

 like the spokes of 

 a wheel. These 

 are called pith 

 rays, or medullary 

 rays. The pith 

 rays divide the 

 wood layer into 

 woody bundles. 



The arrange- 

 ment just given is that present in all dicotyls (cf. 308) and 

 in plants like pines and firs (gymnosperms; cf. 327). In 

 plants having only one cotyledon in the seed, such as corn, 

 the woody bundles are not arranged around a pith, and 

 separated by pith rays, but are scattered through the stem. 



Trees with the four regions just named can grow thicker 

 year by year; the wood is usually added in rings, called 

 annual rings (cf. Fig. 238a). If only one is produced in a 



Sap Wood 



Medullary Ray 



FIG. 238a. Cross Section of a Woody Stem. 



