STEMS 



229 



and is called the cambium layer. It is in this layer that 

 the new cells which make the new wood and bark are 

 formed. 



Steins of this type can increase in diameter. A new 

 layer of wood is formed outside of the old wood. 

 Usually these layers of wood are so distinct that a 

 section of a woody stem shows a series of concentric 

 rings, usually one ring for each year. If a three or 

 four year old twi- is examined, several changes which 

 take place as stems grow older will be noted. A thicker 

 covering composed of waterproof bark is present. 

 The wood cylinder will be found to be much larger, 

 and the pith, smaller. In the stems of some plants, 

 such as lilies, grasses, and palms, the vascular bundles 





Pith 



Rind 



Fibrovascular 

 Bundle 



Fig. 96. Cross section of a corn stem showing the vascular bundles 

 scattered in the pith. 



