ANATOMY OF WOOD. 18 



transverse growth, owing to the activity of the cambium, by a 

 stiffly constructed mass consisting chiefly of wood (Figs. 4 

 & 5, 7), inside which on transverse sections is the pith, and 

 outside, the cortex, while it contains the interposed cambium, 

 the primary medullary rays (a ), and the secondary rays (/>), 

 which have no connection with the pith. 



In the second and all subsequent years, a new woody zone, 

 external and adjacent to the first year's zone, is formed by the 

 cambium ; it surrounds all the growing points of the yearling 

 plant, the leading shoot, the branches and roots (Fig. 6). 



H-. B. 



eonilY'roiH (1). and 



Fit:. I. 



1 li;i'jfi';inis i>i' tnmsvtT-e sections of a two yean' 



broa.lleayed (:>). 



/The firs! annual /one. //The si.M-.md annual /one. M.<i Primary. M. b 

 <ee,, Hilary. medullary rays. <, / S|iriiiL r -\voil. ti ', <l Bummer-wood. Ii 



On section r (Fig. G) of the two concentric annual zones of 

 the plant, two concentric circles appear, the inner circle sur- 

 rounding the medullary sheath with the pith in its centre and 

 forming the first annual ring ; the outer circle surrounds the 

 second year's zone and is the second annual ring. Thus, every 

 year a concentric zone of wood is produced, and by counting 

 these zones on a transverse section of wood its age can be 

 determined. 



The annual zones of certain trees exhibit differences in 

 their anatomical structure, e.g., in oaks and ashes, the vessels 

 at the commencement of each year's zone are wider than those 



