FOEMATION OF WOOD 265 



Once more making a cross section through a 

 still older part of the stem, we shall see that 

 great changes have taken place as growth has 

 proceeded. The central pith has not increased 

 in area, it may even have decayed away en-» 

 tirely ; the cambium has actively formed wood 

 within and bast without, with the result that 

 we are presented with a solid, broad ring of 

 wood ; a ring of thin-walled cambium cells ; 

 a ring of bast, which is smaller in area than the 

 ring of wood ; a ring of cortex surrounded by 

 cork cells which, with the epidermis, form the 

 true bark and are formed from yet another 

 meristematic tissue. Running from pith to 

 cortex are a number of radial Unes of tissue, 

 the primary medullary rays (Fig. 72), and a 

 number of smaller, finer lines which do not 

 extend the whole of the way from pith to cortex, 

 the secondary medullary rays (Fig. 72). The 

 wood is marked with a number of concentric 

 rings, annual rings (Fig. 72), which represent 

 approximately the number of years of growth 

 of the tree. 



Now we must turn our attention to the 

 structure of the various tissues to be found in 

 the stem. We cannot do more than give an 

 outline of the more essential points, but the 

 reader who would learn more of plant structure 

 will find aU he requires in one of the many 



