THE CELL AND TISSUE. 71 



12. Two of the strands begin development simultaneously 

 on the opposite sides of the pith (Fig. 107), in case of the 

 dicotyledonous stem. Later two others appear (Fig. 108), and 



^Cambium 

 Strand _-,^— ^ Epidermis 



Oortex ^%i2cs#*^ ^^^^ Cortex^ 



Fig. 108. Fig. 109. 



very soon numerous strands are present which together form 

 a continuous ring (Fig. 109 ; other parts of this figure, also of 

 Figs. 107 and 108, are the pith, the cortex, and the epidermis). 

 As the wood in each strand is on the pith side, the fusion of 

 the strands results in the formation of a ring of wood adja- 

 cent to the pith. It is evident also that a continuous ring 

 of cambium would be formed, and outside of this a ring of 

 bast. In the following season the cambium renews its growth, 

 the cells are multiplied, and those next to the wood formed 

 the first year are changed to wood cells ; on the opposite 

 side some of the cambium cells become bast. The cells of 

 wood formed in the early spring are usually larger than those 

 formed in the latter part of the growing season, and hence 

 the rings or cylinders of growth for each year are distinctly 

 seen in most trees (Fig. 110). The entire portion exterior to 

 the cambium may be designated as bark. In the outer por- 

 tion of the bast, corky plates or corky layers are developed ; 

 these and the external lifeless bast cells form the dry, often 

 furrowed, dead bark. 



13. The original tissue, or that from which the epidermis 

 and woody strands are developed, is called the ftindamental 



