OF TENNESSEE. 55 



centre in the younger parts of the stem and in the roots ; 

 while in the older parts of the stem the centre is occupied by 

 a more or less considerable cavity, full of air. This cavity 

 results from the central parenchyma becoming torn asunder 

 after it has ceased to grow, by the enlargement of the peri- 

 pheral parts of the stem. Nearer to the circumference than 

 to the centre, lies a ring of woody and' vascular tissue, 

 which, in transverse sections, is seen to be broken up into 

 wedge-shaped bundles, by narrow bands of parenchymatous 

 tissue, which extend from the parenchyma within the circle 

 of woody and vascular tissue (medulla or pith) to that which 

 lies outside of it. Moreover, each bundle of woody and 

 vascular tissue is divided into two parts, an outer and an 

 inner, by a thin layer of small and very thin cells, termed 

 the cambium layer. What lies outside this layer belongs to 

 the bark; what lies inside it, to the wood and pith. 



The cells composing the cambium retain their power of 

 multiplication, and divide by septa parallel with the 

 length of the stem, or root, as well as transverse to it. 

 Thus new cells are continually being added, on the inner 

 side of the cambium layer, to the thickness of the wood, 

 and on the outer side of it, to the thickness of the bark ; 

 and the axis of the plant continually increases in diameter, 

 so long as this process goes on. This is the developement 

 of exogens. 



The soft parts of plants as far as they are exposed to the 

 light, and as far as their epidermis is transparent, are green 

 colored. This green color results from the presence, im- 

 mediately below the epidermic tissue, and imbedded in the 

 parenchyma, of minute, soft granules, called chlorophyll or 

 leaf green. These corpuscles, through the agency of light, 

 have taken their origin in the "protoplasma," a complex 

 chemical compound essentially produced by the union of a 

 few chemical elements, oxygen, hydrogen, carbon, nitrogen, t 

 which the plant absorbes by its roots and leaves, together 

 with some mineral substances from the surrounding earth 



