46 DEPARTMENT OF AGRICULTURE GEORGIA. 



dies, or threads, of woody fibre, etc. , running through th< 

 cellular system, without apparent order; and presenting, 

 on the cross sections, a view of the divided ends of these 

 threads, in the form of dots diffused through the whole ; 

 but with no distinct pith, and no bark which is at any time 

 readily separable from the wood." GRAY. (See Fig. 7.) 



During their growth, the new woody matter is not formed 

 by additions to the outside of the stem next to the bark, 

 but " is intermingled with the old, or deposited towards the 

 centre, which becomes more and more occupied with the 

 woody threads as the stem grows older ; and the increase in 

 diameter, so far as it depends upon the formation of new 

 wood, generally takes place by the distension of the 

 whole." GRAY. 



Exogens, or outside growers, (see Fig. 8) are composed of 

 a multitude of closed cells, within which the vital princi- 

 ple acts in building up the structure of the plant. u ln 

 them and by them its products are elaborated, and all its 

 vital processes carried on." 



"A young, living, vitally active cell consists 1st. Of 

 the membrane or permanent (cell) wall ; 2d. Of a delicate 

 mucilaginous film, lining the wall, called by Mohl the pri- 

 mordial utricle; 3d. Most commonly the centre of the cell, 

 and sometimes the greater part of the cavity, is occupied 

 by the nucleus, a soft solid, or gelatinous body ; and 4th. 

 The space between the nucleus and the lining membrane 

 is filled at first by a viscid liquid called protoplasm, having 

 an abundance of small granules floating in it." GRAY. 



The wall of the cell is composed of carbon, hydrogen 

 and oxygen, and is called cellulose. In addition to car- 

 bon, hydrogen and oxygen, protoplasm contains a consid- 

 erable quantity of nitrogen. 



Plants grow both in length and size by the multiplication 

 of cells by division, beginning in a single cell in the seed, 

 which, by successive divisions and subdivisions, produces 



