3R0WTH BY THE STEM. 25 



only increase by addition of new tubes having the 

 same direction as themselves, as the perpendicular 

 system. 



46. Wood properly so called, and liber or inner 

 bark, consist, in Exogens, of the perpendicular sys- 

 tem, for the most part ; while the pith and external 

 rind or bark are chiefly formed of the horizontal sys- 

 tem. The two latter are connected by cellular tissue, 

 which, when it is pressed into thin plates by the 

 woody tubes that pass through it, acquires the name 

 of medullary rays. It is important, for the due expla- 

 nation of certain phenomena connected with cultiva- 

 tion, to understand this point correctly; and to 

 remember that, while the perpendicular system is 

 distributed through the wood and bark, the horizon- 

 tal system consists of pith, outer bark, and the medul- 

 lary processes which connect these two in Exogens, 

 and of irregular cellular tissue analogous to medul- 

 lary rays in Endogens. So that the stem of a plant 

 is not inaptly compared to a piece- of linen, the hori- 

 zontal cellular system representing the woof, and the 

 woody system the warp. 



47. Whenever the stem is wounded, the injury is 

 repaired by the cellular or horizontal system, which 

 forms granulations that eventually coalesce into 

 masses {fig. 2. a), within which the perpendicular 

 system or woody matter is subsequently developed. 

 Thus the restoration of the communication be- 

 tween the two sides of an annular excision is effect- 

 ed by granulations of the upper and lower lips, and 

 of the medullary rays, which finally run together 



2 



