GROWTH OF STEM. 35 



ference of the stem, whose interior it thus separates into two 

 parts : namely, the bark or the superficial, and the pith or the 

 central, portion ; or, in what are called Endogens, into a super- 

 ficial coatiag analogous to bark, and a central confused mass of 

 wood and pith intermingled. The effect of this, in Exogens, is, 

 to divide the interior of a perennial stem into three parts, the 

 pith, the wood, and the bark. 



Since the cellular tissue of the stem is not sensibly length- 

 ened more in one direction than in another, and as it is that 

 kind of organic matter, which, in stems, chiefly increases later- 

 ally, it is sometimes convenient to speak of it under the name 

 of the horizontal system ; and, for a similar reason, to designate 

 the woody tubes which are plunged among it, and which 

 increase by addition of new tubes having the same direction as 

 themselves, as the perpendicular system. 



Wood properly so called, and liber or inner bark, consist, in 

 Exogens, of the perpendicular system, for the most part ; while 

 the pith and external rind or bark are chiefly formed of the 

 horizontal system. 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 explanation of certain 

 phenomena connected with cultivation, to understand this 

 point correctly, and to remember that, while the perpendicular 

 system is distributed through the wood and bark, the horizontal 

 system consists of pith, outer bark, and the medullary pro- 

 cesses which connect these two in Exogens, and of irregular 

 cellular tissue analogous to medullary rays in Endogens. So 

 that the stem of a plant is not inaptly compared to a piece of 

 linen, the horizontal cellular system representing the woof, and 

 the woody system the warp. 



Whenever the stem is wounded, the injury is repaired by 

 the eellular or horizontal system, which forms granulations 

 that eventually coalesce into masses (Fig. VII. a), within which 

 the perpendicular system or woody matter (b) is subsequently 

 developed. Thus the restoration of the communication between 

 the two sides of an annular excision is effected by granulations 

 of the upper and lower lips, and of the medullary rays, which 



D 2 



