INTRODUCTION 



11 



formation, which leaves the inner cortex intact even in 



old stems. 



i 



Two kinds of specialized elements occur in this layer. 

 The first are small cells set aside for the purpose of con- 

 taining crystals of calcium oxalate. These cells are only 

 found in older stems, and are very frequent in the neighbour- 

 hood of cankers. The second are irregularly branched 

 sclerenchymatous cells with 

 very thick walls (fig. 6, s.e.)- 

 These elements grow longi- 

 tudinally, forcing their way 

 between the parenchymatous 

 cells, and often branch. The 

 branching occurs when a 

 sclerenchymatous cell en- 

 counters a parenchymatous 

 cell broadside, and one branch 

 grows down on each side of 

 the obstructing cell. The 

 function of these cells is 

 obscure. 



There is no distinction be- 

 tween the cortex and pericycle, 

 but in the parenchyma im- 

 mediately abutting on the 

 phloem (and thus presumably 



pericycle) there are here and J*- ^-Longitudinal section of 



J ' phloem: c.c., crystal-containing 



there large intercellular spaces cell ; m.r., medullary ray ; p.p., 



which contain resin. When pMoem parenchyma; aXsderen- 



cnvmatous element ; ts.t., sieve 

 young they are small and tube (x 420). 



spherical, but with the in- 

 creased girth of the stem they become laterally extended 

 and are conspicuous objects in all sections ; they also acquire 

 a poorly developed epithelium. They appear to be associated 

 with some of the larger medullary rays, which, when followed 

 into the xylem, are found to be in contact with resin ducts 

 in the first-year wood. 



The phloem is composed of sieve tubes, crystal-containing 



- c.c. 



- -m.r 



- s.e. 



-p.p. 



