INTRODUCTION 



13 



parts of the phloem. Tetroctohedric crystals of trihydrated 

 calcium oxalate (CaC 2 O 4 .3H 2 O) are occasionally present in 

 the vacuoles of these cells (fig. 6, p.p.). These crystals are 

 larger than those in the small crystal-containing cells. 



The phloem parenchymatous elements occur more or less 

 regularly in tangential rows, giving the effect of annual 

 rings. Also the number 

 of rings is approximately 

 equal to the number of 

 years that the stem has 

 grown, but they are not 

 sufficiently definite to 

 determine an exact cor- 

 respondence. 



(iv) Branched scleren- 

 chymatous elements, like 

 those in the cortex, may 

 also be found in the outer 

 phloem. They seem to 

 replace elements of 

 phloem parenchyma. 



(v) Medullary rays are 

 very numerous, and may 

 be from one to ten cells 

 high. They are at first 

 only one cell in thickness, 



but through cell divisions they may become two or more 

 cells thick*. The cells remain thin walled, and contain 

 protoplasm and nuclei ; among the contents may also 

 be seen numerous resin drops, especially in the vicinity of 

 the cambium, and these drops are apparently conducted 

 outwards to the resin sacs. The dimensions are about 

 5-20 n in tangential, 20 /u in longitudinal, and 40 ju in radial 

 direction, but the cells at the top and bottom of a ray are 

 often extended longitudinally. 



(vi) Resin cysts may also be found in the outer phloem. 

 They have the same form as those in the pericycle, and are 

 made by the expansion of a two-cell-thick medullary ray. 



FIG. 7. Transverse section of phloem 

 ( x 420 ) : c.c., crystal - containing cell ; i. s. , 

 intercellular space ; m.r., medullary ray ; 

 p.p., phloem parenchyma ; s.t., sieve 

 tube. 



