INTRODUCTION 13 
parts of the phloem. Tetroctohedric crystals of trihydrated 
calcium oxalate (CaC,0,.3H,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. 
(s) Medullory kaya are Fic. 7.—Transverse section of phloem 
very numerous, and may (420): c.c.,crystal-containing cell; i.s., 
be from one to ten cells intercellular space ;_m.r., medullary ray ; 
high. They are at first PDs pllcem parenchyma; s.t., sieve 
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 » in tangential, 20 in longitudinal, and 40,» in radial 
direction, but the cells at the top and bottom of a ray are 
often extd@ided 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. 
