PLANT TISSUES 143 
PRIMARY XYLEM (PROTOXYLEM) is that xylem in all conduct- 
ing bundles which directly develops from the plerome. In 
exogenous stems of secondary growth, the primary xylem is in 
small strands adjacent to the pith, and in roots of secondary 
growth it appears as a small, several rayed strand in the center. 
: 
a a 
we ae 4 
a a ek dite as pees | 
Fic. 81.—Bicollateral bundle of the stem of Squash (Cucurbita). X70. P’, 
external phloem; P?, internal or intraxylary phloem; x, xylem; ca, cambium; si, a 
sieve tube. 
SECONDARY XyLeM (METAXYLEM) is xylem layed down 
by the cambium. 
PHtoem! is that part of a vascular bundle that contains sieve 
tubes, phloem parenchyma cells, and often bast fibers. The 
sieve tubes are the most important structural and functional 
elements of the phloem. In Pteridophytes and many Gymno- 
sperms these.and phloem parenchyma are alone present in the 
1 The conducting elements of the phloem which comprise the sieve tubes, 
companion cells and phloem parenchyma constitute the leptome. 
