SPERMATOPHYTES 
241 
the cork. The phellogen layer may be developed at various depths 
in the cortex, and all the cortical Cells outside the cork die, being cut 
off from the supplies within. 
Stele. The plerome cylinder behind the growing point passes 
below into the stele containing the vascular elements. The outermost 
layer of stelar cells, abutting against the endodermis, is called the 
pericyde. The first xylem elements to appear are small in caliber, 
and of the spiral kind (fig. 542), a kind especially adapted to a region 
of rapid elongation. These groups of spiral vessels are called the 
protoxylem (fig. 541), and the later vascular elements form the meta- 
xylem (fig. 541). In case there is a cambium, a secondary xylem is 
FlG. 540. Section of the lenticel of elder. After STRASBURGER. 
formed. In neither metaxylem nor secondary xylem do vessels of 
the spiral kind usually occur, but vessels of larger caliber (fig. 541), 
notably the pitted vessels or dotted ducts so called on account of the thin 
spots left in a generally thickened wall (fig. 544). In gymnosperms 
(except Gnetales) there are no true vessels (tracheae), but tracheids 
(single cells tapering at each end) with thin spots in the wall, so char- 
acteristic in appearance as to be called bordered pits (fig. 547). In 
pteridophytes, this same kind of xylem element is represented by 
tracheids with transversely elongated pits, known as scalariform (ladder- 
like) vessels (fig. 548). 
In forming tracheids or tracheae, the protoplasts of the living cells 
gradually disappear as the characteristic thickening of the wall is formed, 
so that the completed vessels are dead cells. Tracheids are single cells 
