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 

 pericycle. 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 







Fig. 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 



