MEDULLARY RAYS 9 



Tracheary tissue consists of tracheae (ducts or vessels) and tracheids, 

 both of which are found in the xylem region of the fibrovascular bundle 

 and have as their function the conduction of water with mineral salts in 

 solution from the roots upward. The tracheae or ducts are elongated, 

 slightly lignefied tubes with occasional cross- walls and having character- 

 istic thickenings on their inner surface. Tracheae are classified as: 



Annular, with ring-like thickenings. 



Spiral, with spiral thickenings. 



Reticulate, with reticulate thickenings. 



Porous or Pitted with spherical or oblique slit pores. 



Fig. 5. — Cross-section through a portion of a root of Acorus calamus. A. 

 Cortical parenchyma; B. endodermis; C. pericycle; E. phloem F. xylem. At 

 Y, Y, are large tracheal tubes, which were formed last, the narrow tubes near the 

 periphery of the xylem being formed first. At the center of the root, within the circle 

 of vascular buiidles, occur thin-waUed parenchymatous pith cells. {From Sayre 

 after Frank.) 



Tracheids are undeveloped ducts having bordered pores and fre- 

 quently scalariform thickenings. 



Medullary Rays are bands of parenchymatous cells extending radi- 

 ally from the cortex to the pith (primary med. rays) or from a part of the 

 xylem to a part of the phloem (secondary med. rays). 



