GOSSYPIUM 



411 



as the axis matures, with the result that the spiral thickenings 

 become longitudinally stretched, and the thin walls frequently 

 collapse. The metaxylem develops centripetally, consisting of 

 scalariform and scalariform-reticulate elements. At the center of 

 the stele, there are four to eight large tracheae which are some- 

 times separated by smaller xylem elements with scalariform thicken- 

 ings. Complete maturation of the centrally located elements does 



Fig. 2.14. Stages in development of seedling. 



not occur until about the time that secondary thickening is ini- 

 tiated. 



On alternate radii to the protoxylem are four groups of primary 

 phloem that are separated from the central metaxylem elements 

 by parenchyma. The protophloem is parenchymatous and is soon 

 crushed, while the metaphloem consists of sieve tubes, companion 

 cells, and parenchyma. As the maturation of the primary axis pro- 

 ceeds, some of the phloem cells adjacent to the cambium initials 

 become thick-walled, and sieve tubes and companion cells are 

 located at the lateral edges of these groups. (Fig. ii5, A.^ The 

 thin-walled, elongated cells of the pericycle form a single layer 

 external to the phloem region, but this region may be two or three 

 cells in thickness outside the protoxylem elements. 



The endodermal cells form a single layer with well-defined 



