42- 



THE STRUCTURE OF ECONOMIC PLANTS 



tostele of the root, as well as the single vascular strand in the stem 

 of certain hydrophytic dicotyledons; and the term monostele has also 

 been applied as a synonym for protostele. The former term arose 

 prior to the development of the stelar theory, and the latter has 

 been largely superseded by the term -protostele. In radial siphono- 

 steles, the adjacent groups of xylem and phloem occurring on 

 separate radii have been termed radial bundles. Inasmuch as the 



Fig. 13. Transection of the primary root of Pisum showing the triarch primary xylem 

 strand and the development of phloem fibers adjacent to the pericycle. Outside the upper- 

 most protoxylem point a pericyclic ceil has divided tangentially, initiating lateral root forma- 

 tion. 



primary xylem and phloem tissues are definitely separated by non- 

 vascular tissue, it is probably more accurate to reserve the term 

 bundle for those cases in which the two tissues arise from a single 

 provascular strand and to designate the relation of the primary 

 xylem and the phloem in the root as a radial arrangement. 



Ontogeny of the Primary Root. — The primary root, or at 

 least the apical growing point, is developed during the early stages 

 of embryogeny. The degree of development attained prior to 

 germination varies greatly, ranging from an undifferentiated zone 

 of meristem terminating a short conical hypocotyl (Cucurbita), to 



