APIUM GRAVEOLENS 459 



and is initiated by the radial elongation of two adjacent rectangular 

 pericyclic cells. Then an oblique wall comes in at about a 45° 

 angle to the radial wall between the two pericyclic cells involved 

 in the formation of the duct. (Fig. 11, A.^ This results in the 

 cutting of each pericyclic cell into two daughter cells, one of which 

 is large and five- or six-angled, the other small and quadrangular 

 in transection. The two quadrangular cells lie at the outer limit 

 of the pericycle while the five-angled ones extend through the 

 entire width of the pericycle. These four cells have a common 

 angle at a midpoint on the radial wall between the original peri- 

 cyclic cells, and the oil passage is formed by a splitting of the walls 

 along their middle lamellae beginning at this point. This results 

 in a quadrangular passage which is limited externally by two small 

 pentagonal cells and internally by two hexagonal ones. (Fig. 

 11,5.) 



At the time that the pericyclic ducts are being formed, the pri- 

 mary phloem is differentiating; and its position can be determined 

 early in ontogeny by the formation of a primary duct in each 

 phloem group adjacent to the pericycle. These appear to be formed 

 schizogenously in a manner similar to the pericyclic oil ducts except 

 that they are bounded externally by two pericyclic cells and in- 

 ternally by three or more phloem cells so that they are pentagonal 

 in transection. (Fig. 11, C) Before the metaphloem is com- 

 pletely differentiated, the protoxylem begins to mature. This 

 proceeds centripetally from two points centrad to the central 

 pericyclic ducts and continues until a solid diarch xylem strand is 

 formed. The strand is narrow and from five to ten cells from one 

 pole to the other. The protoxylem elements are annular and spiral, 

 while the wall thickenings of the metaxylem are spiral-reticulate 

 and reticulate. The primary xylem and phloem are separated by 

 a zone of interstitial parenchyma. 



Lateral Roots. — The first lateral roots arise from the pericycle 

 at approximately the time that the maturation of the primary 

 xylem has been completed. The root primordia do not form 

 directly outside the protoxylem points, as is frequently the case 

 in lateral root formation; but, instead, are developed in arcs of the 

 pericycle to the right and left of the diarch xylem strand. (Fig. 

 X34.) This may be related to the fact that the arcs of pericyclic 

 cells abutting the protoxylem points are involved in the formation 

 of the primary oil ducts. The origin of the lateral roots at these 



