APIUM GRAVEOLENS 461 



There is an extensive development of secondary phloem which 

 forms a broad zone consisting of groups of sieve tubes, companion 

 cells and fibers, separated by rays of parenchyma. Numerous oil 

 ducts occur in the phloem region which resemble the primary ones 

 except that the number of epithelial cells is ordinarily greater. 



The rapid development of secondary stelar tissue results in an 

 early disorganization and rupturing of the cortical and epidermal 

 regions. The endodermis is broken; and, as disintegration pro- 

 ceeds, the pericyclic cells divide both anticlinally and periclinally 

 so that a zone of parenchymatous tissue is formed. The peripheral 

 portion becomes a periderm, and the phellem is several cells in 

 thickness, being renewed by a phellogen that also cuts off some 

 phellodermal cells on its inner face. 



Vascular Transition. — The transition from the radial exarch 

 arrangement of the stele of the root to the collateral endarch 

 orientation of the vascular tissues in the cotyledons takes place in 

 the upper hypocotyl. The change is abrupt, being accomplished 

 within the space of a few millimeters. The first stage consists 

 of the segmentation of each of the primary phloem groups into 

 two separate groups. At the level at which this segmentation 

 occurs, there is a reorientation of the centrally located metaxylem 

 elements which, instead of forming a continuous spindle-shaped 

 zone from one protoxylem pole to the other, differentiate laterally 

 so that parenchymatous cells occupy the center of the stele. 



At higher levels, there is further lateral differentiation of the 

 metaxylem. The polar sectors of protoxylem are oriented in such 

 a way that the later-formed metaxylem cells are tangentially 

 placed and separated from the former by parenchymatous cells. 

 The phloem groups, each of which has an oil duct, lie in an ap- 

 proximately collateral position with respect to the protoxylem, 

 and lateral branches occupy a similar relation to the primary xylem 

 that with them forms the lateral traces of each cotyledon. The 

 vascular supply to each cotyledon consists of three bundles. The 

 large median bundle, sometimes called a "double bundle" or 

 "triad," is comprised of a protoxylem strand, a portion of the 

 metaxylem lying tangential to it, and two groups of phloem which 

 are collateral with the metaxylem. The two smaller lateral 

 bundles are collateral. 



At the point of divergence of the cotyledons, the bases of the 

 petioles form a continuous collar around the epicotyl enclosing 



