2.^6 THE STRUCTURE OF ECONOMIC PLANTS 



of cells that are more or less rectangular in longitudinal section. About 

 two millimeters above the plerome initials, the primary tissues of the 

 stele begin to differentiate from the elongated procambial cells. Seeliger 

 (17) found the first differentiation to occur in two cells adjacent to 

 the pericycle and 180° from each other. These are the initials of the 

 two groups of primary phloem cells. Protoxylem elements are later 

 differentiated in a plane at right angles to these phloem groups. Differ- 

 entiation begins in the cells bordering the pericycle and progresses 

 toward the center of the axis. The three or four outermost protoxylem 

 elements have annular or spiral thickenings. The metaxylem vessels 

 in the center of the plate are larger in diameter and have reticulate 

 or scalariform thickenings. As soon as the plate is completely differ- 

 entiated, cells on the lateral faces of the same mature as metaxylem 

 vessels or parenchyma." 



Artschwager's account of the primary xylem formation is 

 essentially like that of Lyle, but he regards the protoxylem as 

 being continuous across the entire primary xylem strand and the 

 metaxylem as being lateral to this continuous zone: 



"Differentiation in the protoxylem progresses centripetally until 

 the two protoxylem points meet in the center to form the primary 

 xylem plates. From now on xylem cells mature to the right and 

 left of the xylem plate until all the cells of the primary wood have 

 been formed." 



Further differentiation of the primary phloem is not easy to 

 describe because of the small size of the elements, but Seeliger (17) 

 has reported that the metaphloem differentiates from the pro- 

 cambium one or more cells to the inside of the pericycle and that 

 it consists of sieve tubes, companion cells, and phloem parenchyma. 



The Vascular Transition. — As Artschwager has pointed out, 

 "the change from the exarch condition in the root to the endarch 

 condition in the upper hypocotyl is very abrupt, with the transi- 

 tion region extending over only a few millimeters"; but complete 

 transition of the primary . vascular elements to the collateral 

 endarch arrangement is only attained in the blade of the cotyledon. 

 Lyle (11) described and figured the transition in the Detroit Dark 

 Red variety which occurs chiefly in the region that later forms the 

 fleshy portion of the axis: 



"The lower portion of the hypocotyl is root-like in the arrangement 

 of the vascular tissues, resembling the primary root previously described 

 in this respect. (Fig. 115, A.^ The first indication of a change in 

 the vascular arrangement occurs in the upper third of the hypocotyl 

 where there is a gradual reorientation of the primary vascular tissues 



