5z6 THE STRUCTURE OF ECONOMIC PLANTS 



and left of the protoxylem points in a more or less tangential 

 direction so that each metaxylem group is bifurcated. (Fig. 



2-79' <^' ^0 



Artschwager (i) describes and figures this stage in the transi- 

 tion as a swinging of the two protoxylem groups in an outward 

 direction, "one group following a left, the other a right curve"; 

 but Thiel (14) agrees with the description given above. In the 

 upper hypocotyl, the bifurcation of the metaxylem becomes more 

 pronounced until, at the level of the node, each cotyledonary 

 trace consists of a group of protoxylem cells with metaxylem 

 elements lying on either flank at an approximate right angle to the 

 plane of the protoxylem poles. (Fig. 179, £.) 



While this reorientation of the primary xylem is taking place, 

 definite changes occur in the position of the primary phloem. 

 The central group of cells from each original phloem group diverges 

 gradually toward the center of the stele; and, ultimately, con- 

 stitutes the inner phloem of the bicollateral bundles of the cotyle- 

 dons. (Fig. 179, C, D, £.) The remaining groups of phloem 

 incline tangentially in the direction of the protoxylem points until 

 they occupy a position at the periphery of the stele and adjacent 

 to them. The number of phloem groups increases, and some of the 

 strands come to lie in a position outside the proto- and metaxylem. 

 The inner phloem groups also increase in number; and, as the 

 medullary region enlarges, they occupy a position on the inner face 

 of each double bundle. In this manner, the bicollateral bundle 

 of the cotyledonary trace is established. (Fig. 179, G.) 



The position and direction of development of the proto- and 

 metaxylem at the cotyledonary node is not strictly endarch, 

 although the protoxylem may be nearer the periphery than the 

 metaxylem. (Fig. 2.y^, H.) At the base of the cotyledonary peti- 

 ole, the midvein consists of protoxylem and two groups of later- 

 ally placed metaxylem with adaxial (inner) phloem on one face 

 of the xylem and several strands of abaxial (outer) phloem on the 

 other. In the lamina of the cotyledon, the protoxylem is dif- 

 ferentiated in a more nearly adaxial position with reference to the 

 metaxylem until ultimately the endarch orientation is attained. 

 (Fig. 179, /.) The amount of adaxial phloem gradually decreases 

 toward the distal portion of the midvein; and, in the smaller 

 bundles, it may disappear completely so that they are collateral 

 instead of bicollateral. 



