598 THE STRUCTURE OF ECONOMIC PLANTS 



contrast to the centripetal development and exarch orientation in 

 the root. The relation of the primary phloem and xylem also 

 shifts from the alternate, radial arrangement to the collateral, 

 which ultimately becomes bicollateral with the development 

 of the inner phloem. In this manner, eight bicollateral bundles 

 are formed, separated by narrow medullary rays. (Fig. 313.) 



In C. maxima, the transition region extends from a point some- 

 what below the peg where the axis is exarch and protostelic to 

 a point just above the peg where the vascular tissues form a com- 

 pletely endarch dissected siphonostele. This involves a segment 

 of the axis which is about a centimeter in length in a week-old 

 seedling. The first transitional development is the increase in 

 the number of thickened, parenchymatous cells between the large 

 central vessels. As a result, the pitted vessels are separated into 

 two groups, and the number of large metaxylem vessels is increased 

 to four or more. The narrow band of parenchymatous cells which 

 develops in the intercotyledonary plane increases in width until 

 a definite pith region is formed. (Figs. 311, B, C, and 313, B.) 



As the pith develops, the tangentially orientated metaxylem 

 vessels lie in four groups which occupy positions near the periphery 

 of the stele and alternate with the protoxylem points. They are 

 centrad to the four phloem groups and separated from them by 

 several parenchymatous cells which later function as cambial 

 initials. (Fig. 311, C, D.) This is followed by an increase in 

 the number of reticulate vessels, and a lateral development of large 

 pitted metaxylem elements, so that the primary xylem forms a 

 hollow diamond enclosing the pith. (Fig. 313, C.) At higher 

 levels, the reorientation of the stelar tissues is more abrupt; the 

 pith enlarges, and the first inner phloem is differentiated centrad 

 to the xylem triangles formed by the original proto- and metaxy- 

 lem. (Figs. 3ii, E, and 313, D.) The cells of the inner phloem 

 have end walls that are perforated, and they resemble the sieve 

 tubes of the outer phloem except that they are much larger. 



A millimeter or two higher, each protoxylem point divides 

 and the halves are separated by parenchymatous rays. (Fig. 

 313, F.) Thus, a dissected siphonostele of four transition bundles 

 is formed, each consisting of a tangential band of metaxylem 

 terminated at both ends by annular and spiral protoxylem elements 

 with the primary phloem lying in a position collateral to it. The 

 metaxylem of the bundle consists of small reticulate tracheae, and 



