414 THE STRUCTURE OF ECONOMIC PLANTS 



phases of growth, producing a multilayered periderm that serves as 

 a protective zone for the mature root. 



The Vascular Transition. — The vascular transition in the 

 seedling axis has been investigated by Spieth (31), who used 

 five-day-old seedlings. In a hypocotyl of this age, the primary 

 vascular tissues of the lower portion are completely differentiated 

 and secondary thickening initiated; but, in the upper limits of 

 the same hypocotyl, the maturation of the primary tissues is just 

 beginning, there being little differentiation of vascular elements 

 at the cotyledonary node. The immature condition of the upper 

 hypocotyl and the cotyledonary node as compared with the middle 

 and lower hypocotyl is due to the fact that the former is a region 

 of continued axial elongation resulting from the activity of an 

 intercalary meristem. 



The first evidence of change in the vascular plan occurs in the 

 lower hypocotyl, about one centimeter below the soil surface, 

 in connection with the differentiation of the metaxylem. Instead 

 of forming a central strand, it differentiates in a ring surrounding 

 centrally located parenchymatous cells which are circular in 

 transection and vertically elongated with intercellular spaces 

 at their angles. (Figs. 115, B; 119, 5.) The phloem groups are 

 more extensive at this level and one or two rows of phloem cells 

 immediately inside of the pericycle have thick walls. Sieve tubes 

 and companion cells differentiate laterally in relation to the 

 thick-walled phloem cells, and adjacent to the protoxylem points. 

 Just above this level, the stele enlarges within a very short 

 vertical distance, and four triangular groups of primary xylem cells 

 are differentiated. (Figs. 115,0 119, C) Each group consists of 

 protoxylem at the outer apex of the triangle, and two tangentially 

 oriented arms of metaxylem which lie in an approximate collateral 

 position with reference to the primary phloem. At a higher level, 

 parenchyma is differentiated between the metaxylem arms so that 

 the four distinct groups of primary xylem are separated from each 

 other by medullary rays. Further differentiation results in the 

 formation of tangential bands of metaxylem that alternate with the 

 four transition bundles. (Figs. 115 D; 2.19, D.) The position of 

 the four principal phloem groups remains unchanged, except that 

 there is a large number of thick-walled phloem cells adjacent to the 

 pericycle, and small groups of sieve tubes and companion cells are 

 scattered among these thick-walled elements. 



