i96 THE STRUCTURE OF ECONOMIC PLANTS 



tural difference between the middle and lower hypocotyl, but the 

 basal portion of the latter resembles the primary root except that 

 the epidermal cells produce no root hairs and their outer walls are 

 slightly cutinized. In addition, two or three layers of subepider- 

 mal cortical cells may exhibit some cutinization. The point at 

 which these differences appear has been termed the collet. 



Vascular Transition. — The first transitional changes occur 

 in the lower hypocotyl where the groups of phloem cells are 

 extended circumferentially to form two crescent-shaped sectors. 

 In the middle hypocotyl, there is a separation of each phloem sector 

 into two groups. Coincident with this change in the position of 

 the primary phloem, the metaxylem is differentiated in a lateral 

 direction in relation to the spiral protoxylem vessels rather than 

 in a centripetal one. Thus two broad wedges of scalariform and 

 reticulate metaxylem elements are formed extending from each 

 protoxylem strand, while the central region of the axis consists of 

 undifferentiated pith parenchyma. (Fig. 147, B, C) 



Grassley (7) has described the hypocotyledonary transition in 

 detail. 



"The change from a root structure to stem structure occurs in the 

 upper hypocotyl. The first indication of such a change is in the forma- 

 tion of a central pith which separates the differentiating primary xylem 

 into two distinct units. The primary phloem groups at this level form 

 two broad arcs in the inter-cotyledonary plane, and at a slightly higher 

 level these divide into two tangentially oriented groups, one on either 

 side of the bifurcated xylem units. Each xylem unit and the two groups 

 of phloem adjacent to it constitute a cotyledonary trace. (Fig. 148, 

 C, D.) 



"Procambial strands of the foliar traces of the first two foliage leaves 

 appear in the intercotyledonary plane between the vascular elements 

 of the cotyledonary traces. (Fig. 147, C, L-i, L-z.) The vascular tissue 

 of the hypocotyl at this level forms a siphonostele made up of two coty- 

 ledonary traces and two leaf traces of the foliage leaves. Just above this 

 point, lateral veins are diverged from either side of the cotyledonary 

 traces. These with a second set of branches which diverge from the 

 midrib of the cotyledonary petioles at a higher level form the main 

 veins of the cotyledons. 



' 'The branches of the cotyledonary traces which diverge outwardly 

 from the midrib above the base of the cotyledons are endarch bundles. 

 Complete transition to the endarch collateral arrangement does not 

 occur in the upper hypocotyl. The arrangement of the vascular tissues 

 at the base of the cotyledonary petioles is the same as in the upper hypo- 

 cotyl. (Fig. 147, D.) At a higher level in the petiole, the protoxylem 



