TRITICUM 



i6i 



Anatomy of the Transition Region — First and Second 

 Internodes. — In the first internode, the arrangement of the 

 xylem, phloem, and parenchymatous tissue is transitional, lacking 

 the definite arrangement found in the upper internodes. (Fig. 69.) 

 There are two collateral bundles located on opposite sides of the 

 stele; and the remaining xylem and phloem elements are arranged 



= midrib & 2 lateral bundles of 3rd leaf 

 = = •• • 6 ■ ' ■ 2nd " 



m = coleoptile bundles, cotyledon bundle 

 and common bundle of the axis 



- - ventral scale [ligule) 



coleoptile {1st leaf above cotyledon) 

 2nd internode 



upper limit of fig 71 



1st internode {no elongation) 



epiblast {portion of ligule) 

 nodal plate of the cotyledon 

 lower limit of fig 71 



— primary root stele 

 coleorhiza 



Fig. 70. Diagrammatic reconstruction of portion of seedling showing principal structures 

 present, and vascular inter-relationships of cotyledon with axis, coleoptile, and first foliage 

 leaf. Nodal plate of the cotyledon is main level at which vascular transition takes place and 

 the first internode is also in part a transition region. (After Boyd and Avery.) 



in an irregular fashion, the xylem being exarch, endarch, or indefi- 

 nite in its orientation. The scutellar bundle occupies a cortical 

 position, but the stele and cortex are not limited by a distinct 

 pericycle and endodermis respectively. 



The vascular system of the transition region has been described 

 by Boyd and Avery (3) as consisting of four groups: (i) A bundle 

 connecting the midrib of the second leaf to the cotyledonary 

 plate. Ql) The traces of the scutellum, coleoptile and second 



