TRITICUM 165 



be united, forming broader ones. Tliey are wide at the upper 

 limits of the internode, taper gradually, becoming much nar- 

 rower beneath, and finally disappear entirely near the base of 

 the internode. Percival (10) has noted that the width of the 

 individual bands of chlorenchyma is greatest in the upper inter- 

 nodes, and that they are also more closely grouped so that the 

 upper part of the culm has a uniform green tint. In the three 

 lowest internodes, there is less chlorenchyma, and it occurs chiefly 

 in the upper part of each, being reduced or entirely wanting at the 

 bases. 



The fundamental parenchyma extends from the zone of mechani- 

 cal tissue to the central cavity. It consists of thin-walled cells 

 that are polygonal in transection and elongated in the longitudinal 

 dimension, with those adjacent to the mechanical tissue being 

 longer than the centrally located ones. The cells may become 

 lignified in the lower internodes, and parenchymatous cells which 

 have become thick-walled also constitute a part of the solid dia- 

 phragm at the nodes. 



The vascular tissue in the internodal region consists of two 

 rings of bundles. The outer ring consists of small bundles that 

 alternate with bands of chlorenchyma, while the larger bundles 

 of the inner ring lie in the fundamental parenchyma. (Fig. 73.) 

 The endarch, collateral bundles are surrounded by a sheath of 

 mechanical tissue consisting of slender elongated fibers; but, in 

 the case of the smaller bundles which are embedded in the hypo- 

 dermis, the fibers of the sheath cannot be differentiated from those 

 of the adjacent tissue. The protoxylem consists of one or two 

 annular or spiral vessels; and, in some cases, the innermost element 

 is reinforced with a combination of rings and spiral thickenings. 

 The large, laterally placed, metaxylem vessels are pitted and 

 between them are several small pitted tracheids. The phloem 

 is comprised of slender thin-walled sieve tubes and companion 

 cells. (Fig. 74, A and B.) 



The Course of the Bundles in the Stem. — At each node, some 

 i5 to 35 bundles enter the stem from the leaf sheath. About 

 half of these pass through the node into the outer hypodermal 

 ring, while the remaining somewhat larger bundles penetrate more 

 deeply and become a part of the inner vascular ring. The parallel 

 bundles follow a vertical course through the internode to the next 

 lower node, where there are anastomoses, branchings, and changes 



