THE WHEAT PLANT 



1 



are seen to divide where the leaf becomes wider, the two halves gradually 

 separating into distinct bundles. 



On the other hand, in the upper part of the lamina, 

 some of the finer bundles disappear not by blindly 

 ending in the mesophyll, but by turning to one side 

 and uniting with another ; others somewhat stouter 

 divide in two, the separate parts then curve rapidly 

 to the right and left respectively and become joined to 

 neighbouring parallel bundles as indicated in Fig. 53. 

 On account of these divisions and fusions of 

 bundles, the number found varies with the part of 

 the leaf chosen for examination, being greatest in 

 the middle and least near the tip. 



The vascular bundles from the leaf-blade pass 

 uninterruptedly into the sheath and downwards into 

 F IG 53 Course of the tne n de, where they enter the diaphragm between 

 vascular bundles near two adjoining internodes (pp. 95-97). 



All the bundles are collateral, with the xylem towards 

 the upper surface of the blade, the phloem below. 

 In the xylem of the stout strands there are one or two vessels about 

 20 ,0, in diameter, with annular or spiral thickening, and to the right and 

 left of these are two vessels, usually of wider bore, with narrow elliptical 

 pits. 



In the slender intermediate bundles few or no vessels are visible, 

 although xylem and phloem are both present. 



The very fine anastomosing bundles, which cross from one parallel 

 bundle to another, consist of short tracheids about 6 fj. in diameter, with 

 parallel thin-walled parenchymatous cells about 4 p across accompanying 

 them (Fig. 54). 



Each bundle is surrounded by an inner and an outer sheath (Fig. 55) ; 

 the former (the " mestome " sheath of Schwendener) completely encloses 

 the vascular strand, and is composed of elongated thick-walled cells 



the apex of 

 blade ( x 5). 



a leaf- 



