MAIZE. LEAF. 115 



thinner part of the lamina, lie in a median position 

 between the two epidermal layers. The largest of 

 these correspond in structure to those of the internode, 

 the smaller ones are reduced forms of the same type. 

 Note that the spiral and annular vessels (i.e. protoxylem) 

 are nearer the upper surface of the leaf. 



Between the epidermis on either side, and the larger 

 bundles, are masses of Sclerenchyma, which, together 

 with the bundles, form complete bridges of rigid tissue 

 between the two epidermal layers. 



3. The spaces between the tissues, hitherto con- 

 sidered, are filled with parenchyma (Mesophyll), 

 which may either be (a), green (containing chlorophyll) ; 

 or (&), colourless (without chlorophyll). 



a. The green chlorophyll- containing parenchyma 

 fills up the greater part of the space ; intercellular 

 spaces occur in it. 



I. The colourless parenchyma occurs (i.), as a sheath, 

 without intercellular spaces, surrounding each bundle 

 (bundle-sheath) ; (ii.) as groups of cells immediately 

 below the epidermis ; these are more common towards 

 the central part of the leaf. At the mid-rib this tissue 

 forms the bulk of the structure. 



II. Cut transverse sections of the leaf-sheath, and 

 treat as the above. Compare the arrangement of 

 tissues with that of the lamina, and of the stem. Note 

 that colourless parenchyma preponderates. 



III. Treat a piece of the thin peripheral part of a 

 leaf (which has been previously bleached in alcohol) 

 with potash till it is transparent ; mount in glycerine, 

 and examine under a low power. Observe 



1. The parallel course of the Bundles. 



I 2 



