THE WALLFLOWER 67 



sheaths of closely-fitting parenchymatous cells (see 

 Fig. 24). 



We must now say something as to the tissue 

 which surrounds the vascular bundles in the larger 

 veins of the leaf, especially in the midrib (see Fig. 

 25). The parenchymatous tissue of the midrib 

 contains but little chlorophyll, and closely resembles 

 the cortical tissue of the stem. The cells towards the 

 upper and lower surfaces of the midrib are rather 

 thicker-walled than the rest. The other main veins 

 of the leaf show a somewhat similar structure on a 

 much smaller scale. Together with the midrib they 

 form a mechanical supporting system for the leaf, 

 constituting a stiff framework, between the ribs of 

 which the delicate tissue of the lamina is extended, 

 and thus kept in position. The larger bundles also 

 have a little parenchyma within their sheath. 



Now we will consider the structure of the 

 chlorophyll - containing tissue (mesophyll), of which 

 the thin deep-green lamina chiefly consists. 



On examining a transverse section (see Figs. 25 and 

 26) we see at once that the parts of this tissue towards 

 the upper and lower surface respectively have a 

 different structure. The- upper part of the mesophyll 

 is made up of three or more layers of closely-packed 

 cells, which are elongated in the direction at right 

 angles to the surface of the leaf. These cells contain 

 an immense number of chlorophyll-corpuscles embedded 

 in their protoplasm, and chiefly ranged on the side- 

 walls of the cells (see Fig. 26). From the appear- 

 ance of its upright and regularly arranged cells this 



