THE LEAVES 57 



dicotyledon. In optical surface view the cells appear as circles from 

 20 to 32 p in diameter (, Fig. 48). 



The cells of the chlorophyll-containing tissue in the central part of 

 the leaf are much more irregular in shape and are loosely packed, with 

 larger intercellular spaces between them. 



The assimilating tissue of the leaf-sheath is arranged in parallel 

 strips between the vascular strands, chloroplasts being present to a depth 

 of three or four cells from the outer epidermis only ; the rest of the 

 parenchyma on the inside of the bundles and near the inner epidermis 

 is colourless. 



Extending longitudinally throughout the greater part of a full-grown 

 sheath are large lysigenous cavities between the vascular bundles 

 (/, Fig. 47). 



The ground tissue of the thick nodal part of the sheath is composed 

 of parenchymatous cells somewhat flattened when young and more or 

 less hexagonal in transverse section, with unlignified walls 4 p thick 



(Fig. 58). 



Chloroplasts are present especially in the subepidermal layers. In 

 each cell on the outside of the stereome, and between the vascular bundles, 

 there is a single crystal or cluster of crystals of calcium oxalate (Fig. 58). 

 The crystals appear to be waste products connected with the processes 

 involved in the thickening of the neighbouring cell walls. Starch grains, 

 4 /z in diameter, are also found in the two or three cell layers of the ground 

 tissue on the inner side of the xylem of the bundle. 



(c) The Vascular Bundles. Running through the ground tissue of the 

 blade, somewhat nearer to the lower than the upper surface, are the 

 vascular bundles, three types of which may be recognised, viz. : (i) 

 stout bundles running longitudinally from the base to the apex of the 

 blade, (2) more slender bundles parallel to and lying between the strong 

 ones, and (3) very fine transverse veins which cross more or less obliquely 

 from one longitudinal strand to another. 



The number present in a leaf depends upon the size of the latter and 

 its position on the plant, the upper leaf often possessing more than three 

 times as many bundles as the lowest leaf on the straw. The first foliage 

 leaf of a seedling plant of T. vulgare usually has 11-13 bundles, 3-5 of 

 which may be traced to near the apex. In a full-grown leaf on a well- 

 developed straw there are generally 9-13 stout bundles, and between 

 each pair of these there may be 1-5 slender intermediate strands, the 

 most frequent number being 2 or 3. 



Below are given the number found in the broadest part of the several 

 leaves on a full-grown straw of T. vulgare. 



When the bundles are traced from the base towards the apex, some of 

 them, instead of remaining as single strands throughout their course, 



