8 Botanical Section [PT i 



to give firmness and rigidity to the blade, as well as to reduce 

 its loss of moisture by transpiration. Further, strengthening 

 girder-like bands of similar tissue sometimes connect the vascular 

 bundles with the sclerenchyma bands of the ridges above, or 

 with the bands near the lower surface, or with both (see 

 Fig. 9). The space between the upper and lower epidermis 

 not occupied by the vascular bundles, strengthening bands, 

 etc., is filled up with the chlorophyll- containing tissue, but 

 in some aquatic species, e.g. Glyceria aquatica, Sm., large air 

 cavities also occur. 



Fig. 10 Fig. 11 



Fig. 10. Transverse section of a leaf of Aira ccespitosa x 20. The high ridges 

 are tipped with thick -walled tissue (sclerenchyma) which also forms an almost 

 continuous layer near the lower epidermis. Note the thin-walled motor cells 

 situated at the bass of each groove. 



Fig. 11. Transverse section of leaf of Festuca rubra var. genuina x 30. Note the 

 well-defined groups of strengthening tissue (sclerenchyma) at the keel and 

 margins, and also below each vascular bundle. 



When the blade is of the expanded, ribless type (Fig. 8), the 

 stomata are generally distributed almost equally over both sur- 

 faces, as for example in most of the Poas, Dactylis, etc. 



When ridges and their intervening grooves are conspicuous 

 above, it is found that either all the stomata are situated on the 

 upper surface, as in Festuca ovina and Aira ccespitosa] or that 

 the larger number are on the upper surface as in Lolium sps., 

 and Cynosurus cristatus. In species with marked ribs the stomata 

 are situated on the lower slopes of these ridges, and in some 

 cases are even sunk in the chlorophyll tissue. 



In most grasses there are specially modified epidermal cells 

 found on the upper surface of the blade. They are large in size, 

 thin- walled, and transparent, and are definitely situated, either 



