I. The chlorophyll-tissue, on transverse sections, is arranged in rings round the vascular bundles. There are motor-cells between the ribs, and the stomata are sunk and occur on both faces.
Cynodon Dactylon. The larger lateral nerves have as a rule three smaller ones between each pair, hardly projecting as ribs. Chlorophyll chiefly in a ring round the vascular bundle. Long hairs on lower surface, a few papillæ above. Motor-cells in each shallow furrow. Short cells occur between the long epidermal cells over the bast-region.
The Panicums also come here, and differ according to the disposition of the sclerenchyma sheaths around the bundles.
II. The chlorophyll-tissue is between the vascular bundles, and not confined to rings surrounding them.
A. Conspicuous lacunæ between the vascular bundles. Stomata on both faces. Motor-cells occur.
Lacunæ large and rectangular. Motor-cells confined to a flanking line on each side of the mid-rib.
Glyceria aquatica. Leaves folded and in section V-shaped, hardly keeled, with sclerenchyma at apex. Motor-cells each side of the mid-rib only. The large square or rectangular lacunæ bounded by stellate cells. Papillæ on epidermal cells. Vascular bundles midway between upper and lower surfaces.
Glyceria fluitans. Section V-shaped and keeled, the roof of each polygonal lacuna arched, hence the “ribs" on the upper surface are between the vascular bundles. The latter lie nearer the lower epidermis. The epidermis has papillæ.
Catabrosa aquatica and Hierochloe also come here, the former with small lacunæ, the latter with larger ones chiefly towards the upper surface of the leaf.
Digraphis is also apt to have a few air cavities near the mid-rib.