Page:The Journal of Indian Botany.djvu/782

274 THE JOURNAL OP INDIAN BOTANY. do not alternate with the veins. Sometimes adjacent palisade arcs or girders are connected together by transverse strands of palisade cells. In E. hirsutus there is an extensive tissue of thin-walled parenchymatous cells towards the upper surface on either side of the mid-rib ; this tissue seems to form an articulation tissue which adds to the strength of the system of strands of the articulation tissue below the furrows.

Spongy tissue is not developed in any of the members and thus the ventilating system is quite reduced. The articulation tissue forms a characteristic feature of species of Gramineae ; it is composed of thin-walled cells with a double function — that of collecting water and that of assisting the halves of the leaf-blade to curve upwards. The conical shape of the cells of the upper portion of the strands of the articulation tissue is especially suited to this purpose.

The veins are provided with bundle-sheaths of thick-walled green cubical cells. The bundle-sheaths form complete rings round non-transcurrent veins and arcs on the sides of vertically transcur- rent veins excepting E. aristata in which all the vertically transcurrent veins are provided with complete sheaths. The bundle-sheaths in 0. Thomaeum and L. senegalensis are horse-shoe shaped and are incom- plete on the lower side. As regards the function of bundle-sheaths, they many form the means of inter-communication between the vessel and the assimilatory tissue, as well as assist the latter in photosynthesis. The veins are embedded in D. sanguinalis and in species of Panicum, while they are vertically transcurrent above and below by stereome bundles in D. bispinata, C. villosa, G. Boyleana. The veins are appo- sed to the upper epidermis and are vertically transcurrent below by stereome bundles in 0. Thomaeum. In E. interrupta the larger veins are vertically transcurrent above and below by stereome bundles ; and the smaller veins are apposed to the upper epidermis and are vertically transcurrent below by stereome bundles. In other members the lar- ger veins are vertically transcurrent above and below by stereome bundles except in E. Boyleanus, C. catharticus and E. flagellifera, where aqueous cells are found between the vascular bundles and stereome bundles on one side or both sides ; the smaller bundles are embedded.

The veins either all lie in the same plane where the furrows on both the surfaces are more or less equally deep, or they are placed in two distinct planes where the furrows on the upper surface are deeper than on the lower — the veins below the furrows being situated in the lower plane. This arrangement of veins is well adapted to the curving habit of the leaf-blade.