PLANTS OF THE INDIAN DESERT. 275
The mechanical tissue is composed of stereome bundles on the
lower side of all veins, except in cases where the veins are apposed to
the lower epidermis, and of those on the upper side of vertically trans-
current and embedded veins. In the case of embedded veins the
stereome bundles form I-girders the webs of which are formed by
vascular bundles either unmixed as usually in vertically transcurrent
veins, or mixed. The margins especially when they are bluntly pointed
are strengthened by large stereome bundles.
The stereome bundles in A. funiculata are large and numerous and sometimes form a more or less continuous layer on the lower surface of the leaf- blade (fig. 335). The abundance of strengthening tissue on the lower surface is necessary to protect the leaf-blade against the tension produced by the curving upwards of the leaf-halves.
The stereome bundles on the upper side, supplemented by the articulation tissue, form an adequate strengthening tissue against strains of compression. The abundance of the articulation tissue assists the leaf-halves to regain their normal position, The occurrence of strengthening tissue on the upper surface, as extensive as on the lower, prevents the articulation tissue from performing its function.
The hairy covering on the leaf consists usually of short spiny unicellular clothing hairs with somewhat dilated bases. They are more numerous on the upper surface, usually arising from epidermal cells over the veins and arching over the furrows. Besides spiny hairs, there are long unicellular hairs arising from articulation cells of the epidermis on both sides in P. elegans (fig. 351) and E.flagelli- /era (fig. 346). In E. Boyleanus there occur only long unicellular hairs which arise from articulation cells on the upper surface. Spiny hairs are replaced by short bluntly pointed unicellular hairs in E. aristata and in species of Andropogon. The hairy covering is not found on the axis except in a few cases. In P. elegans and E. hirsutus (fig. 330) there are numerous long unicellular hairs. Short spiny unicellular hairs arise from epidermal cells over stereome bundle in E. flag ellij "era and G. Eoyleana. The absence of a hairy covering on the axis can be accounted for by the highly thickened and silicified outer epidermal walls.
The abundance of spiny hairs is the outcome of a deficient supply of water ; and their usual position above the veins is due to localised extra nourishment. The dilated bases of the spiny hairs are capable of imbibing moisture and so the hair covering has a double function, that of imbibing moisture and that of protecting the surface against strong glare and light which accelerate transpiration.
