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 occur- 

 rence 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. 



