The scantiness of the veins is characteristic of leaves of herbaceous members, while their abundance and anastomosis distinguish the leaves of woody members. Though the veins are numerous in the latter, transpiration is arrested by the thickened and cuticularised outer epidermal walls, by the poorly developed ventilating system and by the abundance of tannin in cells of the mesophyll.
The clothing hairs on the leaf and axis in all members except H. supinum are characterised by calcification of their walls which are consequently verrucose and rough. Some of the epidermal cells In these members undergo a division by vertical walls as in (fig. 210) and elongate as shown in (figs. 207, 208, 209). The walls of the terminal portion are secondarily impregnated with calcium carbonate. The hairs thus formed are unicellular, conical and have a broad thin-walled base. They are found in the leaf and axis of all members except H. supinum. Besides these, there are found other forms of clothing hairs in the different members. In E. aspera (fig. 202), T. indicum (fig. 218) and A. hispidissima (figs. 221, 222, 223) are found unicellular hairs which have a thin-walled bulbous base and a tapering terminal portion with walls, verrucose or smooth as in H. supinum. Hairs in H. undulatum consist of a thin- walled basal cell and of a long appressed terminal cell with verrucose walls. Cystolith-like structures occur in the basal portions of the hairs of S. pauciflorum, A. hispidissima, H. rariflorum and H. undulatum, T. indicum.
It should be observed in all these forms of hairs that the basal portion is bulbous and thin-walled and it may be a means of absorbing water trickling down the terminal portion. Hairs, when not numerous, are more or less appressed as in E. aspera and H. undulatum and they form a suitable light screen.
Glandular hairs are not common. They are found in a very small number in H. supinum, and they consist of a stalk-cell and of a spherical unicellular head.
Structure of the Axis : — The epidermal cells have outer walls thickened and cuticularised in all members except the herbaceous ones. The lateral walls are usually thin and undulated. Some of the epidermal cells in T. indicum and S. pauciflorum are large and vertically elongated (figs. 217, 220); and give rise to a somewhat ribbed appearance to the axis. These cells may have the function of giving rigidity to the epidermal tissue. The primary cortex is generally characterised by an assimilatory tissue which consists either of palisade cells as in H. undulatum (fig. 214), or of arm-palisade tissue as in S. pauciflorum (fig. 220) and A. hispidissima, or of chlorenchyma as in E. Aspera, H. supinum, H. panic ulatum (fig. 216) and T. indicum. There is a two-layered tissue of large thin-walled, colourless tabular
