with verrucose walls. Pericycle of groups of stone-cells. Pith of thin-walled cells.
Heliotropium paniculatura L— Figs. 215, 216. Woody. Meso- phyll formed of palisade tissue on the upper side and of arm-palisade tissue on the lower. Middle tissue present. Hairs unicellular and with verrucose walls. A layer of colourless tabular cells on the inner side of the assimilatory tissue in the axis. Endodermis with granular contents. Pericycle formed of groups of stone-cells. Pith of thin-walled cells with granular contents.
Tricodesma indicum Br. — Figs. 217, 218. Quite herbaceous. Mesopbyll formed of palisade tissue on the upper side and of arm-palisade tissue on the lower. Veins with bundle-sbeaths. Middle tissue absent. Sclerenchymatous pericycle not developed. Pith of thin-walled cells.
Sericostoma pauciflorura Sths. — Figs. 219, 220. Woody. Mesophyll isobilateral. Middle tissue present. Hairs unicellular and with verrucose walls. Pericycle forming a composite ring of stone-cells. A layer of colourless tabular cells on the inner side of the assimilatory tissue in the axis. Pith of thin-walled cells.
Arnebia hispidissima DC. — Figs. 221, 222, 223. Herbaceous. Mesophyll formed of palisade tissue on the upper side and of arm-palisade tissue on the lower. Middle tissue absent. Veins with bundle-sheaths. Hairs unicellular and with verrucose walls. Pericycle formed of isolated stone-cells. Pith of cells with lignified walls.
Structure of the Leaf. — The epidermal cells have the outer walls greatly thickened. The inner walls are thin and more or less undulated. The outer walls are muriculate in E. aspcra and A. hispidissima ; they are convexly arched outwards inT. indicum and S. puvci- florum. Some of the epidermal cells in H, rariflorum and T. indicum are vertically elongated, while the rest are tabular or polygonal ; this peculiarity in shape may be supposed to mark the commencement of unicellular hairs (fig. 206.) These hairs have been observed at different stages of development (fig. 206, 207, 208). It seems that the epidermal cells at these spots divide by vertical walls ; the daughter cells then elongate and present a shape as in (figs. 210, 217). One of these daughter cells then elongates still more and gives rise to a small unicellular hair with a large lumen and with a broad base. These hairs will be discussed in greater detail under the hairy covering. The stomata are usually placed in depressions produced either by the thickened or papillose outer epidermal cells. The guard-cells are in the plane of the surrounding cells. The stomata are more numerous in members with a larger number of veins. In H. paniculatum there is a sort of depression produced by the downwardly pro-
