=Epidermis.= The epidermal cells of the trailing arbutus (Plate 111, Fig. 2) are variable in size. Many of the cells are modified, as guard cells (Fig. 1). =Parenchyma.= The parenchyma cells are round and they are compactly arranged (Fig. 3) on the upper side of the leaf, but on the under side they are arranged in round, small, intercellular spaces (Fig. 5). In some of the intercellular spaces are rosette crystals (Fig. 7). =Under Epidermis.= The under epidermal cells are smaller than the upper epidermal cells. It will be seen that the structure of trailing arbutus leaf is very simple and that its structure is different from that of klip buchu and mountain laurel. The structure of powdered leaves is very variable, yet characteristic for a given species. The leaves from the insect flower plant are collected with the stems, and ground and sold as a substitute for insect flowers. These leaves, when powdered, show the following structure (Plate 112): [Illustration: PLATE 110 CROSS-SECTION MOUNTAIN LAUREL (_Kalmia latifolia_, L.) 1. Hair. 2. Epidermis. 3. Palisade parenchyma. 4. Parenchyma. 5. Under epidermis. 6. Intercellular space. 7. Rosette crystal. 8. Chlorophyll.] [Illustration: PLATE 111 CROSS-SECTION TRAILING ARBUTUS LEAF (_Epigæa repens_, L.) 1. Stomata. 2. Epidermis. 3. Parenchyma. 4. Cell with chlorophyll. 5. Intercellular space. 6. Under epidermis. 7. Rosette crystal.] Both the upper and lower epidermis have stomata (Figs. 1 and 2), but they differ in that the surrounding cells of the upper epidermis are wavy, while the corresponding cells of the under epidermis are similar, though the under epidermis has many attached hairs (Figs. 3 and 4). The T-shaped hairs form the most abundant element of the powder. They are similar in structure to those found on the scales and stem. Fragments of the mesophyll have round cells and contain chlorophyll (Fig. 6). The conducting cells are spiral or reticulate. The different cells of the leaf differ greatly in structure, in amount, and in arrangement. In uva-ursi, boldus, pilocarpus, eucalyptus, and chimaphila leaves the outer walls of the epidermal cell is very thick. In uva-ursi leaves this thick wall appears bluish green when viewed under low power of the microscope. In belladonna, stramonium, henbane, peppermint, spearmint, digitalis, and horehound, the outer wall of the epidermal cells is thin. In witch-hazel, stramonium, coca, phytolacca, and peppermint there is a single layer of palisade parenchyma on the upper surface only of the leaf. In senna there is one layer of palisade parenchyma on the upper and one layer on the under side of the leaf. In matico and tea leaves there are two layers of spongy parenchyma on the upper side of the leaf. In chestnut leaves there are three layers of palisade parenchyma on the upper side of the leaf.