STEMS When studying stems it should first be determined whether they were derived from monocotyledonous or dicotyledonous plants. This fact is ascertained by determining the type of the fibro-vascular bundle. See Chapter XI. The next fact to determine is whether the stem is from an herb or from a woody plant. This fact is readily determined because herbaceous stems have a true epidermis, masses of collenchyma at the angles of the stem. The cortical cells contain chlorophyll, and the pith is very large. Woody stems have a corky layer, a phellogen layer, and the pith is very small except in the very young woody stems. Having determined these facts, a study should be made of the arrangement, form, structure, color, and the cell contents of the different cells in order to determine the species of plant from which the stem was obtained. HERBACEOUS STEMS The great variation in the structure of herbaceous stems is shown in the cross-sections of spigelia (Plate 95); in ruellia (Plate 96); in the charts of powdered genuine horehound, powdered spurious horehound, and in the chart of powdered insect flower stems. CROSS-SECTION SPIGELIA STEM Spigelia stem (Plate 95) has the following characteristic structure: =Epidermis.= The epidermal cells are papillate. =Cortex.= The cortical parenchyma cells consist of tangentially elongated cells which are oval in outline. =Phloem.= The phloem consists of sieve cells, phloem parenchyma, and of bast fibres. [Illustration: PLATE 95 CROSS-SECTION OF STEM OF SPIGELIA MARYLANDICA, L. 1. Papillate epidermis. 4. Phloem. 7. Inner phloem. 2. Cortical parenchyma. 5. Cambium. 8. Pith. 3. Bast stereome. 6. Xylem.] The =sieve cells= are small, and with thin, white, angled walls. The =phloem parenchyma= resembles the sieve cells, but they are larger. The bast fibres are rounded in outline and the walls are thick, white, non-porous, and non-striated. =Cambium.= The cambium cells are rectangular in shape or the walls are collapsed and the cells indistinct. =Xylem.= The xylem contains vessels, wood parenchyma, medullary rays. The vessels are small and angled, the walls are thick and white. =Wood parenchyma.= The cells are variable in size and shape, and the walls are thick. The medullary ray cells are small, narrow, and tangentially elongated. =Internal Phloem.= External to the pith parenchyma are isolated groups of internal phloem consisting of sieve cells. =Pith Parenchyma.= The pith parenchyma cells are oval in form and irregularly placed. The cells contain small, simple starch grains. RUELLIA STEM The cross-section of ruellia stem (Plate 96) is as follows: =Epidermis.= The epidermal cells are variable in shape and very large. There are no cell contents. =Cortex.= The cortex consists of collenchyma and parenchyma cells and stone cells.