The description given above of ruellia root is not typical of all roots, but the structure represents the greater number of the elements that it is possible to find in a root. In many roots, for instance, there are no stone cells, in others no epidermis and no endodermis. In asclepias, aconite, and calumba stone cells occur. In symphytum, chicory, dandelion, burdock, elecampane, pyre thrum, gentian, and senega no stone cells occur. In aconite, althea, asclepias, belladonna, bryonia, columba, ipecac, jalap, krameria, sarsaparilla, scamony, stillingea, and rumex are characteristic starch grains. Symphytum, chicory, dandelion, burdock, elecampane, and pyrethrum contain inulin, but no starch. In saponaria, gentian, and senega neither starch nor inulin occurs.
When studying roots the nature of the epidermis or the periderm must be considered, as also the number of layers of cortical parenchyma; the occurrence, distribution, and amount of stone cells when present; the presence or absence of the endodermis; the occurrence and structure of bast fibres when present; the nature of the cambium cells; the width and structure of the medullary rays, the size of the wood fibres and wood parenchyma, and the nature of the cell contents and the arrangement of the fibro-vascular bundle.
CHAPTER II
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.