Most drug plants contain storage products because they are collected at a period of the year when the plant is storing, or has stored, reserve products. These products are stored in a number of characteristic ways and in different types of tissue.

The most important of the different types of storage tissue that occurs in plants are the storage cells, the storage cavities, and the storage walls.

Storage Cells

Several different types of cells function as storage tissue. These cells, which are given in the order of their importance, are parenchyma, crystal cells, medullary rays, stone cells, wood fibres, bast fibres, and epidermal and hypodermal cells.

Storage Tissue. Cortical Parenchyma

Cortical parenchyma of biennial rhizomes, bulbs, roots, and the parenchyma of the endosperm of seeds store most of the reserve economic food products of the higher plants.

Pith parenchyma of sarsaparilla root (Plate 65, Fig. 4) and the pith parenchyma of the rhizome of memspermun, like the pith parenchyma of most plants, function as storage cells.

Wood Parenchyma

Wood parenchyma, particularly of the older wood, function as storage tissue. The wood parenchyma of quassia, like the wood parenchyma of most woods, contain stored products. In some cases the wood parenchyma contain starch, in others crystals, and in others coloring matter, etc.

In many plants, however, the parenchyma cells contain crystals. The parenchyma cells of rhubarb contain rosette crystals, while the parenchyma cells of the cortex of sarsaparilla and false unicorn root contain bundles of raphides. In every case observed the raphides are surrounded by mucilage. This is true of squills, sarsaparilla, false unicorn, etc. When cells with raphides and mucilage are mounted in a mixture of alcohol, glycerine, and water, the mucilage first swells and finally disappears.

Stone cells with starch of Ceylon cinnamon {Cinnamomum 'eylanicum, Nees.).

Plate 65. 1. Stone cells with starch of Ceylon cinnamon {Cinnamomum 'eylanicum, Nees.). 2. Stone cells with solitary crystals of calumba root (Jateorhiza palmata, [Lam.] Miers). 3. Parenchyma cells, with starch of cascarilla bark (Croton eluteria, [L.] Benn.). 4. Cortical parenchyma with starch of sarsa-parilla root (Smilux officinalis, Kunth). 5. Cortical parenchyma, with starch of leptandra rhizome (Leptandra Virginia, (L.} Nutt.). 6. Crystal cells, with solitary crystals of quebracho bark (Schlechtendal). 7. Hast fibre of blackberry root with starch (Rubus cuneifolius, Pursh.).

Mucilage and Resin.

Plate 66. Mucilage and Resin.

1. Cross-section of elm bark (Ulmus fulva, Michaux) showing two cavities filled with partially swollen mucilage.

2. Mucilage mass from sassafras stem bark (Sassafras variifolium, L.).

3. Mucilage mass from elm bark.

4. Resin mass from white pine bark (Pinus strobus, L.).

Storage Cavities

Particular attention should be given to storage cavities whenever they occur in plants, for the reason that they are usually filled with storage products, and for the added reason that storage cavities are not common to all plants. Storage cavities occur in roots, stems, leaves, flowers, fruits, and seeds.

Crystal Cavities

Characteristic crystal cavities occur in many plants. Such a cavity containing a bundle of raphides is shown in the cross-section of skunk cabbage leaf (Plate 67).

Storage Tissue. Secretion Cavities

In white pine bark there are a great number of secretion cavities which are partially or completely filled with oleoresin. In the cross-sections of white pine bark the secretion cavities are very conspicuous, and they vary greatly in size. This variation is due, first, to the age of the cavity, the more recently formed cavities being smaller; and secondly, to the nature of the section, which will be longer in longitudinal section, which will be through the length of the secretion cavity, and shorter on transverse section. Such a section shows the width of the secretion cavity.

Characteristic mucilage cavities occur in sassafras root, stem bark, elm bark (Plate 66, Fig. 1), marshmallow root, etc. These cavities form a conspicuous feature of the cross-section of these plants. The presence or absence of mucilage cavities in a bark should be carefully noted.

Latex Cavities

The latex tube cavities are characteristic in the plants in which they occur. These cavities as explained under latex tubes are very irregular in outline.

Cross Section of Skunk cabbage Leaf (Symplocarpus faetidus, [L.] Nutt.).

Plate 67. Cross-Section of Skunk-cabbage Leaf (Symplocarpus faetidus, [L.] Nutt.).

1. Crystal cavity.

2. Bundle of raphides.

Oil Cavity

Canella alba contains an oil cavity resembling in form the mucilage cavity of elm bark.

Secretion cavities occur in most of the umbelliferous fruits. For each fruit there is a more or less constant number of cavities. Anise has twenty or more, fennel usually has six cavities, and parsley has six cavities.

In poison hemlock fruits there are no secretion cavities. In certain cases, however, the number of secretion cavities can be made to vary. This was proved by the author in the case of celery seed. He found that cultivated celery seed, from which stalks are grown, contains six oil cavities (Plate 122, Fig. 2), while wild celery seed (Plate 102, Fig. 1), grown for its medicinal value, always contains more than six cavities. Most of the wild celery seeds contain twelve cavities.

Many leaves contain cavities for storing secreted products. Such storage cavities occur in fragrant goldenrod, buchu, thyme, savary, etc.

The leaves in which such cavities occur are designated as pellucid-punctate leaves. Such leaves will, when held between the eye and the source of light, exhibit numerous rounded translucent spots, or storage cavities.

Glandular Hairs

The glandular hair of peppermint (Plate 60, Fig. 3) and other mints consists of eight secretion cells, arranged around a central cavity and an outer wall which is free from the secretion cells. This outer wall becomes greatly distended when the secretion cells are active, and the space between the secretion cells and the wall serves as the storage place for the oil. When the mints are collected and dried, the oil remains in the storage cavity for a long time.

Storage Tissue. Stone Cells

The stone cells of the different cinnamons (Plate 65, Fig. 1) store starch grains; these grains often completely fill the stone ceils.

The yellow stone cells of calumba root (Plate 65, Fig. 2) usually contain four prisms of calcium oxalate, which may be nearly uniform or very unequal in size.

Storage Tissue. Bast Fibres

The bast fibres of the different rubus species (Plate 65, Fig. 7) contain starch. The medullary rays of quassia (Plate 107, Fig. 2) contain starch; while the medullary rays of canella alba contain rosette crystals. In a cross-section of canella alba (Plate 81, Fig. 3) the crystals form parallel radiating lines which, upon closer examination, are seen to be medullary rays, in each cell of which a crystal usually occurs.

The epidermal and hypodermal cells of leaves serve as water-storage tissue. These cells usually appear empty in a section.

The barks of many plants - i.e., quebracho, witch-hazel, cascara, frangula, the leaves of senna and coca, and the root of licorice - contain numerous crystals. These crystals occur in special storage cells - crystal cells (Plate 65, Fig. 6) - which usually form a completely enveloping layer around the bast fibres. These cells are usually the smallest cells of the plant in which they occur, and with but few exceptions each cell contains but a single crystal.

The epidermal cells of senna leaves and the epidermal cells of mustard are filled with mucilage; the walls even consist of mucilage. Such cells are always diagnostic in powders.

Storage Walls

Storage walls (Plates 68 and 69) occur in colchicum seed, saw palmetto seed, areca nut, nux vomica, and Saint Ignatius's bean. In each of these seeds the walls are strongly and characteristically thickened and pitted. In no two plants are they alike, and in each plant they are important diagnostic characters.

Storage cell walls consist of reserve cellulose, a form of cellulose which is rendered soluble by ferments, and utilized as food during the growth of the seed. Reserve cellulose is hard, bony, and of a waxy lustre when dry. Upon boiling in water the walls swell and become soft.

The structure of the reserve cellulose varies greatly in the different seeds in which it occurs in the thickness of the walls and in the number and character of the pores.

Reserve Cellulose.

Plate 68. Reserve Cellulose.

1. Saw palmetto (Serenoa serrulata, [Michaux] Hook., f.).

2. Areca nut (Areca catechu, L.).

3. Colchicum seed (Colchicum autumnale, L.). 3-A. Porous side wall.

3-B Cell cavity above the side wall.

Reserve Cellulose.

Plate 69. Reserve Cellulose.

1. Endosperm of nux vomica (Strychnos nux vomica, L.).

2. Endosperm of St. Ignatia bean (Strychnos ignatii, Berg.).