In Part II the different types of cells and cell contents found in plants have been studied. In Part III it will be shown how these different cells are associated and the nature of the cell contents in the different parts of the plant. These parts are the root, the rhizome, the stem of herbs, bark and wood of woody stems, the leaf, the flower, the fruit, and the seed.

Some fifty-five roots, rhizomes, and rhizomes and roots are official in the pharmacopoeia and national formulary. About 5 of these are obtained from monocotyledonous plants, and 50 from dicotyledonous plants.

In studying the structure of roots and rhizomes, then, it must first be determined whether the root in question is monocotyledonous or dicotyledonous. This fact is ascertained by determining the type of the fibro-vascular bundle. The bundle is of the open collateral type in all rhizomes and roots obtained from monocotyledonous plants, but it is closed, radial, or concentric in the monocotyledonous type.

In both of these groups the cellular plan of structure is similar, the chief variation being the absence of one or more types of cells, the variation in the amount,, in arrangement, in the anatomical structure, in the color, and in the cell contents of the individual cells. These facts will be impressed on the mind while studying the rhizomes and the roots.

Cross-Section Pink Root

The cross-section of pink root (Plate 89) has the following structure:

Epidermis

The epidermal cells are small, nearly as long as broad, and the outer wall is thicker and darker in color than the side and inner walls. The cells usually contain air.

Cortex

The cortical parenchyma cells are very large and somewhat rounded in outline, and the walls are white. There are about twelve rows of these cells, and each cell contains numerous small, rounded starch grains.

Endodermis

The endodermal cells are tangentially elongated, and the walls arc very thin and white. There are two or three layers of endodermal cells; the cells' outer layers are larger than the cells of the inner layers.

Pericycle

The cells forming the pericycle are sieve cells and phloem parenchyma. The sieve cells are small, angled cells with extremely thin, white walls.

The phloem parenchyma resemble the sieve cells, except that they are larger.

Cambium

The cambium cells are rectangular in shape; the walls are thin and white.

Xylem

The xylem is composed of tracheids, wood parenchyma, and wood fibres.

Tracheids

The tracheids are the largest diameter cells of the centre of the root. The walls are thick and the cells are slightly angled in outline.

Wood Parenchyma

The wood parenchyma cells surrounding the tracheids are five to seven, angled, and the walls are not so thick as the walls of the tracheids.

Medullary Rays

The medullary ray cells resemble the structure of the wood parenchyma cells, but they are radially elongated.

Pith Parenchyma

The cells forming the pith parenchyma are larger than the cells of wood parenchyma, but their structure is similar.

Plate 89. Cross-section of Root of Spigelia marytandica, L.

1. Epidermis. 2. Cortical parenchyma. 2. Intercellular space. 3. En-dodermis. 4. Pericycle. 5. Cambium. 6. Xylem. 7. Pith.

Cross-Section Ruellia Root

The cross-section of ruellia root (Plate 90) shows the following structure. It should be carefully noted how the structure differs from that of pink root:

Epidermis

The epidermal cells are angled and variable in size; many of the epidermal cells are modified as root hairs.

Hypodermis

The cells of the hypodermis are one layer in thickness and their structure is similar to the epidermal cells.

Cortex

The cortex contains parenchyma and stone cells. The outer layers of the cortical parenchyma cells are round in outline, and they contain dark-brown cell contents, while the cortical parenchyma cells bordering on the endodermis are small and they are free of dark-brown contents.

Many of the inner parenchyma cells contain amorphous deposits of calcium carbonate.

The stone cells are porous and striated, and the walls are thick and white.

Endodermis

The endodermal cells are tangentially elongated, and the walls are thin and white.

Pericycle

The cells forming the pericycle are the sieve cells, bast fibres, and phloem parenchyma.

The sieve cells are small, angled cells with thin, white walls.

The phloem parenchyma cells resemble the sieve cells, but they are larger.

The bast fibres occur singly or in groups of two or three. They are rounded in outline, and the walls are white, non-porous, and non-striated.

Xylem

The xylem is composed of vessels, wood parenchyma, and wood fibres.

Vessels

The vessels are rounded in outline and few in number.

Wood Parenchyma

The wood parenchyma cells are variable in size and shape, but all the cells are angled in outline.

Medullary Rays

The medullary ray cells are not clearly distinguishable.

Pith Parenchyma

The pith parenchyma cells of the centre of the root resemble the cortical parenchyma cells.

That the structure of rhizomes is similar to the structure of roots is shown by the drawings of spigelia rhizome (Plate 91), and by ruellia rhizome (Plate 92).

Plate 90. Ruellia Root (Ruellia ciliosa, Pursh.).

I. Epidermis with root hair. 2. Parenchyma cells with dark contents. 3. Sclerid. 4. Parenchyma without dark cell contents. 5. Endodermis. 6. Bast fibers and phloem. 7. Cambium. 8. Xylem. 10. Pith.