Basic Plant Anatomy

The various plant structures or morphologies are composed of cells that are aggregated into tissues (Figure 7.8), which are further arranged into organs (Figure 7.9); together, they form the whole plant (Figure 7.1). The specific cells and tissues that are present—specifically, the arrangement of the tissues within the organs—provide the key diagnostic microscopic characters. The ability to identify, differentiate, and describe individual plant cells and tissues is important for the botanical microscopist. The tissues that develop from the various cell types include parenchyma, collenchyma, sclerenchyma, epidermis, vascular tissue (xylem and phloem), secretory tissue, and meristematic tissue (described later) (Table 7.1).

As the plant ages, the structures of roots and stems change dramatically as the plant transitions from primary growth to secondary growth. Primary growth refers to the development of the basic plant body that arises from groups of cells with high potential for cell division. These groups of cells are present in the embryo in a seed and are

seed enc a seed enc

Exocarp

Embedded crystals in endocarp fibers

FIGURE 7.6 Structure of fruit in transverse section. (a) Crataegus laevigata showing exocarp (exc), mesocarp (mec), endocarp (enc); (b) section of Senna alexandrina fruit showing exocarp, mesocarp, and endocarp with fibers and prism crystals. (Images courtesy of Prof. Dr. Reinhard Länger, AGES PharmMed, Vienna, Austria.)

FIGURE 7.7 Cypsela of Echinacea purpurea. (a) Schematic of transverse section of Echinacea purpurea cypsela exocarp, mesocarp, and seed; (b) transverse section of Echinacea purpurea cypsela exocarp, mesocarp, fibrous layer, secretory ducts, and embryo tissues with oil droplets. (Images courtesy of Prof. Dr. Reinhard Länger, AGES PharmMed, Vienna, Austria.)

FIGURE 7.7 Cypsela of Echinacea purpurea. (a) Schematic of transverse section of Echinacea purpurea cypsela exocarp, mesocarp, and seed; (b) transverse section of Echinacea purpurea cypsela exocarp, mesocarp, fibrous layer, secretory ducts, and embryo tissues with oil droplets. (Images courtesy of Prof. Dr. Reinhard Länger, AGES PharmMed, Vienna, Austria.)

FIGURE 7.8 Schematic of plant tissues. (Image courtesy of Prof. Dr. Reinhard Länger, AGES PharmMed, Vienna, Austria.)
FIGURE 7.9 Schematic of plant organs showing primary and secondary growth characteristics. (Image courtesy of Prof. Dr. Reinhard Länger, AGES PharmMed, Vienna, Austria.)

called meristems. They can be found, for example, on the tip of a stem or a root.

Secondary growth refers to the increase in diameter of the plant axis by vascular and cork cambia following the initiation of a lateral meristem. Secondary growth is also called woody growth because the vascular cambium produces secondary xylem to the inside of the stem or root, which we know as wood, and secondary phloem to the outside, which forms part of the bark of a stem or root. Dicotyledons and gymnosperms are capable of undergoing secondary growth; monocotyledons are not and therefore do not produce true wood. The structure of their stems or roots does not undergo dramatic changes with age.

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