Plant Form and Function
26.1 Plant Tissues
1. The tissues of a flowering plant are meristems, ground tissue, dermal tissue, and vascular tissue, which includes phloem and xylem. The plant body consists of a shoot and a root.
2. Meristems are localized collections of cells that divide throughout the life of the plant. Apical meristems located at the plant’s tips provide primary growth, and lateral meristems add girth, or secondary growth.
3. Most of the plant body is ground tissue. It includes parenchyma cells, which can divide and store substances. A chlorenchyma cell is a type of parenchyma cell that photosynthesizes. Collenchyma supports growing shoots and sclerenchyma supports plant parts that are no longer growing. Sclerenchyma includes sclereids, which form hard coverings, and fibers, which form strands.
4. Dermal tissue includes the epidermis, a single cell layer covering the plant. The epidermis secretes a waxy cuticle in aerial plant parts. Gas and water exchange occur through epidermal pores (stomata), which are bounded by guard cells. Trichomes are epidermal outgrowths, and include root hairs.
5. Vascular tissue is specialized conducting tissue. Xylem transports water and dissolved minerals from roots upwards. Xylem cells are elongated with thick walls and are dead. They include the long, narrow, less-specialized tracheids and the more specialized, barrel-shaped vessel elements.
6. Phloem transports dissolved carbohydrates and other substances throughout a plant and includes sieve cells and, in flowering plants, the more specialized sieve tube members. Pores of sieve tube member cells cluster at sieve plates, allowing more efficient nutrient transport. Companion cells help transfer carbohydrates.
26.2 Anatomy of a Plant
7. A stem is the central axis of the shoot and consists of nodes, where leaves attach, and internodes between leaves, where the stem elongates. Vascular bundles in the stem contain xylem and phloem, which are scattered in monocots and form a ring in dicots. Between a stem’s epidermis and vascular tissue lies the cortex, made of ground tissue. Pith is ground tissue in the center of a stem. Stem modifications include stolons, thorns, succulent stems, tendrils, and tubers.
8. Simple leaves have undivided blades, and compound leaves form leaflets, which may be pinnate (with a central axis) or palmate (extend from a common point). Leaves are the sites of photosynthesis.
9. Leaf epidermis is tightly packed, transparent, and nonphotosynthetic. Veins may be in either netted or parallel formation. Leaf ground tissue includes palisade and spongy mesophyll cells.
10. Leaf modifications include tendrils, spines, bracts, storage leaves, insect-trapping leaves, and cotyledons.
11. Leaves are shed from an abscission zone in response to environmental cues.
12. Roots absorb water and dissolved minerals. A plant’s first root is the radicle. Taproot systems have a large, persistent major root, whereas fibrous root systems are shallow, branched, and shorter-lived. A root cap protects the tip as an apical meristem replaces cells lost during rapid extension. The apical meristem also produces cells that differentiate into the root’s epidermis, cortex, and vascular tissues.
13. The root cortex consists of storage parenchyma and endodermis. Some roots are specialized for storage or adapted to low-oxygen environments.
26.3 How Does a Plant Increase in Girth?
14. Secondary tissues increase a plant’s girth.
15. Two lateral meristems, the vascular cambium and cork cambium, produce outward growth. The vascular cambium produces secondary xylem and secondary phloem. The cork cambium produces cork and other tissues that, along with secondary phloem, comprise bark.