Introductory Plant Biology 8/e Stern
Chapter 6: Stems
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Chapter 6: Stems
1. Stem structure and function need to be examined to understand practical uses of stems.
2. The shoot system of plants is usually erect, but some stems may be horizontal or modified for climbing or food/water storage.
3. Leaves of woody twigs may be arranged alternately, oppositely, or in a whorl. Nodes are stem regions where leaves are attached; internodes occur between nodes. Most leaves have petioles and blades. Axillary buds occur in leaf axils. Most buds are protected by bud scales. Terminal buds occur at twig tips. Terminal bud scales, when they fall, leave bud scale scars that help one determine the age of the twig.
4. Stipules are paired appendages present at the base of some leaves; when they fall off, they leave small scars on the twig. When whole leaves fall, they leave leaf scars on the twig, with tiny bundle scars within the leaf-scar surfaces.
5. Each stem has an apical meristem at its tip that produces tissues resulting in increase in length. Leaf primordia develop into mature leaves when growth begins. Three primary meristems develop from an apical meristem: the protoderm gives rise to the epidermis; the procambium produces primary xylem and primary phloem; and the ground meristem produces pith and cortex.
6. As leaves and buds develop from primordia, traces of xylem and phloem branch off from the main cylinder, leaving leaf gaps or bud gaps.
7. A vascular cambium, producing secondary tissues, may arise between primary xylem and phloem. Secondary xylem cells include tracheids, vessel elements, and fibers. Secondary phloem cells include sieve tube members and companion cells.
8. In many plants, a cork cambium producing cork and phelloderm cells develops near the surface of the stem. Cork cells, which are part of the outer bark (periderm), have suberin in their walls. Suberin is impervious to moisture, and the outer bark, therefore, aids in protection. Lenticels in the bark permit gas exchange.
9. Primary vascular tissues and the pith, if present, constitute the stele. Protosteles have a solid core of xylem, usually surrounded by phloem; siphonosteles are tubular, with pith in the center; eusteles have the vascular tissues in discrete bundles.
10. Dicotyledons (dicots) are plants whose seeds have two seed leaves (cotyledons), while monocotyledons (monocots) have seeds with one seed leaf. Herbaceous dicots have vascular bundles arranged in a ring in the stem.
11. Woody dicots have most of their secondary tissues arranged in concentric layers. The most conspicuous tissue is wood (secondary xylem). In broadleaf trees, spring wood usually has relatively large vessel members, while summer wood has smaller vessels and/or a predominance of tracheids.
12. An annual ring is one year’s growth of xylem. A tree’s age and other aspects of its history can be determined from annual rings. Rays, which function in lateral conduction, radiate out from the center of the trunk. Older wood toward the center (heartwood) ceases to function when its cells become plugged with tyloses. Younger, functioning wood (sapwood) is closer to the surface. A tree’s functions are not particularly affected by the rotting of its heartwood.
13. The wood of cone-bearing trees consists primarily of tracheids, and resin canals are often present. The wood of conifers has no fibers or vessels and is called softwood, while the wood of woody dicots is called hardwood. In woody plants, older tissues composed of thin-walled cells become crushed and functionless, and some are sloughed off.
14. Laticifers are latex-secreting cells or ducts found in various flowering plants. The latex of some plants has considerable commercial value.
15. Monocot stems have scattered vascular bundles and no cambia. The parenchyma tissue is not divided into pith and cortex. Each vascular bundle is surrounded by a sheath of sclerenchyma cells. Numerous bundles and a band of sclerenchyma cells and thicker-walled parenchyma cells just beneath the surface of monocot stems aid in withstanding stresses.
16. Palm trees are monocots that become large because their parenchyma cells continue to divide. Other monocots develop a secondary meristem that produces parenchyma cells and secondary vascular bundles. Grasses have at the base of each internode intercalary meristems that contribute to rapid increases in length. Several commercially important cordage fibers are obtained from monocots.
17. Specialized stems include rhizomes, stolons, tubers, bulbs, corms, cladophylls, and tendrils. Such stems may have adventitious roots.
18. The dry part of wood consists primarily of cellulose and lignin. Resins, gums, oils, dyes, tannins, and starch are also present. Properties of wood that play a role in its use include density, specific gravity, and durability.
19. Logs are usually cut longitudinally along the radius (quartersawed) or perpendicular to the rays (tangentially, plain-sawed, or slab cut). Knots are bases of lost branches that have become covered over by new wood; they usually weaken the boards in which they occur.
20. About half the timber produced in the United States is used as lumber. Sawdust and waste are converted to particle board and pulp for paper, synthetics, and linoleum. Other timber is used for cooperage, charcoal, railroad ties, boxes, tool handles, and so forth. Developing countries use a greater proportion of their timber for fuel.
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