Stems

1. Stems Chapter 6

2. Outline • External Form of a Woody Twig • Origin and Development of Stems • Tissue Patterns in Stems • Herbaceous Dicotyledonous Stems • Woody Dicotyledonous Stems • Monocotyledonous Stems • Specialized Stems • Wood and Its Uses

3. External Form of A Woody Twig • Stem meristem produces shoot systemwithbranches and leaves • Woody twig consists of axis with attached leaves • Node -area of stem where leaves attach • Alternate or spiral • Opposite -attached in pairs • Whorled -in groups of 3 or more • Internode - stem region between nodes • Leaf has flattened blade and usually attached to twig by petiole

4. External Form of A Woody Twig • Axil -angle between petiole and stem • Axillary bud located in axil • Become branches or flowers in flowering plants • Bud scales protect buds • Terminal budat twig tip • Growth makes twig longer • Number of groups of bud scale scars tells age of twig • Stipules - paired, often leaflike appendages at base of leaf

5. External Form of A Woody Twig • Deciduous trees and shrubs (lose all leaves annually) - After leaves fall, have dormant axillary buds with leaf scars below • Bundle scarsmark food and water conducting tissue within leaf scars

6. Origin and Development of Stems • Apical meristem at stem tip • Increases stem length • Dormant before growing season begins • Protected by bud scales and by leaf primordia • Leaf Primordia- tiny embryonic leaves that develop into mature leaves Longitudinal section through stem tip

7. Origin and Development of Stems • Apical meristem cells form 3 primary meristems • Protoderm - gives rise to epidermis • Procambium - produces primary xylem and phloem • Ground Meristem - produces pith and cortex, both composed of parenchyma cells Longitudinal section through stem tip

8. Origin and Development of Stems • Leaf primordia and bud primordia develop into mature leaves and buds • Traces branch off from cylinder of xylem and phloem, and enter leaf or bud • Trace - strand of xylem and phloem • Each trace leaves gap filled with parenchyma in cylinder of vascular tissue, forming leaf gap or bud gap

9. Origin and Development of Stems • Narrow band of cells between 1° xylem and 1 °phloem may become vascular cambium • Vascular cambium produces 2°xylem toward center and 2°phloem toward surface

10. Origin and Development of Stems • Corkcambium (= phellogen) produces cork cells with suberin and phellodermcells • Reduce water loss and protect stem against injury • Lenticels - parenchyma cells in cork for exchange of gases

11. Tissue Patterns in StemsSteles • Stele - central cylinder of 1°xylem, 1°phloem, and pith (if present) • Protostele - solid core, phloem surrounds xylem • Primitive seed plants, whisk ferns, club mosses and ferns • Siphonosteles - tubular with pith in center • Common in ferns • Eusteles - discrete vascular bundles • Flowering plants and conifers

12. Tissue Patterns in Stems • Cotyledons -seed leaves attached to embryonic stems • Store food needed by young seedling • Dicotyledons (Dicots) - flowering plants developing from seeds with 2 cotyledons • Monocotyledons (Monocots) - flowering plants developing from seeds with single cotyledon

13. Tissue Patterns in StemsHerbaceous Dicotyledonous Stems • Annuals - plants that die after going from seed to maturity within 1 growing season • Usually green, herbaceous plants • Most monocots are annuals, but many dicots are also annuals • Tissues largely primary

14. Tissue Patterns in StemsHerbaceous Dicotyledonous Stems • Herbaceous dicots - discrete vascular bundles arranged in cylinder • Vascular cambium between 1°xylem and 1° phloem • Adds 2° xylem and 2°phloem Dicot stem

15. Tissue Patterns in StemsWoody Dicotyledonous Stems • Wood - 2° xylem • Differences in wood: • Vascular cambium and cork cambium active all year: • Ungrained, uniform wood produced • Some tropical trees

16. Tissue Patterns in StemsWoody Dicotyledonous Stems • If wood produced seasonally: • In spring: Relatively large vessel elements of 2°xylem produced - Spring Wood • After spring wood: Fewer, smaller vessel elements in proportion to tracheids and fibers - Summer Wood • In conifers, vessels and fibers absent • Tracheids in spring larger than later in season

17. Tissue Patterns in StemsWoody Dicotyledonous Stems • One year’s growth of xylem = Annual Ring • Vascular cambium produces more 2°xylem than phloem • Bulk of trunk = annual rings of wood • Indicates age of tree • Indicates climate during tree’s lifetime • Vascular Rays- parenchyma cells functioning in lateral conduction of nutrients and water • Xylem Ray - part of ray within xylem • Phloem Ray - part of ray through phloem

18. Tissue Patterns in StemsWoody Dicotyledonous Stems Cross section of young stem with secondary growth

19. Tissue Patterns in StemsWoody Dicotyledonous Stems 3-D view of dicot wood

20. Tissue Patterns in StemsWoody Dicotyledonous Stems • Tyloses -protrusions of adjacent parenchyma cells into conducting cells of xylem • Prevent conduction of water • Resins, gums, and tannins accumulate, and darken wood, forming heartwood • Heartwood - older, darker wood in center • Sapwood - lighter, still-functioning xylem closest to cambium

21. Tissue Patterns in Stems Woody Dicotyledonous Stems • Softwood - wood of conifers • No fibers or vessel elements • Hardwood - wood of dicot trees • Resin Canals - tubelikecanals scattered throughout xylem and other tissues • Lined with specialized cells that secrete resin • Common in conifers • Some tropical flowering plants • Frankincense Resin canals in pine

22. Tissue Patterns in Stems Woody Dicotyledonous Stems • Bark - tissues outside vascular cambium, including 2°phloem • May consist of alternating layers of crushed phloem and cork Cross section of young stem with secondary growth • Laticifers - ducts found mostly in phloem that have latex-secreting cells • Rubber, chicle (chewing gum), morphine

23. Tissue Patterns in Stems Monocotyledonous Stems • Monocots stems - no vascular cambium nor cork cambium • No 2°vascular tissues or cork • 1°xylem and phloem in discrete vascular bundles scattered throughout stem • Xylem closer to stem center and phloem closer to surface • Parenchyma (ground tissue) surrounds vascular bundles Cross section of monocot stem

24. Tissue Patterns in Stems Monocotyledonous Stems • Typical monocot vascular bundle: • 2 large vessels with several small vessels • First xylem cells stretch and collapse • Leave irregularly shaped air space • Phloem = sieve tubes and companion cells • Vascular bundle surrounded by sheath of sclerenchyma Monocot vascular bundle

25. Specialized Stems • Rhizomes - horizontal stems that grow below-ground and have long to short internodes • Irises, some grasses, ferns • Runners - horizontal stems that grow above ground and have long internodes • Strawberry • Stolons - produced beneath surface of ground and tend to grow in different directions • Potato

26. Specialized Stems • Tubers - swollen, fleshy, underground stem • Store food • Potatoes - eyes of potato are nodes • Bulbs - large buds surrounded by numerous fleshy leaves, with small stem at lower end • Store food • Onions, lilies, hyacinths, tulips

27. Specialized Stems • Corms - resemble bulbs, but composed almost entirely of stem tissue, with papery leaves • Store food • Crocus and gladiolus • Cladophylls - flattened, leaf-life stems • Greenbriars, some orchids, prickly pear cactus Prickly pear cactus

28. Wood and Its Uses • In living tree, 50% of wood weight comes from water content • Dry part of wood composed of about 60-75% cellulose and about 15-25% lignin • Density - weight per unit volume • Durability - ability to withstand decay • Tannins and oils repel decay organisms • Knots - bases of lost branches covered by new annual rings produced by cambium

29. Wood and Its Uses • Wood Products • ½ of U.S. and Canadian wood production used as lumber, primarily for construction • Sawdust and waste - particle board and pulp • Veneer - thin sheet of desirable wood glued to cheaper lumber • Pulp- second most widespread use of wood • Paper, synthetic fibers, plastics, linoleum • In developing countries, ½ of cut timber used for fuel • Less than 10% in US and Canada

30. Review • External Form of a Woody Twig • Origin and Development of Stems • Tissue Patterns in Stems • Herbaceous Dicotyledonous Stems • Woody Dicotyledonous Stems • Monocotyledonous Stems • Specialized Stems • Wood and Its Uses