Stems Chapter 6

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

External Form of A Woody Twig  Stem meristem produces shoot system with branches 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

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 bud at 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

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 scars mark food and water conducting tissue within leaf scars

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

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

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

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

Origin and Development of Stems  Cork cambium (= phellogen) produces cork cells with suberin and phelloderm cells Reduce water loss and protect stem against injury Lenticels - parenchyma cells in cork for exchange of gases

Tissue Patterns in Stems Steles  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

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

Tissue Patterns in Stems Herbaceous 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

Tissue Patterns in Stems Herbaceous 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

Tissue Patterns in Stems Woody Dicotyledonous Stems  Wood - 2° xylem  Differences in wood: Vascular cambium and cork cambium active all year: −Ungrained, uniform wood produced −Some tropical trees

Tissue Patterns in Stems Woody 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

Tissue Patterns in Stems Woody 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

Tissue Patterns in Stems Woody Dicotyledonous Stems Cross section of young stem with secondary growth

Tissue Patterns in Stems Woody Dicotyledonous Stems 3-D view of dicot wood

Tissue Patterns in Stems Woody 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

Tissue Patterns in Stems Woody Dicotyledonous Stems  Softwood - wood of conifers No fibers or vessel elements  Hardwood - wood of dicot trees  Resin Canals - tubelike canals 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

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

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

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

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

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

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

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

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

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