Plant Organs: Stems Chapter 7

LEARNING OBJECTIVE 1 Describe three functions of stems

Stem Functions Support Conduct Produce new living tissues leaves and reproductive structures Conduct water, dissolved minerals, carbohydrates Produce new living tissues at apical meristems at lateral meristems (secondary growth)

Variation in Stems

LEARNING OBJECTIVE 2 Relate the functions of each tissue in an herbaceous stem

Tissues in Herbaceous Stems 1 Epidermis protective outer layer covered by water-conserving cuticle Vascular tissues Xylem conducts water and dissolved minerals phloem conducts dissolved carbohydrates (sucrose)

Tissues in Herbaceous Stems 2 Storage tissues Cortex and pith ground tissue

LEARNING OBJECTIVE 3 Contrast the structures of an herbaceous eudicot stem and a monocot stem

Herbaceous Eudicot Stems Have vascular bundles arranged in a circle (in cross section) Have a distinct cortex and pith

Monocot Stems Have scattered vascular bundles Have ground tissue instead of distinct cortex and pith

Monocot Stems

(a) Cross section of a corn (Zea mays) Epidermis Ground tissue Vascular bundles Ground tissue Phloem Xylem Air space Figure 7.4: Arrangement of tissues in a monocot stem. Bundle sheath Corn seedling (a) Cross section of a corn (Zea mays) stem, showing the scattered vascular bundles. (b) Close-up of a vascular bundle. The air space is where the first xylem elements formed. The entire bundle is enclosed in a bundle sheath of sclerenchyma for additional support. Fig. 7-4, p. 134

LEARNING OBJECTIVE 4 Distinguish between the structures of stems and roots

Differences Between Stems and Roots 1 Unlike roots, stems have nodes and internodes, leaves and buds Unlike stems, roots have root caps and root hairs

KEY TERMS NODE INTERNODE Area on a stem where one or more leaves is attached; stems have nodes, but roots do not INTERNODE Stem area between two successive nodes

KEY TERMS BUD An undeveloped shoot that contains an embryonic meristem May be terminal (at tip of stem) or axillary (on side of stem)

A Woody Twig

Terminal bud scale scars Node One year’s growth Terminal bud scale scars Node Internode Axillary bud Leaf scar Node Figure 7.2: The external structure of a woody twig. The age of a woody twig is determined by counting the number of groups of terminal bud scale scars (don’t count those on side branches). How old is this twig? Lenticels Terminal bud scale scars Bundle scars Fig. 7-2, p. 131

Differences Between Stems and Roots 2 Internally herbaceous roots possess an endodermis and pericycle stems lack a pericycle and rarely have an endodermis

Differences Between Stems and Roots

LEARNING OBJECTIVE 5 Outline the transition from primary growth to secondary growth in a woody stem List the two lateral meristems, and describe the tissues that arise from each

Primary Growth: Eudicot

Vascular bundles Pith Cortex Epidermis Sunflower Epidermis Cortex (a) Cross section of a sunflower (Helianthus annuus) stem, showing the organization of tissues. The vascular bundles are arranged in a circle. Sunflower Epidermis Cortex Phloem fiber cap Pith ray Figure 7.3: Primary growth in a eudicot stem. Phloem Vascular cambium Vascular bundle Xylem Vessel element Pith (b) Close-up of a vascular bundle. The xylem is toward the stem’s interior, and the phloem toward the outside. Each vascular bundle is “capped” by a batch of fibers for additional support. Fig. 7-3, p. 132

KEY TERMS VASCULAR CAMBIUM A lateral meristem that produces secondary xylem (wood) to the inside and secondary phloem (inner bark) to the outside

Secondary Growth Occurs in woody eudicots and conifers Produced by vascular cambium between primary xylem and primary phloem

Vascular Cambium 1 Is not initially a solid cylinder of cells becomes continuous when production of secondary tissues begins

Vascular Cambium 2 Certain parenchyma cells between vascular bundles retain ability to divide connect to vascular cambium cells in each vascular bundle form a complete ring of vascular cambium

Dividing Vascular Cambium

Second division of vascular cambium forms a phloem cell. 1X 2X 3X 4X 2P 1P 1X 2X 3X 2P 1P 1X 2X 2P 1P Time Secondary xylem Secondary phloem Figure 7.6: Radial view of a dividing vascular cambium cell. To study this figure, start at the bottom and move up. Note that the vascular cambium (blue cell) divides in two directions, forming secondary xylem (X) to the inside and secondary phloem (P) to the outside. These cells, which are numbered in the order in which they are produced, differentiate to form the mature cell types associated with xylem and phloem. 1X 2X 1P Second division of vascular cambium forms a phloem cell. 1X 1P Division of vascular cambium forms two cells, one xylem cell and one vascular cambium cell. 1X Vascular cambium cell when secondary growth begins. Vascular cambium cell Fig. 7-6, p. 136

KEY TERMS CORK CAMBIUM A lateral meristem that produces cork parenchyma to the inside and cork cells to the outside Cork cambium and the tissues it produces make up the outer bark of a woody plant

Cork Cambium Arises near the stem’s surface Is either a continuous cylinder of dividing cells or a series of overlapping arcs of meristematic cells that form from parenchyma cells in successively deeper layers of the cortex and, eventually, secondary phloem

Development: Woody Eudicot

(a) At the onset of secondary growth, vascular cambium arises in the Primary xylem Epidermis Cortex Primary phloem Vascular cambium Pith (a) At the onset of secondary growth, vascular cambium arises in the parenchyma between the vascular bundles (that is, in the pith rays), forming a cylinder of meristematic tissue (blue circle in cross section). Figure 7.5: Development of secondary growth in a eudicot stem. Vascular cambium and the tissues it produces are shown; cork cambium is not depicted. (The figures change in scale from part a to part c because of space limitations; pith and primary xylem are actually the same size in all three diagrams, but the change in scale makes those tissues appear to shrink.) Fig. 7-5a, p. 135

and secondary phloem on the outside. Remnant of cortex Remnant of epidermis Remnant of primary phloem Secondary phloem (inner bark) Secondary xylem (wood) Periderm (outer bark) Remnant of primary xylem Remnant of pith Vascular cambium Figure 7.5: Development of secondary growth in a eudicot stem. Vascular cambium and the tissues it produces are shown; cork cambium is not depicted. (The figures change in scale from part a to part c because of space limitations; pith and primary xylem are actually the same size in all three diagrams, but the change in scale makes those tissues appear to shrink.) (b) Vascular cambium begins to divide, forming secondary xylem on the inside and secondary phloem on the outside. Fig. 7-5b, p. 135

(outer bark; remnants of primary phloem, cortex, Periderm (outer bark; remnants of primary phloem, cortex, and epidermis are gradually crushed or torn apart and slough off) Secondary xylem (wood) Secondary phloem (inner bark) Figure 7.5: Development of secondary growth in a eudicot stem. Vascular cambium and the tissues it produces are shown; cork cambium is not depicted. (The figures change in scale from part a to part c because of space limitations; pith and primary xylem are actually the same size in all three diagrams, but the change in scale makes those tissues appear to shrink.) Remnant of primary xylem Remnant of pith Vascular cambium (c) A young woody stem. Vascular cambium produces more secondary xylem than secondary phloem. Fig. 7-5c, p. 135

3-Year-Old Stem

Secondary xylem (wood) Pith Primary xylem Annual ring of secondary xylem Secondary xylem (wood) (a) LM of cross section of basswood (Tilia americana) stem. Note the location of the vascular cambium between the secondary xylem (wood) and secondary phloem (inner bark). Vascular cambium Secondary phloem Periderm and remnants of primary phloem, cortex, and epidermis Expanded phloem ray Xylem ray Remains of epidermis Cork Cork cambium Expanded phloem ray Figure 7.7: Three-year-old stem in cross section. It may be easier to study the parts labeled on the sketch and then locate them on the micrograph. (b) Sketch of a pie-shaped segment of the cross section. The primary phloem is not labeled because it is crushed beyond recognition. Cortex Secondary phloem Vascular cambium Secondary xylem (third year) Xylem rays Secondary xylem (first year) Primary xylem Pith Fig. 7-7, p. 137

Onset of Secondary Growth

Cortex Phloem fiber cap Primary phloem Secondary phloem Vascular cambium Secondary xylem Primary xylem Figure 7.8: Onset of secondary growth. This cross section through part of a magnolia (Magnolia) stem shows a vascular bundle that was split apart by secondary growth. Compare this vascular bundle with the one in Figure 7-3b, which has primary growth only. Cross section of twig Vascular cambium Pith Fig. 7-8, p. 138

Variation in Bark

Lenticels

Lenticel Cork cells Cork cambium and cork parenchyma (phelloderm) Figure 7.10: Structure of a lenticel. This cross section through the periderm of a calico flower (Aristolochia elegans) stem shows a lenticel. Cork cells Cork cambium and cork parenchyma (phelloderm) Calico flower Fig. 7-10, p. 141

Heartwood and Sapwood

Heartwood Sapwood Fig. 7-11, p. 141 Figure 7.11: Heartwood and sapwood. This cross section through a butternut (Juglans cinerea) trunk reveals the darker heartwood in the center of the tree and the lighter sapwood. The sapwood is the functioning xylem that conducts water and dissolved minerals. Fig. 7-11, p. 141

Tree-Ring Dating

Long, slender core of wood extracted by a boring tool Vascular cambium Pith Annual rings Bark Tilia (basswood) Long, slender core of wood extracted by a boring tool Vascular cambium Outer bark Annual rings Pith Tree-ring dating. Scientists develop a master chronology by using progressively older pieces of wood from the same geographic area. They can then accurately determine the age of a wood sample by using a computer to match its rings to the master chronology. Sample from a living tree 1950 1940 1932 Outermost ring is the year when the tree was cut. Sample from a dead tree in the same forest 1940 1932 1931 1926 Matching and overlapping older and older wood sections extends dates back in time Sample from an old building in the same area as the forest 1931 1926 1920 1918 p. 143

Annual Rings

Secondary phloem Vascular cambium Late summerwood Annual ring of xylem Figure 7.12: Annual ring. This portion of a basswood (Tilia americana) stem, in cross section, shows one annual ring, or growth increment. Note the differences in size between the vessel elements of springwood and late summerwood. Springwood Cross section of 3-year-old Tilia stem Late summerwood of preceding year Fig. 7-12, p. 144

Sections

(b) Tangential section Annual rings (a) Cross section Ray (c) Radial section (b) Tangential section Figure 7.13: A block of wood showing (a) cross, (b) tangential, and (c) radial sections. Both tangential and radial sections are longitudinal sections. A tangential section is cut in a plane that does not pass through the center of the stem but instead passes at a right angle to a radius. A radial section is cut in a plane that passes through the center along a radius of the stem. Rays and annual rings are distinctive for each section. Rays Annual rings Annual rings Rays Fig. 7-13, p. 144

KEY TERMS DEFORESTATION The temporary or permanent clearance of large expanses of forests for agriculture or other uses

Rain Forest Distribution

KEY TERMS VINE A plant with a long, thin, often climbing stem

LEARNING OBJECTIVE 6 Contrast the various stems that are specialized for asexual reproduction

KEY TERMS RHIZOME A horizontal underground stem that often serves as a storage organ and a means of sexual reproduction Example: iris

A Rhizome

Rhizome Adventitious roots Fig. 7-14a, p. 147 Figure 7.14: Modified stems. Fig. 7-14a, p. 147

KEY TERMS TUBER The thickened end of a rhizome that is fleshy and enlarged for food storage Example: white potato

A Tuber

Rhizome Figure 7.14: Modified stems. Tuber Roots Fig. 7-14b, p. 147

KEY TERMS BULB A rounded, fleshy underground bud that consists of a short stem with fleshy leaves Example: onion

A Bulb

Bulb Fleshy leaves Stem Adventitious roots Fig. 7-14c, p. 147 Figure 7.14: Modified stems. Stem Adventitious roots Fig. 7-14c, p. 147

KEY TERMS CORM A short, thickened underground stem specialized for food storage and asexual reproduction Example: crocus

A Corm

Axillary bud Leaf scars Corm (modified stem) Old corm (last year’s) Figure 7.14: Modified stems. Old corm (last year’s) Adventitious roots Fig. 7-14d, p. 147

KEY TERMS STOLON An aerial horizontal stem with long internodes; often forms buds that develop into separate plants Example: strawberry

Stolons

New Scale leaf shoot (at node) Stolon (runner) Adventitious roots Figure 7.14: Modified stems. Stolon (runner) Adventitious roots Fig. 7-14e, p. 147

Animation: Layers in a Woody Stem CLICK TO PLAY

Animation: Growth in a Walnut Twig CLICK TO PLAY

Animation: Annual Rings CLICK TO PLAY