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Unformatted text preview: d tissue Primary phloem
Primary xylem Monocots Secondary Growth in Woody Stems
• Diameter growth in existing organs.
Conifers & many magnoliid & eudicot flowering plants.
Similar in both roots & stems. Onset of Secondary Growth
(p. 617-619; 555, 583-584) First Step: Formation of the vascular cambium.
• Undifferentiated procambium between xylem & phloem.
– Main source in basswood (Tillia)-type stems.
• Pith ray Parenchyma.
– Important in elderberry (Sambucus)-type stems.
10 Vascular bundle
Once formed, V.C. periodically produces new layers of 20
xylem (inside) & 20 phloem (outside). Sequence of Primary & Secondary Stem
(See important chart on P. 635; 583) Ground meristem
Protoderm Procambium 1) early primary tissue differentiation
near apical meristem Primary & Secondary Stem Development
Pith Cortex Pith ray 1o Xylem Procambium
2) Mature primary stem tissues Primary & Secondary Stem Development Fascicular cambium Interfascicular cambium
3) Vascular cambium forms Primary & Secondary Stem Development
Pith Cortex 2o phloem 1o Xylem 2o Xylem
Epidermis Vascular cambium
1o phloem 4) First season of secondary growth Primary & Secondary Stem Development 2o phloem
5) Secondary growth continues: After 2 seasons Wood = Secondary Xylem
Annual Rings: p. 632; 585
• spring wood: fast growth, large cells, light.
• summer wood: slow growth, Small, dense cells, dark.
Summer growth Previous year’s growth Focus Plant 3: Wood; Pinus sp. (pines)
110 species mostly in temperate zone & mountainous
areas of tropics.
½ all species native to Mexico, C. America, &
Economically Important for timber & pulpwood.
Driving force in historic development of E. TX
Major part of local economy today.
East TX is sometimes called the “Pineywoods”. Historic logging of Pine in Texas Loblolly Pine in Texas Characteristics of Pine
Woody plants: trees, shrubs.
Evergreen, Needle-shaped leaves in bundles.
Seeds borne on woody cones.
pollen on separate pollen cones. Male (pollen) cones Female (ovulate, seed) cones The 3 Native Pines of East Texas Loblolly pine
species Shortleaf pine
short (<4”) needles
Most common on
drier, sandy sites
drier, Longleaf pine
Long (>10”) needles
historically. Monocot (Bamboo, Palm, Greenbriar)
Lignified woody tissue, but thickness all result of primary
growth at & near apical meristem.
Bamboo Greenbriar Fossilized palm “wood”
Note scattered bundles Palm The Periderm (p. 619; 585)
Originates: Cork cambium (=phellogen).
Lenticils: areas of cork with airspaces...
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