1. Outlines of Previous Lecture
Primary Growth of Roots and Shoots
Apical meristems – herb (entire body), woody (younger parts)
Roots – root caps, zones of division, elongation, maturation
Vascular Tissues – Stele : Xylem from the center, Phloem in wedges
Monocots – Xylem and phloem alternating rings and central parenchyma cells
Ground Tissues – Cortex, Endodermis; lateral roots from pericycle
Shoots – Dome shaped apical meristems
Two leaf primordia
Axillary buds form lateral shoots
Epidermis, vascular tissue and lateral shoots from axillary buds
Dicots – xylem faces the pith, phloem to cortex, ring form
Monocots – vascular bundles scattered
Leaves – Parenchyma, collenchyma, sclerenchyma
Palisade mesophyll, spongy mesophyll

2. Secondary Growth of Plant Body

3. Woody plants 20% of all dicot species (oaks, maples)
5% of all monocot species
All gymnosperms (pines, firs, redwoods)

4. Secondary growth - the growth in thickness
Secondary growth commences where primary growth has stopped
older regions of all gymnosperm species and many eudicots, but rarely in monocots
lateral meristems
stems and roots of woody plants, but rarely in leaves
The secondary plant body
vascular cambium – xylem (wood) and phloem
cork cambium – cork cells (tough and thick cells)

5. Vascular Cambium and Secondary Vascular Tissue
Cylinder of meristematic cells one cell thick
Increases in circumference
lays down successive layers of secondary xylem to its interior and secondary phloem to its exterior
develops from
undifferentiated cells and
parenchyma cells that regain the capacity to divide

8. Secondary growth of dicot stems

11. Vascular cambium produces secondary vascular tissue in woody stems and roots

12. Vascular cambium in Roots
typical gymnosperm or woody eudicot root,
forms in segments between
the primary phloem,
the lobes of primary xylem, and
the pericycle,
eventually becoming a cylinder

13. Vascular cambium in stem
gymnosperm or woody eudicot stem
the vascular cambium forms in a layer
between the primary xylem and primary phloem and
in the ground tissue between the bundles
The meristematic bands within and between the vascular bundles unite
to become a continuous cylinder of dividing cells
Fusiform initials
Ray initials

14. The Vascular Cambium (Cont.)
Fusiform initials
tapered (fusiform) ends
oriented parallel to the axis of a stem or root.
produce elongated cells
tracheids,
vessel elements, and
fibers of the xylem,
sieve–tube members,
companion cells,
parenchyma, and
fibers of the phloem

15. Ray initials
shorter cells
oriented perpendicular to the stem or root axis
vascular rays—radial files consisting mainly of parenchyma cells

16. The Vascular Cambium (Cont.)
Vascular rays
living avenues
move water and nutrients between the secondary xylem and secondary phloem
store starch and other organic nutrients
xylem ray – located in the secondary xylem
phloem ray – located in the secondary phloem

17. Layers of secondary xylem (wood)
accumulated over the years
tracheids, vessel elements, and fibers
Gymnosperms – tracheids
Angiosperms – tracheids and vessel elements
Both types of cells - thick, lignified walls that give wood its hardness and strength
Early wood (Spring wood)
Tracheids and vessel elements that develop early in the growing season,
typically in early spring (have relatively large diameters and thin cell walls)
Late wood (Summer wood)
Tracheids and vessel elements produced later in the growing season,
during late summer or early fall (thick–walled cells, add more support)

18. Secondary xylem & tree rings

19. springwood
summerwood

20. Heartwood Sapwood The older layers of secondary xylem
no longer transport water and minerals (xylem sap)
Closer to the center of a stem or root
Sapwood
The outer layers of secondary xylem
still transport xylem sap
Only the youngest secondary phloem, closest to the vascular cambium, functions in sugar transport
As a stem or root increases in circumference, the older secondary phloem is sloughed off

21. heartwood
sapwood

22. Cork Cambium and the Production of Periderm
Stem – arises in the outer cortex
Roots arises in the outer layer of the pericycle
During the early stages of secondary growth,
epidermis pushed outward
split, dry, and fall off the stem or root
Replaced by two tissues produced by the first cork cambium
Phelloderm – thin layered parerchymatous cells, forming interior of the cork cambium
Cork cells – accumulate to the exterior of the cork cambium
Periderm
A cork cambium and the tissues it produces

23. Periderm Impermeable to water and gases
Lenticels – small, raised areas in periderm, more space between cork cells
Living cells of woody stem or root may exchange gases with the outside air
Unlike the vascular cambium, cells of the cork cambium do not continue to divide;
no increase in its circumference

24. Outer bark (periderm)

26. Periderm (outer bark)

27. Periderm

28. Lenticels
Lenticels

29. Lenticels

30. thickening of a stem or root splits the first cork cambium,
loses its meristematic activity and
differentiates into cork cells
A new cork cambium forms to the inside, resulting in another layer of periderm
As this process continues, older layers of periderm are sloughed off,
evident in the cracked, peeling bark of many tree trunks

31. Bark includes all tissues external to the vascular cambium (outward direction),
secondary phloem (produced by the vascular cambium),
the most recent periderm, and
all the older layers of periderm