Secondary Growth in Stems (and Roots)

1. Secondary Growth in Stems (and Roots)

2. Annuals, Biennials, & Perennials • Annuals – single growing season • Only seed bridges gap between growing seasons • Biennials – two growing seasons • 1st season – root, short stem and rosette of leaves • 2nd season – flowering, fruiting, seed formation • Perennials – many growing seasons • Herbaceous – dormant underground parts • Woody – usually stop growing for a time • Deciduous vs Evergreen

3. Primary Growth Primary Meristems Primary Tissues Epidermis Protoderm Apical Meristem Ground Tissues (Cortex) Ground Meristem Primary Phloem Primary Xylem Procambium Secondary Growth Vascular Cambium Cork Cambium Secondary Meristems Cork Phelloderm Secondary Phloem Secondary Xylem Secondary Tissues

4. Vascular Cambium • Meristem cells highly vacuolated • Fusiform initials – vertically oriented • Ray initials – horizontally oriented

5. Vascular Cambium • 2º xylem and 2º phloem – periclinal divisions of fusiform and ray initials and derivatives • i.e., cell plate is parallel to root/stem surface • Derivative on outside  phloem cell • Inside …..?

6. Vascular Cambium • Axial System -- vertically oriented • Xylem and phloem from fusiform initials • Radial System (vascular rays) – horizontal • Xylem and phloem from ray initials • Variable in length • Pathways -- 2º xylem ↔ 2º phloem • Storage – starch, proteins & lipids • Synthesize – secondary metabolites

7. Vascular Cambium • Technically – only the cambial initials • Derivatives – may stay meristematic • Difficult to separate derivatives from initials • Some – Vascular Cambium = “initials & derivatives” • Others – Cambial Zone

8. Vascular Cambium • Adds to 2º Xylem – increases diameter • Cambium displaced outward – increases circumference • Anticlinal divisions produce new initials • Maintains fairly constant ratio of fusiform initials to ray initials periclinal anticlinal

9. Vascular Cambium • Dormant in winter; reactivates in spring • Auxins stimulate resumption of cambial activity • During expansion – bark can be easily stripped • Bark – everything outside vascular cambium • Tropical plants tend to not be seasonal

10. Effect of 2º Growth on Stem • Vascular cambium • Fascicular cambium -- undifferentiated procambium • Interfascicular cambium – interfascicular Parenchyma • Cylinder formed with rays • More xylem formed (think about it …..)

11. 2º Growth – Roots vs Stems Much more xylem produced Unlike 1º growth, 2º growth between root and stem contiguous – no transition zone

12. Periderm • Formation – after initiation of 2º xylem & phloem • Replaces epidermis • Three parts • Cork cambium • Phellogen – meristem that produces periderm • Cork • Phellem – protective tissue outside cork cambium • Phelloderm • Living parenchyma – inside cork cambium

13. Periderm -- Cork • Divisions of cork cambium  cork • Cork cell walls  suberin & wax • Impermeable to water & gases • Dead after differentiation • May also lignify

14. Periderm -- Phelloderm Living at maturity No suberin Look mostly like cortical parenchyma Inner cells in radial rows

15. Lenticels • All tissue needs gas exchange • Formation begins with first periderm cells • Generally around stomata • Also on fruits • Apples & Pears (those spots!)

16. Platanus stem Lenticels Lenticels

17. Bark Outer bark – dead Inner bark -- living Everything outside vascular cambium! Before 2º phloem – bark is solely 1º tissue After 1 year – remaining 1º tissue, 2º phloem, periderm, and any tissue outside of periderm

18. Bark & Wood • Vascular cambium adds 2º phloem to bark & 2º xylem to wood. • Soft-walled cells (sieve elements & parenchyma) • More 2º xylem accumulates • More made • Old 2º phloem separated by periderm (and sloughed)

19. Bark and Periderm • First periderm can remain active • Inactive phloem parenchyma -- meristematic • Can form new periderms to compensate for growth • Appearance of bark – how new periderms are formed

20. 2° Phloem • Active phloem -- Functional/Conducting • Generally only current year is active • phloem parenchyma and phloem in rays can remain alive • Only outer bark completely dead

21. 2° Growth in Roots • Just like stems • Formation of 2° tissues • Formation of periderm (mostly cork cambium) • Not seen in monocots or herbaceous dicots • Rays present • Only 1° phloem fibers • Periderm like stems

22. Seriously Cool photo of harvesting cork from a cork oak – Quercussuber

23. Note the structure of the twigs. Pay particular attention to both the Terminal bud scale scars as well as the bundle scars.

24. Wood – 2º Xylem Shelter Fire – heat/cooking Protection – weapons and walls Furniture Tools Toys Paper Transportation ?????

25. Wood Block Sections Pith Tangential Radial Transverse Radial Transverse Radial Bark Branch Tangential

26. Hardwoods vs Softwoods • Differences are structural • Hardwoods – angiosperms • Softwoods – gymnosperms • Lack vessels

27. Gymnosperm Wood • Simple structure – no vessels • Lesser amounts of axial/wood parenchyma • Tracheids dominant cell type • Resin ducts • Both axial and radial systems • Result from trauma? • Protection from herbivory

28. Gymnosperm tracheids • Large circular bordered pits on radial walls • Most abundant on the ends • Pit-pairs have torus

29. Pinuswood faces • Notice rays are one cell wide • Resin duct does alter! • Rays 1-15 (or more) cells tall

30. Angiosperm wood • Typically has vessels • More varied structures – more cell types • Rays • 1 to many cells wide • 1 to hundreds of cells tall • Can be seen with the naked eye • About 17% of wood volume • Conifers only ~ 8%

31. Angiosperm wood • Cellular arrangement not orderly like gymnosperms • Enlargement of vessels • Elongation of fibers • Displacement by vessel elements • notice slide 29 and slide 32 • Gymnosperms – fig 26-20 • Angiosperms – fig 26-24

32. Quercuswood faces Notice rays! Notice presence of other cell(s) and cell types

33. Growth Rings • Seasonal activity in vascular cambium • Incremental growth in 2º phloem not always easily discernable • One year’s growth – annual ring • False annual rings – abrupt changes in water etc.

34. Trees as Rain Gauges • Rings different! • Wide? Lots of rain! • Lots of environmental information • Bristlecone pine • Tree begins ~6260 BC; shaded band ~4240-4210 BC

35. Growth Ring Information • Differences in porosity/density of wood • Early wood – less dense with wider cells & thinner walls • Late wood – narrower cells & thicker walls • Change during the year is gradual

36. Pinus strobus Wood Rings Which way to the Vascular Cambium?

37. Pinus strobus Wood Rings Growth Vascular Cambium

38. Early and Late Wood • Ring-porous wood – vessels/pores of early wood larger than late wood • Diffuse-porous wood – vessels fairly uniform in diameter • Which is better at conducting water? • Up to 10X better …..

39. Sapwood & Heartwood • With age – increasingly non-functional • Oils, gums, resins, tannins  colored • Heartwood • Sapwood – lighter conducting wood • Heartwood theorized to hold toxic/ inhibitory 2º metabolites. • Not visible in all trees!

40. Sapwood & Heartwood • Tyloses formed in non-functional vessels • Outgrowth from parenchyma • White oak – transverse (left) & longitudinal • Red oak not have!

41. Comparison of wood with and without tyloses

42. Diagram - how wood warps, etc.

43. Reaction Wood • Developmental response by a leaning branch/ stem • Gymnosperms – compression wood • Angiosperms – reaction wood • Increased cambial activity

44. Reaction Wood • Compression Wood • Pushes wood straight • More lignin & less cellolose • Greater lengthwise shrinkage •  10X more than regular wood (regular -- 0.1-0.3%) • Tension Wood • Pulls wood straight • Gelatinous fibers • Lengthwise shrinkage < 1%

45. Reaction wood in a conifer

46. Knots Knots are remnants of branches

47. Pine branches inside trunk

48. Tree Growth • Relationship between apical growth and lateral growth • Radial section & partial transverse section