1. Softwood Microanatomy
FW1035
Lecture 4
Bowyer et al – Chapter 4
Softwood Microanatomy
Softwoods
General information
Overview of structural cells
Wood appearance
Importance of pitting
Component cells and tissues
Longitudinal tracheids
Resin canals
Rays
Longitudinal Parenchyma
Crossfield pitting

2. Softwoods Tree Appearance Distribution Needle or scale-like foliage
Trees have ‘excurrent’ form (dominant trunk with lateral branches)
Distribution
Worldwide, less common than hardwoods
North America has a large standing inventory

3. Softwoods - Why are we Interested in them?
Softwoods are the foundation of the forest products industry in North America
structural construction lumber (light and straight grained)
pulp and paper - long fiber cells
plywood industry

4. Cell Types in Conifer Wood
Longitudinal Tracheids
Dominant cell (90-95% of wood volume)
Elongated times their diameter (aspect ratio)
Occur in radial rows (or “ranks”)
Denser latewood from smaller radial diameter (flattening of the cell) and thicker wall
Rays - generally uniseriate, except when a ray contains a resin canal
Resin canals - Found in 4 genera
Pinus (Pines)
Picea (Spruces)
Larix (Larches)
Pseudotsuga (Douglas-fir)

6. Diagram of Pine Wood Microanatomy
E = Epithelial cells
Diagram of Pine Wood Microanatomy

7. Softwood Structure
Can you recognize and name all the indicated features?

8. Longitudinal Tracheids
Compose the bulk of xylem (90-95%)
Shape and proportions are similar amongst softwood species
Radial diameter varies with position in growth ring
Tangential diameter remains constant and leads to judgements of texture
Conductive and support functions

9. Spiral Thickening in Longitudinal Tracheids
Also called “helical thickening”
Usually smooth inner cell walls
Some species have spiral thickening on inner walls
Fine, helical ridges
Found in Douglas-fir and western yew, plus some other minor species

10. Earlywood / Latewood Transition
Abrupt Transition
Gradual Transition

11. Pitting in Softwoods
Fluids transported between cells through pits - no vessels like hardwoods
3 types of pit pairs
Bordered pits are the most common pit type in softwoods- bulk of xylem is composed of longitudinal tracheids
Appearance of half-bordered pit pairs between ray parenchyma and longitudinal tracheids is important for microscopic ID - “crossfield pitting”

12. Simple Pit Pairs

13. Bordered and Half-Bordered Pit Pairs

14. Bordered Pit Pair Structure - Conifers

15. Bordered Pit Pair Aspiration
Response to injury, drying, or conversion of sapwood to heartwood
Fibers of the margo are flexible enough to distort in response to pressure
Torus moves and contacts the inside surfaces of the pit aperture and seals the pit pair
Earlywood pit pairs aspirate more easily than latewood pits

16. Aspirated Earlywood/Latewood Bordered Pits

17. Longitudinal Parenchyma
Also called strand parenchyma
Appear as chains of short, vertically oriented cells
Formed by fusiform cambial initials

18. Longitudinal Parenchyma
Appear as thin walled cells on the cross section
May have dark-colored contents
Compose as much as 1-2% of the wood volume in:
Redwood (Sequoia spp.)
Cedars (Thuja spp.)
Pines (Pinuss spp.)
Typically, only visible with hand lens in redwood and bald cypress.

19. Resin Canals
Conduct resin secreted by specialized parenchyma cells called epithelial cells lining the canal opening
Seals wounds from insects or mechanical damage
Occur oriented in the longitudinal direction and in the radial direction (within fusiform rays)

20. More on Resin Canals Occur in: Useful for wood ID
Pinus spp.
Larix spp.
Picea spp.
Pseudotsuga menziesii
Useful for wood ID
May need hand lens to see
Appearance varies with presence/absence of resin

21. How can you tell if a wood sample has resin canals?
(Fusiform Ray)
How can you tell if a wood sample has resin canals?
Longitudinal resin canal

22. More on Resin Canals Occurrence:
Pines - large and numerous (all growth rings and even distribution)
Spruces, larches, Douglas-fir - more sporadic distribution
may not be in all rings
commonly in tangential groups
Pines have thin walled epithelial cells (easily damaged in cutting)
Douglas-fir

23. Traumatic Resin Canals
May occur in species that do not normally have them
Response to injury
Appear as single, continuous line
Different appearance from normal resin canals

24. Conifer Rays Almost always uniseriate (except fusiform rays)
Non-fusiform rays are composed of ray tracheids and/or ray parenchyma cells
homogeneous
heterogeneous
tracheids usually at top and bottom of a heterogeneous ray
Multiseriate Rays in Redwood

25. Heterocellular Rays

26. Fusiform Rays Horizontal resin canals
Occur in all species that have longitudinal resin canals
Resin canal within a ray
May contain three cell types
Ray parenchyma
Ray tracheids
Epithelial cells

27. Dentate Ray Tracheids - Hard Pines

28. Dentations All ray tracheids are conductive cells, form a bordered pit
Dentations occur in ray tracheids of hard pines.
They are localized wall thickenings that appear tooth-like as a consequence of slide preparation.
When the cell is considered as a whole, dentations are actually just irregular ringlike structures that are similar to spiral thickenings in longitudinal tracheids.

29. What is the “cross-field” in cross-field pitting?
Intersection of a longitudinal tracheid and a ray parenchyma cell produces a half-bordered pit
In radial view, in conjuction with ray tracheids, we see distinctive pitting patterns
Cross-field

30. Types of Cross-Field Pitting