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

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

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

Cell Types in Conifer Wood Longitudinal Tracheids Dominant cell (90-95% of wood volume) Elongated 75-100 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)

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

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

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

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

Earlywood / Latewood Transition Abrupt Transition Gradual Transition

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”

Simple Pit Pairs

Bordered and Half-Bordered Pit Pairs

Bordered Pit Pair Structure - Conifers

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

Aspirated Earlywood/Latewood Bordered Pits

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

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.

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)

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

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

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

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

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

Heterocellular Rays

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

Dentate Ray Tracheids - Hard Pines

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.

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

Types of Cross-Field Pitting