Lecture #5 - Anatomical Structure of Softwoods, part I

Forestry 280Iowa State University

Cross-Section of Eastern White Pine

Cell Types of Coniferous WoodsLongitudinal Cells

Tracheids – Tracheids conduct water/solutes and provide mechanical support.

Two Types: Longitudinal

Tracheids Strand Tracheids

XS

Radial Section (RS)

Longitudinal Tracheids (LTs) Elongated cells with

thickened walls and tapering, pointed ends

Closed at the ends, i.e., are “imperforate” as opposed to the vessel elements found in hardwoods.

Side walls of tracheids contain pits.

Longitudinal Tracheids, continued

Mature-wood softwood tracheids are typically between 3-5 mm long and have a l/d ratio of about 100; thus, the average diameter of such cells would be 30-50 micrometers in (tangential) diameter.

The radial diameter of LTs decreases from earlywood to latewood.

LTs typically comprise 90 to 94% of softwood volume.

Strand Tracheids (STs)

Strand tracheids are short cells that are arranged in longitudinal strings or “strands”

STs have one or two “square ends”

They contain bordered pits. They are usually found near a resin canal.

Parenchyma

Parenchyma conduct and store food and other substances. Longitudinal Parenchyma Epithelial Cells

Longitudinal Parenchyma Relatively thin-walled

cells Have simple pits Form strands with

tapered ends May contain darkly

staining substances When present,

longitudinal parenchyma are not abundant.

Epithelial Cells Epithelial cells form

the periphery of a structure called a resin canal (RC)

May be thin- or thick-walled

Collectively, they are called “epithelium”

They excrete resin into the RC

Epithelial Cells, continued “Normal” resin canals are found in four

domestic genera, namely Pinus (pines), Picea (spruces), Larix (larches), and Pseudotsuga (Douglas fir)

“Traumatic” resin canals (i.e., those arising in response to an injury to the tree) may be found in these and other coniferous genera. Longitudinal only.

Transverse Cells Ray Tracheids Ray Parenchyma Epithelial cells

Transverse Section of Sugar Pine

Ray Tracheids Have bordered pits Relatively thick-

walled cells Interior walls are

either smooth or dentate

Not universally present (i.e., not found in all genera).

Ray Parenchyma Universally present

in all genera Have simple pits Generally thin-

walled, but some may have relatively thick, lignified walls

End-walls may be smooth, nodular or indentured

Epithelial Cells These surround

transverse resin canals

May be thin- or thick-walled

Are part of a composite structure known as a “fusiform ray”