The Microscopic Structure of Softwoods

Cells that look superfieially like axial parenchyma but in fact have bordered pits and assist with eonduetion are termed strand tracheids. Strand traeheids arise by the subdivision of axially-elongated eells that might otherwise have developed into normal undivided traeheids. Strand tracheids have no living eontents at funetional maturity. Some evolutionists believe that they represent an intermediate stage between the traeheid and the parenchyma cell. In some woods, such as larch and Douglas fir, they replaee the parenchyma in the latewood. 

Radially-elongated parenehyma eells make up most rays. These eells resemble the axial parenchyma cells being reetangular in shape with more or less transverse endwalls. They have moderately thiekened walls perforated by small simple pits. 

The cells are physiologically alive in the sapwood. However, near the sapwood-heartwood boundary the depletion of oxygen and the formation of embolisms accompany the hydrolysis of starch to sugars that in turn breakdown, oxidize and polymerize to yield polyphenolics. Extractives are formed and deposited. This is the onset of heartwood formation. 

---