Figure 1
Possible flow paths in conifer wood. Images are from spruce (Picea abies) transverse (left) and tangential (right) sections (black scale bar is 100 µm). The insert shows an enlarged tracheid with typical bordered pits of conifers. Tangential flow (red solid line) can pass through pits in a rather straight line as all tracheids have pits to their neighbours on their radial cell walls (only some are visible in the section). These pits may also serve for radial flow (purple dashed line), but there are very few pits in tangential walls and few tracheids are connected to more than one tracheid on one side. The path would therefore be more tortuous and resistance is very high in latewood tracheids and at the ring boundary (purple circle7). Radial flow thus appears to pass mainly through the rays with high resistance (brown, dotted line). Right: rays (black arrows) are c. 100–400 µm high and may make the path of least resistance somewhat longer, but will not add very much resistance to radial flow (red arrow). Axial flow (green) needs to pass through pits much less frequently.
