I'm curious about this mainly because i saw it being mentioned on woodworking.SE's posts, and some other sites.
Essentially, I always thought that wood could have "wood" movement, especially over time (e.g: aging) or when other factors such as moisture or molds are taken into account.
Taking into account the example I mentioned above (aging, moisture, mold) is it still true that wood movement is unlikely? Even longitudinally?
TL;DR: It depends, but generally we can ignore longitudinal movement.
The idea is that compared with the movement across the grain movement along the grain is much smaller, and is often negligible for the purposes of planning for that movement in a build.
This is especially true at the scales woodworkers work with. I'm sure examples of longitudinal movement across giant spans in wood bridges meant that at that scale it would have to be taken into consideration. But this is generally not the case for most woodworking.
But, the cross-grain movement is so large it is the one we often care about most, or at all. But it is moving on those other axes as well. At the end of the day some of this movement might be less than the margin of accuracy we can get when working wood, in which case we simplify and say it doesn't move along that axis.
