Here's the current state of affairs. Note that now the sun has started moving northward, a slant of late afternoon light cuts across the courtyard every day.A closeup view shows the left-hand two small trees. On the left in the picture is actually the second tree from the left in the overall picture. Just in from of the trunk of the largest tree is the apex of the nearly bare left-hand-most tree. If you look very closely, you'll see a hint of green between the bud scales.
While the small seedlings (about 3-years old) definitely break bud first at the apex, the older tree has been sprouting somewhat irregularly. The terminal apex has some activity, yet several lower branches are also showing signs of movement. In one case a downward bending branch has most of the growth on its uppermost bud near the down turn.This seems like a case of apical dominance combined with basipetal transport. Somehow, the plant "knows" which way is up. The uppermost bud becomes the source for auxin that suppresses the buds below, even if they are distal to the uppermost bud.
Blakeslee et al. have recently done work on the cellular mechanism of auxin transport.
On a cellular level, directional auxin transport is primarily controlled by an efflux carrier complex that is characterized by the PIN-FORMED (PIN) family of proteins.Looks like these are the proteins that form the basis for positive gravitropism. There is a theory that statoliths (starch grains) in the cells sink to the bottom of the cell and somehow influence PIN proteins in pumping auxins out of the downward side. A nice description that shows how complex this might be is at John Kimball's biology website.
For now, it looks like it'll be a couple more weeks before we have a leafy green forest.
