Decisions, decisions....

It seems that every time I look at a White Fir, I see something different.  I have two young ones growing where I store our canoe, and what struck me on this particular afternoon was the junctions of branches and the main trunk.  These two trees were like storybook trees, so symmetrical and undamaged, kind of like the ideal Christmas tree.  Then the curiosity took hold.  I noticed, as in the top photo, some of these junctions hosted 5 branches, and others, as in the second photo, hosted 4.  I got to wondering how the trunk "decides" to grow a branch in a particular place.  I've seen some three's and six's, and my son said he's seen a 7, but on this day the 4's and 5's got all my attention.  It stirred a memory of a scientific paper I read in college many years ago.  It was titled "Homologous Appendages in Homarus americanus."  It must have made a strong impression for me to remember it after all these years.  It turns out the American lobster, which normally has 10 legs, four pairs of small ones along either side of the carapace plus the two large pincher claws pointing forward.  In this case, a lobster with an injury to the carapace, something like a puncture wound, grew a new leg out of the wound that was the same type as the 8 already there.  The chemistry or heredity that allows this to happen must be similar in some ways to what allows lizards to grow new tails, and perhaps halves of an earthworm to grow new "other" halves.  I think this falls under the relatively new branch of biology abbreviated Evo Devo.  Without getting too deeply into the status of the science behind this, I was really enjoying the look of these two trees, and then the close-up (third photo) shows another intriguing feature of white fir - the base of each needle shows a quarter twist where it leaves the branch.  There's something beautiful about this, and it, too,  set me to wondering: how does a branch "decide" when and where to grow a needle?