I've noticed that branches stemming from the trunk of a tree are capable of doing so at all 360 degrees about the axis of the trunk. However, branches stemming from the trunk's primary branches, or from any branch that is already horizontal, seem to have a strong tendency to do so horizontally - forming sort of a plane along their parent branch's axis. This is especially noticeable in some conifers like the eastern hemlock or white pine. I would like to understand what causes this to happen. Does a branch's phyllotaxis change based on its orientation or is the cause more external? Can anyone explain?
It's likely to be genetically controlled and does not seem to apply to temperate fruit trees. I'm constantly pruning off vertical secondary branches that will become water sprouts.
Genetically-controlled overall, yes, but the more proximate cause has to do with plant hormones, e.g., Apical control of branch growth and angle in woody plants .
Well its an interesting article but I don't see how apical control would cause this…maybe its in there and I'm missing it. I'm not talking about suppressed lateral branches drooping to the sides. I'm wondering why the very bases of lateral shoots, on horizontally growing branches, appear mostly on the sides, and rarely on the top or bottom. Like this:
On hemlocks. As Vitog mentioned, that doesn't seem to be much of a rule for all trees. And even for conifers, this one I posted that might or might not be a Juniper seems to be doing a lot of top branching: http://forums.botanicalgarden.ubc.c..._lulukurd_cutler_20151208_123556c-jpg.137880/
I'm aware that it is not a rule for all trees. It's not even a strict rule within the trees it does occur. I'm just wondering why it seems to be a strong tendency for certain trees. Its hard to find pictures but I've even seen it on deciduous decurrent trees.
Start reading from page 193 in this book for information about different hypotheses surrounding diageotropism.
Wow. Not only did I learn a new word (diageotropic - growing at right angles to the line of gravity), but it seems clearly not related to a search for light, if it even happens underground with roots.
Hehe, yea my first time hearing of diageotropism. That does look like an interesting read, unfortunately the pages you mentioned don't appear to be available for free so I may have to consider buying it. Thanks guys.
Google Books search is always like that. The link only lasts a few times. Make note of the title and maybe do a separate search for the title and the word, and perhaps you'll get a link to the pages.
Seems like part of it is genetic like someone mentioned. If a twig extends outward, and buds are genetically in certain places, isn't that where the next twigs or shoots must originate? And if each species varies, then branching varies. Seems a good light gathering development too. I was thinking of how some trees have alternate or opposite buds, twigs and leaves, and the leave seem to often space evenly to the sides and lay flat facing upward. Hormones are involved.
For the best exploration I know of the spatial geometry of plants, and other things, try 3 videos by my favourite math teacher - Vi Hart. Here's part 1 of "Doodling in Math Class": Be patient, all will become clear by the time you get to part 3.