Minor Mystery

What do you notice about the two sides of this sugar maple?

At first glance, I took the difference between the left and right sides of this majestic sugar maple to be simply a trick of the light.  Then I realized the left side really was fuller and darker than the right, but thought it might get more light on the left.  But the photo is taken from dead south, and there's nothing nearby tall enough to shade it.  I've noticed this difference now two years running.  Hmmm.

I'm still not sure of the explanation, but I have a hypothesis.

All plants except mosses and liverworts  get water from roots to crown through a tissue called xylem, that runs up through wood of the trunk.  Xylem is composed in flowering plants mostly of dead cells like stacks of barrels with the ends knocked out, making long pipes called vessels.  (If you look closely at the end of a recently-cut log you can often see the ends of these vessels like little pores in the wood.)  The vessels run from roots to trunk to branch to twig to leaf stem to leaf vein, and supply all the cells with water and nutrients.  There is some communication from vessel to neighboring vessel, but probably not much.

My hypothesis is that the vessels from roots on the right hand side of the tree mostly serve branches on that same side, and so with the left.  Notice that the right side is right next to pavement: either there are few roots alive under that pavement, or the roots there get less water because the pavement prevents most of it from infiltrating into the soil.  This has starved the right side of water and probably nutrients, and led to relatively sparse leaf growth on that side.

Well, it's just a hypothesis.  But I haven't found a better one.

The large holes in red are the water-carrying vessels seen end-on.

http://pysssec3biology2010.wikispaces.com/

Beautiful scanning electron micrograph of xylem with large vessels above,

light micrograph of xylem cross section below.

https://simple.wikipedia.org/wiki/Xylem_vessel

Nice cartoon of the whole process, from water entry into root to evaporation from leaf.

https://www.boundless.com/biology/textbooks/boundless-biology-textbook/plant-form-and-physiology-30/

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Meanwhile, it's the second week in August and the drought only deepens.  Our area has only received a fraction of the average spring and summer rainfall (much less than I reported in a previous post).  This particular tree has lost keys and leaves.  Nearly all the black and red oaks in the neighborhood have aborted and dropped acorns on the ground.  This time last year was pretty dry also, but I didn't see fallen acorns then.

It's too early in the season to be seeing fallen sugar maple leaves.

 The red oak acorns are falling unripe, even though they only have a few weeks to go.

These scarlet oak acorns are getting thick on the sidewalk beneath.