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Apical Dominance
Hormones from the apex (tip) of poison-oak stems
circulate just under the bark all the way to the roots
of the plant. Buds for side branches are formed at
regular intervals along the sides of the stems, but
remain dormant. However if the tip of a branch dies, the
highest surviving bud, and often also the top bud of the
previous year’s growth, take over.
The
bud at each node is set to begin growing at a 69 degree
angle to the stem. When the dead tissue has been shed,
that leaves a funny crook in the stem, called an
“elbow”:

In the hotter, drier locations in
Morro Bay each year poison-oak grows through the plants
that shade it in spring and then die back, typically to
three inches within the shading plant’s crown; but as
much as six inches inside during droughts:

Click image for larger version.
Elbows reveal the
growth history of particular plants for years:

Click image for larger version.
A “Devil’s Pitchfork”
(My term)

The combined effects
of phototropism and apical dominance can be seen in this
specimen, extracted from a small oak tree at Los Osos
Oak Reserve: |