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:

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Elbows reveal the growth history of particular plants for years:

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A “Devil’s Pitchfork”
(My term)