This whole thread confused me, but kitsune's modified ascii graphic has
finally clued me in. From what I know about root formation (not that
much, but I did have a plant physiology course once.... decades ago),
there is no reason you can't get two areas of root formation on the
same branch. It happens all the time on shrubs that "scramble", that
is, that root on low branches that touch the ground. Roots that emerge
from stems are called "adventitious roots" and form from the inner bark
layer. The formation of these roots are influenced by hormones (auxin
and ethylene) and energy (sugar). Ethylene is produced by (among other
things) wounding, so the combination of making a wound and keeping it
smothered in sphagnum will allow ethylene to accumulate. Sugar and
auxin will come down from the branch tip. In a normal single girdle
air-layer, sugar and auxin will accumulate in the "upper lip" of the
girdle (unable to go any farther because the phloem has been severed),
and roots formation will be in that upper lip. In a double girdle such
as Kitsune has diagrammed, the lateral branch between the two girdle
sites has been transformed into the new "apical tip" for that segment,
and sugar and auxin should travel pretty uniformly to both girdle
sites. Hope that makes sense.....