Then, reading my newest book, "Pocket Bonsai," which, generally
speaking, is the exception that proves the rule in the accuracy
of the horticultural info presented (in a book on Bonsai), I
read in the section on air layering:

"Don't worry about the moss drying out: as the new roots take in
moisture, the xylem remains active and will be continually
pumping water into the moss!" (!?)

I'm not at all sure what this xylem pump is doing BEFORE roots
form, but maybe this is one of the reasons my moss says damp
enough. (?)

I have no idea what the author of the book is talking about. He may be
saying that the parent roots of the tree are supplying water to the
new roots that are forming in the air-layer; this might be true as
long as there are leaves present above the air-layer area to act as
transpiration pumps. But I wouldn't count on this; roots won't form
on an airlayer unless the area is protected from evaporation, and the
best way to make sure the sphagnum (or whatever) is still damp is to
check it occasionally and moisten it.

Nina