The Curator of Woody Plants, at the New York Botanical Garden, says that ivy
growth on the side of a building is not governed by hydraulics (as we had all
surmised a few months ago).  Rather, water is drawn up the ivy stem by a
couple of processes I don't really get, including hydrogen bonding and
cell-to-cell transfer.

Mr. Curator says that the limiting factor is probably plant vigor - the older
and taller the plant, the thicker the stems, the slower the transmission, and
things just sort of s l o w   d o w n (Hey, Mrs. Ralph: a little deja vu
here?)   The tallest tree in the world, a California redwood, is over 360
feet high, a height limited by the strength of the tree structure.  But
that's not really a problem for ivy - Mr. Curator says that ivy doesn't have
to be that strong, since its structure is the building to which it is
attached (think World Trade Center, Ralph).

Mr. Curator says he has seen ivy in forest situations growing at least 100
feet high, and helpfully notes that the lucky little plants generally have
plenty of food: "Oh, there's calcium in mortar, and calcium is an important
plant food." Heartwarming!

Oh, Ralph, I forget to tell you Mr. Curator's name:  Woody Forrest.

Sign me,  Weed Whacker