An effect of gravity on bud-burst in balsam fir.

Bud-burst on first order lateral branches of Abies bafsamea L. (balsam fir) was delayed when the branches were rotated 180 degrees about their long axis. This was not a consequence of injury caused by the treatment because buds rotated 180 degrees on inverted plants flushed at the same time as the controls, whereas flushing of all other buds was delayed. Buds thus appear to be more vigorous when maintained in the same orientation to gravity in which they are formed and the site of gravitational stimulus perception appears to be the bud itself. Except on the leading shoot, leaves from inverted buds turned so that their adaxial surface faced upward, unless there was intense illumination from below. However, both anisophylly and positioning of leaves on lateral shoots were apparently predetermined because the shorter, more forward pointing leaves appeared below the longer distichous leaves on shoots from inverted buds. Shoots with normally oriented leaves appeared the next season.