An essay on juvenility, phase change, and heteroblasty in seed plants

Phase change (the change from nonreproductive to reproductive status) and h eteroblasty (ontogenetic changes in vegetative metamers) are two determinan ts of longitudinal asymmetry in plants. These concepts are critically impor tant to understanding the regulation of plant development as well as morpho logical evolution and life-history variation. Since Goebel, the two have be en conflated. This article questions how phase change and heteroblasty are delimited and explores some of the problems that arise in the explicit or i mplicit link between them, given that several lines of evidence indicate th at they are distinct and independent facets of plant development. It is sug gested that problems are perpetuated through use of the terms "juvenile" an d "adult" to describe both phenomena.