DEVELOPMENTAL GENETICS IN LOWER PLANTS 811
Figure 5 Cell lineages in the development of the gametophyte of Physcomitrella patens. Transitions be-
tween stages that are connected by a broken arrow do not require cell division. The branched arrows with sin-
gle lines represent developmental steps involving a cell division; the arrows indicate the two products of the di-
vision. Stages connected by a double-lined arrow require more than one cell division. The () sign beside a
signal indicates that it is required for or enhances the frequency of the transition, the () sign, that it decreases
the frequency of the transition. (From Ref. 3.)