Figure 3.

Schemes of the Fate of Successive Primordia during Short Day–Induced Bud Set in Poplar and of the Morphology of the Bud at Accomplished Bud Set.

(A) Dormancy induction is illustrated with an apex at the perception of the critical daylength (top left) and a bud after bud set has been completed (top right and bottom). The planes of sectioning for longitudinal (L) and radial (R) sections are indicated by solid lines, and the apical meristem is indicated by the dashed line through the bud. At the perception of the critical daylength, the apex of the plant already contains committed primordia that will develop into leaves (primordia 5 to 0). Only those primordia initiated after the perception of the critical daylength will change morphogenetic fate (primordium −1 and younger ones). After bud set has been completed, the following units of organs are found inside a P. tremula × P. alba bud (in successive order from outside to inside): one abortive bud scale leaf, consisting of two sometimes fused bud scales (primordium −1; absl); one incipient bud scale leaf, consisting of two bud scales and a small embryonic leaf lamina (primordium −2; ibsl [not always found]); approximately seven embryonic leaves sensu stricto, consisting of two stipules and an embryonic leaf lamina (primordia −3 to −9; el); and approximately two not yet developed leaf primordia (primordia −10 and −11; lp).

(B) Morphology of the bud after bud set has been completed. As a result of the helical phyllotaxis of the bud, radial sections (R) reveal approximately one-third more organs than longitudinal sections (L). The plane of radial sections is situated just above the apical meristem (see bud in [A]); therefore, the enlarged stipules contributing to bud scales (primordia 1 and 0) usually are not revealed. In the schemes of radial and longitudinal sections, embryonic leaf laminae are dark gray and stipules and bud scales are light gray. In the radial section, organs 0 and −1 out of primordia 0 and −1 are numbered (to conform with the primordium numbers in [A]) to illustrate the fact that at the end of bud formation, a different number of organs may persist from different primordia.