The shoot apex in eastern white pine: its structure, seasonal development, and variation within the crown

Abstract

Structure of the shoot apex in eastern white pine (Pinus strobus L.) was similar to that described for other members of the genus. The overwintering bud contained all primordia for the next season's growth, and the apical meristem exhibited ginkgoid cytological zonation. Seasonal development at the shoot apex was divided into three phases: dormancy, extension of overwintering bud accompanied by formation of new cataphylls, and formation of primordia in the axils of some cataphylls. Terminal shoot apices decreased in size and number of primordia with lower position in the crown. Height/diameter ratio of the apical meristem increased with lower crown position, and cytological zonation was absent in apices in the lowest part of the live crown. Such variation within individual trees makes it difficult to describe or compare species on the basis of limited sampling. By late summer or early fall of the present study, dwarf shoot primordia possessed sheath scale and needle primordia. But primordia of flower buds and lateral vegetative buds could not be distinguished from them. Past work with soft pines has shown similar results. Hard pines, on the other hand, have been shown to differentiate flower bud primordia during the season of their formation.