Cellular adjustments in the vascular cambium leading to spiral grain formation in conifers

Radial files of tracheids having changing orientations were traced through serial tangential sections to deduce events in fusiform cambial cells (FCCs) associated with spiral grain formation. Within an induction bridge of phloem and cambium sloping diagonally across a stem girdle in Pinus contorta Dougl. ssp. latifolia, repetitive, homologously sloping, pseudo-transverse anticlinal divisions in FCCs were a primary event contributing to FCC orientation changes. Imperfect periclinal divisions (IPDs) in FCCs also contributed. In Picea glauca (Moench.) Voss, IPDs and FCC elongation interacted to change tracheid orientation during natural spiral grain development. In both natural and induced reorientations, small numbers of adjoining FCCs changed their orientation abruptly while the greater population of FCCs changed orientation less rapidly. These sites of abrupt reorientation are termed "microdomains."not available