Spiral grain formation by induced reorientation of the conifer cambium

Processes involved in the reorientation of fusiform cambial initials related to spiral grain formation were investigated. Spiral grain was induced to form in a vigorous, nursery-growing sapling of Pinus strobus L. by constructing narrow diagonal bridges of phloem and cambium across ring girdles of stem internodes in the early spring. After 5 months, the bridges were prepared for light microscopy of transverse, radial and serial tangential sections, and the reorientation process investigated. As a result of this first investigation, further investigations to determine the time required for the cambium to reorient and the effect of wounding upon the reorientation process were begun: in late summer, 18 vigorous, field- growing saplings of Pinus contorta Dougl. were each given two phloem-cambium bridges across ring girdles of two internodes on the same day; the upper of each bridge was oriented diagonally and the lower parallel to the long axis of the tree stem in each case, the lower bridge to serve as a control to the wound response. Periodically, the two bridges of one of each of these 18 trees were then harvested; serial tangential sections of xylem and transverse, radial, and serial tangential sections of the cambial zone were then prepared and investigated. By tracing radial files of mature xylem tracheids in serial tangential sections through the zone of reorientation, the step-by-step changes which occurred in the cambium as it changed its alignment were deduced for each species and have been illustrated diagrammatically. Relatively small, localized regions of the vascular cambium, termed "microdomains," were found to form the vanguard to the reorientation process, followed by similar reorientation of the intervening regions subsequently. Microdomains were found to begin by one, or sometimes two or more, adjacent fusiform cells successively subdividing to result in a number of shorter fusiform cambial cells. The subdividing divisions were oblique and parallel anticlinal divisions. Accentuation of the reorientation occurred when some of the short fusiform cambial initials failed, permitting adjacent ones to elongate into and occupy the vacated space. In the elongation process, the numerous parallel oblique dividing walls formed a template which guided the extending cells into the new alignment. Following late-summer girdling, after 50 days reorientation was still in the microdomain stage within the diagonal bridge cambium of Pinus contorta; vertical bridges showed no change in the axial alignment of their fusiform cambial cells throughout the duration of the experiment. Wound responses found to be cornnon to both the diagonal and vertical bridges, as well as to both species, included: swelling of ray parenchyma, true transverse divisions in xylem mother cells, formation of xylary axial parenchyma and traumatic longitudinal resin canals and callus tissue. It has been concluded that a number of activities are equally necessary within the vascular cambium of Pinus before a group of adjacent fusiform cambial initials become realigned. These include subdivision by anticlinal division of each of the initials into a number of shorter initials; formation of oblique anticlinal dividing walls which are parallel and in the direction of the orientation to be adopted; failure of some of the shortened fusiform initials following division; and occupation of the vacated space by the surviving initials as they are guided by the parallel dividing walls into the new alignment.