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Inclined to take the strain
As students, many of us learned about the negative gravitropism of shoots by laying a potted plant on its side and then observing the differential growth that restores the growing axis to the vertical. However, for woody plants, the problem is not so easily solved because radial secondary growth from the vascular cambium leads to the formation of a thick, rigid secondary xylem, as discussed by Yamashita et al. (Nagoya, Japan, pp. 487-493). Nevertheless, woody plants do have some capacity for righting themselves if a stem becomes inclined away from the vertical and this is achieved by mechanical bending. To investigate this, the authors used 2-year-old pot-grown saplings of the gymnosperm Cryptomeria japonica. These were inclined at angles of up to 50° from vertical; growth and biomechanical responses were monitored. Gymnosperms respond to being away from vertical by forming compression wood on the underside of the stem. This is characterized in particular by thicker secondary walls and higher lignin content. The authors used a strain gauge to measure the released strain of the compression wood: the compression wood was “released” from its integration in the stem by making incisions through the new growth. A positive reading on the strain gauge indicated compressive growth stress. The thickness of the compression wood and the magnitude of the longitudinal released strain both increased up to approx. 30° inclination but then plateaued. The same was true of wall thickness and lignin content of the compression-growth layer. These data, and especially those on longitudinal released strain, correlated very closely with the bending moment, which again increased up to a 30° inclination. It is concluded that in gymnosperms, as in angiosperms, the development of reaction wood forces the stem back towards the vertical. An angle of 30° is rather critical; at inclinations greater than this, righting the stem takes somewhat longer.
Professor J. A. Bryant
University of Exeter, UK
j.a.bryant{at}exeter.ac.uk
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