Anatomy of stem internodes from developing Populus trichocarpa Nisqually-1 shoot apex. IN2, IN3, IN4, IN5 and IN9 are Toluidine Blue-O stained cross sections from stem internodes 2 to 9 (plastochron index) of an actively growing young tree under field conditions in Corvallis, Oregon. The IN3 stem section shows well developed primary vascular tissue. IN9 shows well developed secondary growth with well lignified xylem tissue. IIP, IIX - secondary phloem and xylem respectively; VC - vascular cambium; IP, IX - primary phloem and primary xylem respectively. Scale bar for IN2-IN9 whole stem cross sections is 0.5 mm; that for IN3 and IN9 stained partial cross sections is 50 um.