Earlywood formation was observed in 60-year-old forest-grown red oak trees in southern Michigan. Extreme care in removing samples from the cambial region of the main stem at 1.4ensp;m and 18ensp;m, and from small branches at about 24ensp;m, permitted the following conclusions. First vessel elements were initiated in the second or third xylem derivative radially removed from the previous year's latewood, possibly in overwintering derivatives, simultaneously throughout the bole and branches of the tree, some 2 weeks before bud enlargement. Vessel elements enlarged first in the tangential dimension (to about 200ensp;mu;.) within a few days after initiation of differentiation. Enlargement in the radial direction required up to 2 weeks to grow 300ensp;mu;, occurring as the entire xylem mother cell zone was displaced outward by cambial growth to either sidemdash;tangentiallymdash;of the vessel element. The duration of earlywood formation was about 10 weeks, while the duration of shoot elongation was less than 2 weeks. First earlywood vessels were fully mature about 5 weeks after initiation, coinciding with the unfolding of first leaves. All foliage was mature several weeks before complete maturation of later formed earlywood vessels. Detailed stem analysis and bark peeling studies revealed that stem sections clear of branching contained few lateral junctions between axial vessels. There were many such junctions where twigs joined larger limbs and where limbs joined the main stem; all such junctions were between adjacent vessels from the same limb.
展开▼
