Occurrence of Reaction Wood in Branches of Dicotyledons and Its Role in Tree Architecture

Abstract

Lateral branch and young leader axes of 122 species of dicotyledonous trees in 46 families, displaying a variety of architectural models, were surveyed for presence of reaction wood (RW) as defined by gelatinous wood fibers (G-fibers). Distinct G-fibers occurred in 56 species, mostly in the upper half of the axis. There was a positive correlation between changes in axis orientation and position of the RW in 22, or one-fourth, of the species showing changes in branch orientation. Many species show an increase in branch angle; i.e., branches become more horizontal with age. Among these, Hevea (Rauh's architectural model), Endospermum, and Terminalia (both Aubreville's model) first produce RW on the lower side near the pith and later on the upper side. The erection of a lateral branch, i.e., a decrease in branch angle, to form the new leader in Koriba's model is correlated with RW on the upper side of the branch in Hura, but not in other species with this model. Many species with Troll's model, in which the arched leader becomes vertical, have RW on the upper side of the leader axis, e.g., Hymenaea, Fagus, Banara, Celtis, Ulmus, Piscidia, and Brownea. In some species with Koriba's or Troll's models, the wood on the upper side is thinner walled and less lignified, and the phloem rays are longer radially and more dilated tangentially than on the lower side of the axes. Thus, in some trees the normal reorientation of woody axes is related to the distribution of G-fibers, which cause longitudinal tensile stress, and/or other anatomical differences within those axes. This suggests an active, endogenous role for RW in the establishment of the architecture of these examples, in addition to its previously known supportive role in horizontal and displaced axes.