ABSTRACT
The secondary xylem of Dicotyledons with its four to six constituent cell types is the most complex tissue in terms of differentiation. Casting therefore seems to offer access to new information from the angle of the histogenesis of the xylem. The secondary xylem that develops in the galls has apparently highly irregular forms, as partly described by R. Aloni et al. on galls of castor-oil plant. In the secondary xylem of Dicotyledons, vessels are formed either entirely in the wood of one axis or straddle two adjacent axes. The secondary xylem formed in the cortical strands is continuous with that of the mistletoe stem and produced by the extension of the cambium of the stem. There are no comparative investigations into the morphology of vessel ends. Among the Dicotyledons in which vessels have been examined, the frequency of branching seems higher in the plane tree than in species with simple perforations, such as tomato or walnut.
