Seedling establishment in Psittacanthus (Loranthaceae)

Abstract

The haustorium of Psittacanthus allenii Woodson and Schery, in contrast to other primary mistletoe haustoria, originates subterminally from the flanks of an extremely short radicle, the summit of which is occupied by the persistent remnant of the multicellular suspensor. The standard features of Santalalean haustoria, such as collapsed zones, glands, and repetitive intrusive organs, are present. The host cambium is displaced upward from below the successful intrusive organ, but continues its activity. The bulbous base of the mistletoe forces the expanding host cambium to fold back, resulting in the formation of a funnel-like wooden cup or woodrose. In subsequent years the host cambium deposits additional increments on the outside of the woodrose, each one enlarging the latter's diameter. The upper, internal margin of the woodrose provides an absorptive front of the newest host wood with the last marginal increment of vascular bundles of the haustorium itself. The haustorium of Psittacanthus, although not a modification of the most terminal portion of the radicular apex, may nevertheless be regarded as a primary haustorium, as it arises from the apical area of the radicular pole.