Seedling developmental anatomy of an undescribed Malaccotristicha species (Podostemaceae, subfamily Tristichoideae) with implications for body plan evolution

The seedling development of an undescribed Malaccotristicha species was observed by using seedling culture and microtomy to infer the evolution of body plan with a focus on the root, which is a developmentally leading organ of most Podostemaceae. The young seedling has a small primary shoot apical meristem and a primary root apical meristem. The shoot meristem develops into a plumular ramulus, and the root meristem, into a cylindrical radicle with no root cap. The radicle transforms to a dorsiventral, flattened, capped primary root. An adventitious root develops endogenously on the lateral side of the hypocotyl and is similar to the primary root. This is a new pattern in Podostemaceae. Comparison of this and described patterns of Podostemaceae (and the sister-group Hypericaceae) suggests that the radicle was lost in the early evolution of Podostemaceae and instead adventitious roots replaced it as a leading organ.