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Foliage leaves of Marathrum foeniculaceum Humb. & Bonpl. (Podostemaceae–Podostemoideae) resemble pinnate compound leaves at first sight. But the similarity is deceptive, since studies of the leaf structure reveal the ensiform (sword-like) shape of the blade. The ribbon-shaped central rachis-like portion represents the parallel-veined ensiform blade that extends in the median plane direction. Pinnae, typically developing from the leaf margin of the blade in transverse position relative to the mother shoot axis, do not occur here, but instead, bi-pinnate-like appendages arise alternately at the adaxial and abaxial edge of the ensiform blade. The pinnate-like structures are accessory structures. These project only from one side (“front side”) of the ensiform blade. The appendages produce repeatedly forked threads that end in pubescent filaments. The trichomes are of the Zeylanidium olivaceum type. The occurrence of similar structures in species of Marathrum and the related genera Apinagia and Mourera are discussed. |
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