Nephelieae pollen (Sapindaceae): Form, function, and evolution

Two main types may be distinguished within Alectryon pollen: the colporate type and the parasyncolporate type. These types are not clear-cut, but merge into each other through a complete series of intermediate forms. The colporate type has usually a colporate apertural system, a high P/E value (av. 0.97), a low A/E value (0.32), and striate to striate-rugulate ornamentation. The parasyncolporate type shows usually a parasyncolporate apertural system, a low P/E value (av. 0.76), a high A/E value (av. 0.46), and striate-rugulate to rugulate ornamentation. In addition, there is some difference in endoaperture and nexine morphology, and the relative length of the peripheral columellae of a mesocolpium. An endoaperture in a colporate grain often shows acute or acuminate lateral sides or a fastigium; these characters were never observed in parasyncolporate grains. Colporate pollen has usually a less regular endexine/foot layer boundary than parasyncolporate pollen. Colporate pollen may have relatively short columellae or no distinguishable columellae at all along the colpi, whereas in parasyncolporate pollen the peripheral columellae are always relatively long.