POLLEN MORPHOLOGY AND THE RELATIONSHIPS OF AËTOXYLON, AMYXA, AND GONYSTYLUS TO THE THYMELAEACEAE

The pollen of three closely related genera, Aetoxylon, Amyxa, and Gonystylus is compared in SEM and TEM with that of Thymelaeaceae, s. s. The Thymelaeaceae have spherical, pantoporate grains with a crotonoid tectum in which the basic subunit is tnangular in shape and forms a continuous triangular array. Thin section (TEM) and fractures (SEM) revealed that these subunits are attached to a ringlike network of horizontal rods. Within the Thymelaeaceae, the triangular subunits vary in the number of subdivisions and degree of fusion and form a morphological continuum. Aetoxylon, Amyxa, and Gonystylus also have spherical, pantoporate pollen but with a tectum in which almost all of the distinction of the subunits appears to have been lost. The structure of the exine in Aetoxylon, Amyxa, and Gonystylus, however, is unique thus far within the angiosperms. Thin section revealed a thick tectum with a layer of short or even granular columellae, then a thin, discontinuous layer from which larger columellae appear to hang. There is no evidence of an endexine even in the region of the apertures. The distinctive exine structure would support the treatment of Aetoxylon, Amyxa, and Gonystylus as a separate family, Gonystylaceae, allied to the Thymelaeaceae. AETOXYLON (Airy Shaw) Airy Shaw, and Amyxa Van Tiegh., each with a single species in Borneo, and Gonystylus with 25 species distributed through most of Malaysia, the Solomon Islands, and Fiji, have been treated as a separate family, Gonystylaceae, by Hutchinson (1959), or included in the Thymelaeaceae by Domke (1934), Wagenitz (1964), Airy Shaw (Willis, 1966), and Cronquist (1981), all of whom acknowledge the distinction of these three genera by segregating them as a subfamily, Gonystyloideae. Aetoxylon, Amyxa, and Gonystylus can be distinguished from the remaining Thymelaeaceae by a combination of characters: capsule fruit, numerous petals, elongate style, and pellucid-punctate leaves. These genera also lack the internal phloem that characterizes the vast t Received for publication 6 August 1984; revision accepted 5 February 1985. We thank J. L. Bittner, S. G. Braden, and W. Chissoe for their fine technical assistance. We are also grateful for the pollen samples sent by the late Dr. Jan Muller. This study was supported in part by a grant from Fluid Research from S. Dillon Ripley, and National Science Foundation Grants BSR-8002585 and BSR-8216349. majority ofthe Thymelaeaceae, as well as most of the order Myrtales. Archangelsky (1971 ), who described the pollen morphology of the Thymelaeaceae and allied families (1971), referred to their predominant crotonoid pattern as stellate. He considered the majority of the Thymelaeaceae to have typical stellate sculpturing, but described the Gonystylaceae as having cryptostellate sculpturing, and cited the "absolutely special type of their exines as a further criterion" for their treatment as an independent family. Of further interest, Archangelsky suggested that stellate sculpturing was one of the complex variables of simple reticulate sculpturing. As part of an extensive study of pollen morphology in the orders Myrtales (Patel, Skvarla and Raven, 1983, 1984) and Euphorbiales (sensu Cronquist, 1981; Nowicke, 1984; Nowicke and Skvarla, 1984), the pollen of more than 70 species representing 39 genera in the Thymelaeaceae have been examined. Aetoxylon sympetalum, Amyxa pluriconis, and four species of Gonystylus were included (see Table 1).