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Pollen Characters in Relation to the Delimitation of Myrtales

Varsha C. Patel, John J. Skvarla and Peter H. Raven
Annals of the Missouri Botanical Garden
Vol. 71, No. 3, The Order Myrtales: A Symposium (1984), pp. 858-969
DOI: 10.2307/2399170
Stable URL: http://www.jstor.org/stable/2399170
Page Count: 112
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Pollen Characters in Relation to the Delimitation of Myrtales
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Abstract

Pollen grains representative of the Lythraceae (including subfamilies Punicoideae, Sonneratioideae, and Duabangoideae), Trapaceae, Oliniaceae, Combretaceae, Alzateaceae, Rhynchocalycaceae, Penaeaceae, Crypteroniaceae, Melastomataceae, Myrtaceae (including Psiloxylaceae and Heteropyxidaceae), and Onagraceae, the 11 families constituting the order Myrtales, were examined with scanning (SEM) and transmission (TEM) electron microscopy. With omission of the Trapaceae, Myrtaceae, and Onagraceae, the remaining families have enough similarities to be grouped together palynologically. Heterocolpate pollen characterizes the Lythraceae, Rhynchocalycaceae, Combretaceae, Oliniaceae, Penaeaceae, Crypteroniaceae, and Melastomataceae. In the latter five families, subsidiary colpi (pseudocolpi) were noted in all taxa examined except Adelobotrys, Allomorphia, Astronia, Bredia, Oxyspora, and Tococa of the Melastomataceae and Buchenavia, Laguncularia, and Strephonema of the Combretaceae. With the exceptions of the latter four genera these taxa possess intercolpar concavities. Subsidiary colpi and intercolpar concavities are considered to be functionally equivalent, their distinction being the larger size of the intercolpar concavities. Subsidiary colpi are equal to the number of apertures except in Lythraceae where Ammannia, Nesaea, and Crenea have twice the number; intercolpar concavities are also isomerous with apertures. In Oliniaceae, the subsidiary colpi are located on just one polar face and hence considered as half subsidiary colpi while Alzateaceae may have incipient subsidiary colpi. The pollen of Lythraceae subfamilies Sonneratioideae, Duabangoideae, and Punicoideae possess meridional ridges which alternate with the apertures; they are prominent in the former two subfamilies and weakly defined in the latter. Meridional ridges are also present in some Lythraceae lacking subsidiary colpi. Exine sculpturing in the mesocolpia is variable throughout the order with Crypteroniaceae, Alzateaceae, Oliniaceae, and Penaeaceae basically psilate; Melastomataceae basically striate and rugulate; Combretaceae echinate, reticulate, rugulate, striate, and psilate; Lythraceae subfam. Lythroideae striate, psilate, verrucate, and granular; Lythraceae subfam. Punicoideae basically granular-microrugulate; and Lythraceae subfam. Sonneratioideae and Duabangoideae basically verrucate-rugulate. Commonly, the subsidiary colpi (and intercolpar concavities) and colpi have different sculpturing than the mesocolpial regions. Exine structure is essentially of the post and beam construction with the fundamental ektexine and endexine stratification layers. In all Combretaceae and some Melastomataceae the foot layer is strikingly delineated as domes, whereas in Alzateaceae the columellae layer follows a zig-zag course. The Oliniaceae and Penaeaceae are distinct throughout the order with remarkably thickened tectum and foot layer and negligible columellae; Acanathella of the Melastomataceae is somewhat similar, differing only in a better developed columellae layer. In general, Lythraceae have the most diverse pollen at both sculptural and structural levels; Combretaceae also have considerable diversity, primarily exomorphically, and Strephonema is the only taxon noted in the order with a reticulate sculpture; Melastomataceae are diverse exomorphically, with the pollen of Miconia melanotricha shed as tetrads and that of Tococa spadiciflora as polyads, the only example of polyads in the order other than Ludwigia (Onagraceae); Oliniaceae, Penaeaceae, and Crypteroniaceae show rather uniform intrafamilial morphology. The monogeneric Trapaceae with protruding and domed apertures and meridional ridges resulting from exine folds is totally unique in Myrtales. One sample shows linked pollen grains suggestive of polyads while sections through the ridge area show an extremely thick endexine and loosely organized ektexine reminiscent of some Onagraceae. Myrtaceae, with pollen oblate-elliptic in lateral view and triangular in polar view, are also without a counterpart in Myrtales. Based on the nature of the colpi, three distinctive pollen groups are evident: (1) longicolpate, (2) syncolpate and parasyncolpate with and without intercolpar concavities, and (3) brevicolpate and brevissimicolpate. Myrtus communis and Psidium littorale shed pollen in tetrads as well as in monads and are the only tetrads recorded in Myrtales other than Onagraceae and Miconia melanotricha (Melastomataceae). The pollen morphology of Psiloxylon and Heteropyxis, genera of questionable taxonomic placement, suggests that they fit within the Myrtaceae. The final family, Onagraceae, is also very distinctive in Myrtales. The viscin threads, tetrads and polyads, exceptionally thick endexine, essentially spongy-paracrystalline ektexine with columellae absent or greatly reduced, absent foot layer, protruding apertures, and 2-aperturate grains (in Fuchsia) distinguish this family, which with a few superficial similarities to Trapaceae and Penaeaceae differs from all others in Myrtales.

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