Comparative pollen morphology and taxonomic ... - CNCFlora

y-two formation in different plans as in a crosshatch pattern
(Plate I, 1–2).
In the genus Actinocephalus, A. bongardii, and A. denudatus, have
pollen grains with a furrow in a single spiral pattern, but A. divaricatus
and A. ramosus have two spirals in varied patterns (Plate I,10–12).
The pollen grains of Paepalanthus have apertures that generally
trace a single spiral (Plate IV,7–9), but do show significant variation in
the number of spirals, while the genus Philodice has grains with
various types of apertures in spiral form but no one predominant type
(Plate V, 2–10). Among the species of Syngonanthus studied, pollen
grains could be 2-zonasulcate (Plate VI, 7–9) or could have spiral
apertures in various arrangements (Plate VII, 2–4).
The interapertural exines are of varying widths, with the
narrowest seen in Blastocaulon (~7 µm) and in Eriocaulon (~8 µm),
while the widest strips were noted in Syngonanthus (up to 17 µm).
The greater number of studied taxa has unornamented apertural
membranes, except for the pollen grains of Eriocaulon and Syngonanthus
arenarius. Eriocaulon are ornamented with obtuse microspines
on this membrane (Plate I, 5), whereas the pollen grains of
Syngonanthus arenarius have spines and microspines on the furrow
membrane (Plate VII, 1). At the apertures of the pollen grains of
Rondonanthus, the apertural membranes are not ornamented (Plate
VI, 6), as is the case in Actinocephalus (Plate II, 5), Blastocaulon (Plate II,
11), Lachnocaulon (Plate III, 4), Leiothrix (Plate IV, 3), Paepalanthus
(Plate V, 1) and Philodice (Plate V, 11).
3.3. Exine — structure and ectexine ornamentation
The exine is usually thin, varying in thickness from 1 to 3 µm. The
thickest exine was observed in specimens of the genera Eriocaulon
and Rondonanthus, whose average exine thickness is 2 µm; in the
other genera, the average exine thickness is 1 µm (Table 2).
The layer stratification of the exine is discernible, with the nexine
being thicker than the sexine in Eriocaulon and some species of Syngonanthus.
The exine is undifferentiated in Leiothrix spp., showing no
individualization of the layers under LM, with the exception of
L. flavescens and L. plantago, whose sexine and nexine are quite
distinct. The same is observed in Blastocaulon albidum. In Paepalanthus,
the exine stratification can be either undifferentiated or
differentiated into sexine and nexine and, in the latter case, the
analyses show that the exine can have a thicker sexine, or that the
sexine and nexine could be of equal thickness.
Due to the thinness of the exine in the pollen grains of the majority
of the species analysed, the exine layers (sexine and nexine) were not
measured individually, although this does not prevent a qualitative
analysis of the relative thicknesses of the two layers (Table 4).
The ectexine surface is irregular (Plate III, 10–11), sometimes
presenting areas with microperforations (Plate II, 2;Plate IV, 3)or
being slightly microrugulate (Plate III,5–6,10; Plate V,1;Plate VI,11–12).
The ectexine surface has two morphological types of supratectal
processes: granulose and (micro)spinose (the latter have tapered
apices). Due to their small size, both types are more easily visualized
under SEM. In spite of the heterogeneity in their size, the granules
appear to be more uniform than the microspines. The granules vary in
diameter from 0.22 to 0.64 µm and are present in the majority of the
genera studied. They are generally distributed among the microspines
and along the edge of the interaperture exine strip in Actinocephalus
(Plate II, 6),Lachnocaulon (Plate III, 5),Paepalanthus and Syngonanthus
(Plate VII, 6). They were not observed in Eriocaulon, Philodice or Tonina.
The (micro)spines show a wide range of variation in size, being
between 0.2 and 2.09 µm in length, with a majority being acute (Plate II,
5; Plate III, 11;Plate V, 12) while others are more obtuse (Plate I, 5;
Plate V,1;Plate VII, 1). The acute or obtuse shape is related to the size of
the supratectal processes, with the larger processes (spines) generally
being acute (Plate II, 5; Plate III, 11), while the smaller processes
(microspines) are generally obtuse (although some are occasionally
R.L.B. de Borges et al. / Review of Palaeobotany and Palynology 154 (2009) 91–105
acute) (Plate I,5;Plate VI,12;Plate VII, 1). The largest spinose processes
are observed in Paepalanthus chrysolepis and P. planifolius (Table 3).
4. Discussion
4.1. Pollen morphological considerations
The pollen grains in Eriocaulaceae present few variations with
regard to size or shape, being small to medium and mainly spheroidal.
Large variations in the size of the pollen grains at the intra-specific
level were seen among the specimens studied, which could be
attributed to the different degrees of maturation of the pollen grains
derived from different flowers on the same inflorescence. Giulietti
(1996), for example, reported that the maturation of the flowers in the
inflorescences of the genus Leiothrix is centripetal, a condition that
has been seen in the entire family. Coan et al. (2007a) pointed out that
flowers of many different ages occur on the same inflorescence in
Syngonanthus caulescens.
The sulcate feature, colpate according to some authors (Furness, 1988;
Rull, 2003), the typical apertural pattern of the family, was observed in all
species included here. Thanikaimoni (1965) described several apertural
arrangements for pollen grains of the Eriocaulon species, and the patterns
seen here are very similar to those described by that author.
Apertures having a design resembling the pattern on a tennis ball
are quite unique, and are described here for the genera Leiothrix and
Lachnocaulon, although Thanikaimoni (1965) also observed this
apertural type in two different genera, Syngonanthus and Tonina, as
well as a very similar pattern in the pollen grains of Lachnocaulon. The
genera Actinocephalus, Blastocaulon, and Paepalanthus have very
similar aperture patterns, with apertures in a single spiral (helicoidal)
or tracing various designs. However, the pollen4 grains of all species of
Syngonanthus sect. Eulepis and of S. imbricatus [S. sect. Thysanocephalus
after Ruhland (1903) and S. sect Eulepis after Lazzari (2000)]
have a 2-zonasulcate apertural pattern — the first time this pattern has
been reported for this family.
On many occasions, even under SEM, it is difficult to distinguish
between supratectal processes to determine which are microspines
and which are, in fact, granules (Plate I,6;Plate II,2;Plate III,6;Plate IV,
12). The following species have pollen grains that were entirely
microspinose: Actinocephalus cabralensis, Blastocaulon scirpeum,
Lachnocaulon engleri, Paepalanthus leucocephalus and P. subtilis. All of
the other taxa have exines that varied from microspinose to spinose,
with acute spines and acute to obtuse microspines, with the exception
of the species of Leiothrix that had both acute spines and microspines.
The morphology of the grooved spines of Tonina fluviatilis, unique
in the family, was very similar to those described by Halbritter et al.
(2007) for Thymelaea passerina (L.) Coss. & Germ. (Thymelaeaceae).
The representatives of Eriocaulon, the basal group of the family
according to M. J. G. Andrade (unpubl. data.), generally have three
apertures and a large apertural area. The number of apertures
diminishes and the interapertural exine strips are wider among the
genera of the subfamily Paepalanthoideae, which indicates a decrease in
the apertural area. In the genus Syngonanthus, a significant decrease in
the apertural area has been noted, mainly in the species of S. sect. Eulepis,
and in S. imbricatus, which have zonasulcate apertures and,
consequently, have smaller apertural areas than the others.
4.2. Taxonomic considerations
The contribution of pollen morphological characteristics to taxonomic
studies of the Eriocaulaceae can be seen at many levels. Although
the family can be considered stenopalynous, as recorded by Erdtman
(1943, 1952), Thanikaimoni (1965), andSantos et al. (2000), the pollen
characteristics presented here suggest that they have taxonomic value.
The pollen characteristics of the Eriocaulaceae are sufficient to
distinguish them from the other families of the order Poales.
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