Pollen morphology and exine ultrastructure of selected species of Waltheria L. (Byttnerioideae-Malvaceae)

Review of Palaeobotany and Palynology 221 (2015) 204–210

Contents lists available at ScienceDirect

Review of Palaeobotany and Palynology journal homepage: www.elsevier.com/locate/revpalbo

Pollen morphology and exine ultrastructure of selected species of Waltheria L. (Byttnerioideae-Malvaceae) Marileide Dias Saba a,⁎, Francisco de Assis Ribeiro dos Santos b a b

Laboratório de Estudos Palinológicos, Departamento de Educação–Campus VII, Universidade do Estado da Bahia (UNEB), Senhor do Bonfim, Bahia, 48970-000, Brazil Laboratório de Micromorfologia Vegetal, Departamento de Ciências Biológicas, Universidade Estadual de Feira de Santana (UEFS), Feira de Santana, Bahia, 44036-900, Brazil

a r t i c l e

i n f o

Article history: Received 7 February 2014 Received in revised form 17 June 2015 Accepted 3 July 2015 Available online 31 July 2015 Keywords: Palynology Heterostyly Pollen dimorphism Malvaceae Sterculiaceae

a b s t r a c t The pollen morphology of five species: Waltheria albicans Turczaninow, Waltheria brachypetala Turczaninow, Waltheria cinerascens Auguste Saint-Hilaire, Waltheria martiana Bentham ex J.G. Saunders, and Waltheria viscosissima Auguste Saint-Hilaire were studied by light microscopy (LM), scanning electron microscopy (SEM), and transmission electron microscopy (TEM), focusing on apertural type and exine. Pollen grains are medium to large, isopolar, oblate spheroidal to prolate spheroidal, 3(-4)-5(-6)-zonocolporate, costate, and fastigiate. Exine ornamentation is microreticulate, homobrochate, suprareticulate (longistyled morph), and microechinate to echinate (brevistyled morph). Sexine thickness is equal to or greater than the nexine. In TEM, the sexine consists of four layers, having an inner sexine 1, and outer sexine 4. An ultrathin section of exine revealed a columellate-like intine on pollen grains of W. cinerascens. © 2015 Elsevier B.V. All rights reserved.

1. Introduction Waltheria Linnaeus is a tropical and subtropical genus of Malvaceae with approximately 60 species; 53 species are found in the Americas (Saunders, 2007), and 20 are endemic to Brazil (Saunders, 1993). Currently the genus, along with Hermannia L., Melochia L., and Dicarpidium F. Muell., comprises the tribe Hermannieae of the subfamily Byttnerioideae (Malvaceae), characterized by flowers with five stamens and staminodes which are absent or reduced (Bayer et al., 1999). Morphologically, the genus is characterized by a unicarpellar gynoecium, lateral, simple style, and penicillate stigma (Esteves, 1986). According to Saunders (1993), approximately 40 species of Waltheria exhibit distyly. Ganders (1979) defined heterostyly as a heteromorphism that is genetically controlled, composed of two (distyly) or three (tristyly) morphs that differ in the length of the style and stamens. In Malvaceae, heterostyly is common in the genera Melochia and Waltheria (Byttnerioideae), which also exhibit pollen dimorphism. The pollen grains of the brevistyle morph are microechinate, while those of the longistyle morph are suprareticulate. Pollen grains of both morphs also differ in apertural number and pollen size (Köhler, 1973, 1976; Dorr and Barnett, 1989; Miranda and Andrade, 1989; Saunders, 1993; Saba et al., 2004; Saunders and Pozner, 2007). The pollen morphology of the Waltheria species has been described previously, based on observations using light microscopy (LM) ⁎ Corresponding author. Tel.: +55 74 3541 8935; fax: +55 74 3541 8905. E-mail addresses: [email protected] (M.D. Saba), [email protected] (F.A.R. Santos).

http://dx.doi.org/10.1016/j.revpalbo.2015.07.007 0034-6667/© 2015 Elsevier B.V. All rights reserved.

by Erdtman (1952), Chaudhuri (1969), Sharma (1970), Köhler (1971, 1976), Melhem et al. (1976), Moncada and Salas (1983), Miranda and Andrade (1989), Palacios-Chávez et al. (1990, 1991), Roubik and Moreno (1991), Saunders (1993), Saba and Santos (2000, 2003), Saunders and Pozner (2007), and using scanning electron microscopy (SEM) by El Ghazali (1993) and Saba et al. (2004). No observations in transmission electron microscopy (TEM) were previously published. In the present study, the pollen morphology of five species of Waltheria (Waltheria albicans Turcz., Waltheria brachypetala Turcz., Waltheria cinerascens A. St.-Hil., Waltheria martiana Benth. ex J.G. Saunders, and Waltheria viscosissima A .St.-Hil.) was investigated using light microscopy (LM), scanning electron microscopy (SEM), and transmission electronic microscopy (TEM), with emphasis on the differences in pollen morphology from heterostyled morphs. 2. Materials and methods Flower buds were collected from selected herbarium specimens (CEPEC, EAC, HRB, HUEFS, and SPF, abbreviations follow Index Herbariorum; Thiers, B., continuously updated). Prior to treatments, flower buds were examined for heterostyly and dehydrated in pure acetic acid. Pollen grains were treated by the acetolysis method (Erdtman, 1960). After acetolysis, each sample was split into two subsamples, for observations in LM and SEM. For LM, the pollen grains were mounted on slides with glycerin jelly. The measurement of the main morphometric parameters (equatorial and polar diameters) was made, whenever possible, for 25 pollen grains, while the other parameters (diameter of the apertures and

M.D. Saba, F.A.R. Santos / Review of Palaeobotany and Palynology 221 (2015) 204–210

205

Table 1 Morphometric characters of the pollen grains of Waltheria L. speciesa. Species/specimens

PD x ± Sx

Waltheria albicans Turcz. (Plate I, 1-9) Saunders & Carvalho 3120 (CEPEC) (L) 35.1 ± 0.3 Saunders & Carvalho 3117 (CEPEC) (L) 33.9 ± 0.4 Fernandes & Nunes s.n. (EAC 8517) (B) 43.6* Waltheria brachypetala Turcz. (Plate I, 10–12) Saunders & Carvalho 3116 (CEPEC) (L) 35.3 ± 0.3 Hatschbach & Kummrow 48064 (CEPEC) (L) 33.5 ± 0.4 Waltheria cinerascens A. St.-Hil. (Plate II, 13–17) Harley et al. 20540 (SPF) (L) 41.8 ± 0.3 Harley et al. 18570 (CEPEC) (L) 37.7* Waltheria martiana Benth. ex J.G. Saunders (Plate III, 18–19) Harley 16683 (CEPEC) (L) 49.5* Queiroz & Nascimento 3480 (HUEFS) (B) 58.1* Waltheria viscosissima A. St.-Hil. (Plate III, 20–26) Hatschbach & Kummrow 48065 (CEPEC) (L) 40.1 ± 0.5 Mattos-Silva et al. 2738 (HUEFS) (L) 36.7* Longa & Campos 3 (HRB) (B) 43.5 ± 0.6

ED

EDpv

P/E

Ecto

Endo

Sex

Nex

31.5–36.0 31.5–37.5 45.0–51.0

1.06 1.05 0.89

22.5 × 1.1 22.3 × 1.6 3.5 × 3.8

3.8 4.1 –

1.4 1.4 1.4

0.8 0.8 1.2

35.3 ± 0.3 35.6*

31.5–37.5 30.0–39.0

1.00 0.95

21.2 × 2.5 18.8 × 2.4

4.2 2.8

1.2 1.2

0.6 0.6

39.0–45.0 34.5–40.5

42.6* 36.8*

40.5–43.5 36.0–39.0

0.99 1.01

25.6 × 3.6 17.2 × 1.6

4.5 3.2

2.0 1.8

0.6 1.0

47.8* 59.8*

45.0–54.0 57.0–66.0

46.0* 60.0*

43.5–48.0 55.5–64.5

1.03 0.97

31.7 × 2.3 –

5.3 –

1.0 1.0

1.0 1.0

36.4 ± 0.5 35.6* 48.2 ± 0.6

31.5–39.0 33.0–39.0 39.0–52.5

35.4* 34.0* 46.8 ± 0.6

32.3–40.5 31.5–36.0 37.5–51.0

1.10 1.03 0.90

15.2 × 2.5 16.0 × 1.0 –

5.6 4.0 3.5

1.0 1.4 1.9

0.6 1.0 0.6

R

x ± Sx

R

x ± Sx

R

31.5–37.5 30.0–34.5 39.0–48.0

33.1 ± 0.3 32.3 ± 0.4 48.9*

30.0–36.0 28.5–34.5 43.5–52.5

33.3 ± 0.3 34.3 ± 0.4 48.5*

31.5–39.0 30.0–37.5

35.2 ± 0.4 35.3 ± 0.5

31.5–37.5 31.5–39.0

39.0–45.0 36.0–40.5

42.1 ± 0.3 37.0*

46.5–54.0 54.0–63.0 34.5–42.7 33.0–42.0 36.0–46.5

a PD = polar diameter; ED = equatorial diameter; EDpv = equatorial diameter in polar view; Ecto = length × width of the ectoaperture; Endo = height of the endoaperture; Sex = sexine; Nex = nexine; x = arithmetic mean; s = medium standard; R = range variation; L = longistyle morph; B = brevistyle morph; *n b 25; measurements in μm and indices in absolute numbers.

exine thickness, sexine, and nexine) were measured in ten randomly selected pollen grains. For SEM, acetolyzed pollen grains were rinsed in an ethanol series up to 100%, pipetted onto specimen stubs. The specimens were air dried, coated with gold, and photographed with a Zeiss LEO 1430 VP microscope (SEM Lab, Biological Sciences Department, Universidade Estadual de Feira de Santana). For TEM, the most commonly widespread species were selected. In this case, a longistyled morph of W. cinerascens was used. Closed, mature anthers were fixed for 48 h in glutaraldehyde (2.5%) solution in 0.1 M sodium phosphate, pH 7.4. The material was postfixed in osmium tetroxide (OsO4) dissolved in 1% buffered solution, dehydrated in ascending acetonic series, and then infiltrated in Epon resin. The ultrathin sections were obtained using an ultramicrotome, placed on copper grids and contrasted with uranyl acetate and 7% aqueous lead citrate. The samples were then analyzed on a Zeiss M 109 (Platform for Electron Microscopy, Gonçalo Moniz Research Center—FIOCRUZ). The pollen terminology follows Punt et al. (2007). 3. Results and discussion In this study, brevistyled and longistyled specimens were investigated of Waltheria albicans, W. martiniana, and W. viscosissima, and only longistyled specimens of W. brachypetala and W. cinerascens, according to the vouchers available in herbaria. The analyses of pollen developed in this study have confirmed the data found in the literature about pollen dimorphism between heterostyle species Waltheria. All Waltheria species generally have medium to large-sized pollen grains (Table 1), oblate spheroidal to prolate spheroidal, amb subcircular to polygonal. Pollen grains are angulaperturate, with 3(-4)-5(-6) apertures (colporus). Exine is microreticulate, suprareticulate in longistyled morph, and microechinate to echinate in brevistyled

morph (Plates I–III, Table 2). Sexine has the same thickness as that of the nexine or it is greater. In TEM, the sexine consists of four layers (sexines 1, 2, 3, 4), and intine exhibits a columellate-like structure. Pollen diameters of the brevistyled morphs are comparatively larger compared to specimens of the longistyled morphs (Table 1), confirming previous observations for heterostyled species (Köhler, 1973, 1976; Miranda and Andrade, 1989; Saunders, 1993; Saba et al., 2004; Saunders and Pozner, 2007). 3.1. Aperture Waltheria exhibits a colporate apertural system. In the pollen of longistyled morphs, the ectoapertures are elongated with tapered ends (Plate I, 3; Plate III, 18), while in the brevistyled morphs, the ectoapertures are considerably shorter, narrower, and slightly larger than the endoaperture (Plate I, 9; Plate III, 25; Table 1). The endoapertures can be both lalongate to circular. The lalongate form has upper and lower parallel margins, with indistinct equatorial ends. Apertural heteromorphism was observed in all species studied (within the same sample). However, variation is greater in the pollen of brevistyled morphs, which have more apertures (4–6) compared to those of the longistyled morph (3) of the same species (Table 2). Pollen grains with three and five apertures are more predominant. In pollen grains with more than three apertures, the apertures are arranged in a sinuous line in equatorial view (Plate I, 6). In some pollen grains, there is a separation of the layers of the exine in the apertural region (Plate I, 1), forming a fastigium. There are reports of the presence of annuli in pollen apertures of brevistyled morphs of W. viscosissima (Miranda and Andrade, 1989) and W. belizensis (Saunders and Pozner, 2007). There is certainly a terminological confusion since an annulus is a pollen feature restricted to a porus (Punt et al., 2007). The thickening corresponds to the costa (Plate I, 6 and 12, Plate II, 15)

Plate I. Pollen grains of Waltheria L. 1–4 5–9 10–12

W. albicans Turcz. (longistylous): 1—Polar view (optical section). 2—LO analysis. 3—General view (SEM). 4—Surface (SEM). W. albicans Turcz. (brevistylous): 5—Polar view (optical section). 6—Equatorial view (optical section). 7—LO analysis. 8—Surface (SEM). 9—Aperture detail (SEM). W. brachypetala Turcz. (longistylous): 10—Polar view (optical section). 11—LO analysis. 12—Equatorial view (optical section).

Plate II. Pollen grains of Waltheria L. (see on page 4) 13-17

W. cinerascens A. St.-Hil. (longistylous): 13—Polar view (optical section). 14—LO analysis. 15—Equatorial view (optical section). 16—Radial section across apertural region (TEM). 17—Radial section across mesocolpium (TEM). (In = intine; S1 = sexine 1; S2 = sexine 2; S3 = sexine 3; S4 = sexine 4).

206

M.D. Saba, F.A.R. Santos / Review of Palaeobotany and Palynology 221 (2015) 204–210

M.D. Saba, F.A.R. Santos / Review of Palaeobotany and Palynology 221 (2015) 204–210

Plate II (caption on page 2).

207

208

M.D. Saba, F.A.R. Santos / Review of Palaeobotany and Palynology 221 (2015) 204–210

Plate III. Pollen grains of Waltheria L. 18–19 20–23 24–26

W. martiniana Benth. ex J.G. Saunders (longistylous): 18—General view (SEM). 19—Surface (SEM). W. viscosissima A. St.-Hil. (longistylous): 20—Polar view (optical section). 21—Equatorial view (optical section). 22—Surface (SEM). 23—Detail the structure of the exine (SEM). W. viscosissima A. St.-Hil. (brevistylous): 24—General view (SEM). 25—Surface (SEM). 26—Detail the structure of the exine (SEM).

Chaudhuri (1969) described heterocolpate pollen grains in Waltheria indica L. (= W. americana), while Palacios-Chávez et al. (1990, 1991) and Salgado-Laboriau (1973) described the pollen of this species as pantocolporate. None of these apertural types were observed in any species treated here, where in our study, were observed as 3(-4)5(-6)-zonocolporate. 3.2. Exine Waltheria pollen grains has two patterns of ornamentation: suprareticulate in longistyled morphs and microechinate to echinate

brevistyled morphs. Under LM and SEM, the suprareticulum differs among species with smaller lumina and thin muri, such as in W. brachypetala (Plate I, 11) and in W. viscosissima. Wider lumina and thicker muri were observed in W. albicans, W. cinerascens, and W. martiniana (Plate I, 2, 4; Plate II, 14; Plate III, 19). The spines, located around a microreticulum, are heterogeneous in size, shape and diameter. In W. albicans (Plate I, 7–9) and W. viscosissima (Plate III, 24–26), the (micro)spines have a broad base and acute apex and are densely distributed throughout the pollen surface. The results presented here corroborate those of Köhler (1971, 1976), who divided Waltheria into two pollen types: suprareticulate and

M.D. Saba, F.A.R. Santos / Review of Palaeobotany and Palynology 221 (2015) 204–210

209

Table 2 Morphological characters of pollen grains of Waltheria L. speciesa. Species floral morphology

Size

Shape Amb

Aperture

Exine

Colpori number Endoaperture Waltheria albicans Turcz. Longistylous Brevistylous

M M

PS OS

Waltheria brachypetala Turcz. Longistylous M OS-S Waltheria cinerascens A. St.-Hil. Longistylous M OS-PS Waltheria martiana Benth. ex J.G. Saunders Longistylous M (L) OS Brevistylous L OS Waltheria viscosissima A. St.-Hil. Longistylous M Brevistylous M (L)

a

PS OS

Subcircular Circular to polygonal

3 (4)5(6)

Lalongate, rectangular Lolongate

Suprareticulate, heterobrochate Echinate, spines heterogeneous distributed homogeneously throughout the pollen grain; microreticulate surface in the interspinous region

(Sub)circular to polygonal 4(5)

Lalongate, rectangular

Suprareticulate, heterobrochate

(Sub)circular to polygonal (4)5(6)

Lalongate, rectangular

Suprareticulate, heterobrochate

Circular Circular

3 6

Lalongate, rectangular Lolongate

Suprareticulate, homobrochate Echinate, spines densely distributed throughout the pollen grain

Subcircular Circular to polygonal

3(4) (5)6

Lalongate, elliptical Lolongate to circular

Suprareticulate, heterobrochate Microechinate, microspines distributed homogeneously throughout the pollen grain; perforated surface in the interspinous region

Size: M = medium, L = large; shape: OS = oblate spheroidal, PS = prolate spheroidal, S = spheroidal.

microechinate. Köhler (1976) also associated the pollen dimorphism to heterostyly within the genus. Ultrathin sections of W. cinerascens pollen in longistyled morphs exhibited sexine stratification in four layers (Plate II, 16–17), such as (I) sexine 1, which is the compact and continuous footlayer; (II) sexine 2, which is defined as the infratectum which is formed by thicker columellae; (III) sexine 3, which is defined as the tectum that is thinner and with few and sparse perforations; (IV) and finally sexine 4, which is formed by muriform thickenings on sexine 3. When observed frontally, these thickenings appear as a reticulate pattern. Below the sexine, there is a very thin endexine, and the unusual columellate-like intine (Plate II, 17). The intine is thick, with columellae closer to each other than the columellae on the sexine. Miranda and Andrade (1989) and Saba and Santos (2003) observed the presence of nexines 1 and 2 in Waltheria species, which may have been misinterpretations of the sexine structure. The nexine, as observed in TEM, is a thick, columellate-like layer under the thin endexine. Other pollen descriptions of Waltheria, besides the mentioned studies above, confirmed most of observations in this study (Erdtman, 1952; Sharma, 1970; Melhem et al., 1976; Moncada and Salas, 1983; Roubik and Moreno, 1991; El Ghazali, 1993; Saba and Santos, 2000; Saunders and Pozner, 2007). It is noteworthy that many of the differences among these studies are due to misinterpretations of exine ornamentation and terminological confusion.

for his help with English text. This research was funded by CNPq to FARS.

4. Conclusions

Bayer, C., Fay, M.F., De Bruijn, A.Y., Savolainen, V., Morton, C.M., Kubitzki, K., Alverson, W.S., Chase, M.W., 1999. Support for an expanded family concept of Malvaceae within a recircunscribed order Malvales: a combined analysis of plastid atpB and rbcL DNA sequences. Bot. J. Linn. Soc. 129, 267–303. Chaudhuri, S.K., 1969. In: Santapau, H. (Ed.), Contribution to the pollen morphology of Sterculiaceae. J. Sen Memorial volume. J. Sen Memorial Committee & Botanical Society of Bengal, Calcutta, pp. 229–238. Dorr, L.J., Barnett, L.C., 1989. A revision of Melochia section Physodium (Sterculiaceae) from Mexico. Brittonia 41 (4), 404–423. El Ghazali, G.E.B., 1993. A study on the pollen flora of Sudan. Rev. Palaeobot. Palynol. 76, 99–345. Erdtman, G., 1952. Pollen morphology and plant taxonomy – angiosperms. Almqvist and Wiksell, Stockholm. Erdtman, G., 1960. The acetolysis method. A revised description. Sven. Bot. Tidskr. 54, 561–564. Esteves, G.L., 1986. A Ordem Malvales na Serra do Cipó, Minas Gerais, Brasil (Master´s dissertation) Instituto de Biociências—Universidade de São Paulo, São Paulo. Ganders, F.R., 1979. The biology of heterostyly. N. Z. J. Bot. 17, 607–635. Köhler, E., 1971. Zur pollenmorphologie der gattung Waltheria L. (Sterculiacaeae). Feddes Repertorium 82 (2), 125–153. Köhler, E., 1973. Über einen bemerkenswerten pollendimorphismus in der gattung Waltheria L. Grana 13, 57–64. Köhler, E., 1976. Pollen dimorphism and heterostyly in the genus Waltheria L. (Sterculiaceae). In: Ferguson, I.K., Müller, J. (Eds.), The evolutionary significance of the exine. Academic Press, London, pp. 147–161.

Pollen dimorphism in Waltheria can be attributed to heterostyly. Pollen grains of brevistyled specimens have microechinate to echinate exine, with more apertures and larger size than those from longistyled morphs that have suprareticulate ornamentation, corroborating previous studies of Köhler (1971, 1973, and 1976). Furthermore, the sexine is four layered in longistyled morph of W. cinerascens, as observed in TEM, besides the columellate-like intine. Acknowledgments The authors thank the Fundação Oswaldo Cruz (FIOCRUZ), mainly Marcos André Vannier and Claudio Pereira, of the Platform for Electron Microscopy, and the Gonçalo Moniz Research Center, for their assistance in TEM analyses. Many thanks to the herbaria (CEPEC, EAC, HRB, HUEFS and SPF) that provided specimens for pollen analyses. Thanks to Dr. Madeline Harley (Royal Botanic Garden, Kew) for useful comments and suggestions and to Dr. Vernie G. Sagun (University of Connecticut)

Appendix A. Specimens examined Waltheria albicans Turcz.: BRAZIL. Bahia: Xique-Xique, 17/III/1990 Saunders & Carvalho 3120 (CEPEC); Morro do Chapéu, 16/III/1990, Saunders & Carvalho 3117 (CEPEC). Maranhão: São João dos Patos– Barão de Grajaú, 23/IV/1980 Fernandes & Nunes s.n. (EAC). Waltheria brachypetala Turcz.: BRAZIL. Bahia: Morro do Chapéu, 16/III/1990 Saunders & Carvalho 3116 (CEPEC); Andaraí, 19/VI/1984, Hatschbach & Kummrow 48064 (CEPEC). Waltheria cinerascens A. St.-Hil.: BRAZIL. Bahia: Andaraí, 24/I/1980, Harley et al. 20540 (SPF); Andaraí-Mucugê, 12/II/1977, Harley et al. 18570 (CEPEC). Waltheria martiana Benth. ex J.G. Saunders: BRAZIL. Bahia: Delfino, 04/III/1974, Harley 16683 (CEPEC); Jacobina, 20/VIII/1993, Queiroz & Nascimento 3480 (HUEFS). Waltheria viscosissima A. St.-Hil.: BRAZIL. Bahia: Andaraí, 19/VI/1984, Hatschbach & Kummrow 48065 (CEPEC); Lençóis, 18/V/1989, MattosSilva et al. 2738, (HUEFS); Salvador, 05/IX/1996, Longa & Campos 3 (HRB). References

210

M.D. Saba, F.A.R. Santos / Review of Palaeobotany and Palynology 221 (2015) 204–210

Melhem, T.S., Silvestre, M.S.F., Lucas, N.M.C., 1976. Pollen morphological studies in Sterculiaceae. Hoehnea 6, 23–32. Miranda, M.M.B., Andrade, T.A.P., 1989. Pólen das plantas do Nordeste Setentrional do Brasil III. Sterculiaceae. Acta Bot. Bras. 3 (2) (Suppl. 1), 281–292. Moncada, M., Salas, E., 1983. Polen de las plantas melíferas em Cuba. Instituto de Botanica, Centro de Informacion y divulgacion Agropecuario, La Habana. Palacios-Chávez, R., Arreguin, S.M.L., Quiroz, D.L., 1990. Morfologia de los granos de polen de la familia Sterculiaceae de la Estacion de Biologia Chamela, Jalisco. Palynol. Palaeobot. 2 (1), 62–81. Palacios-Chávez, R., Ludlow-Wiechers, B., Villanueva, R.G., 1991. Flora palinologica de la reserva de la biosfera de Sian Ka’an Quintana Roo, Mexico. Centro de Investigaciones de Quintana Roo, Chetumal. Punt, W., Hoen, P.P., Blackmore, S., Nilson, S., Le Thomas, A., 2007. Glossary of pollen and spore terminology. Rev. Palaeobot. Palynol. 143, 1–81. Roubik, D.W., Moreno, P.J.E., 1991. Pollen and spores of Barro Colorado Island. Missouri Botanical Garden, United States of America.

Saba, M.D., Santos, F.A.R., 2000. Morfologia polínica de espécies de Sterculiaceae do Pico das Almas (Bahia–Brasil). Rev. Univ. Guarulhos – Geociências V, 201–204 (especial). Saba, M.D., Santos, F.A.R., 2003. Morfologia polínica de Sterculiaceae das Dunas do Abaeté, Salvador—Bahia. Sitientibus Série Ciências Biol. 3 (1-2), 109–114. Saba, M.D., Santos, F.A.R., Esteves, G.L., 2004. Palinotaxonomia das tribos Byttnerieae DC., Hermannieae DC. e Helictereae DC. (Malvaceae s.l.) da flora da Bahia, Brasil. Hoehnea 31 (2), 189–214. Salgado-Laboriau, M.L., 1973. Contribuição à palinologia dos cerrados. Academia Brasileira de Ciências, Rio de Janeiro. Saunders, J.G., 1993. Four new distylous species of Waltheria (Sterculiaceae) and a key to the Mexican and Central American species and species groups. Syst. Bot. 18 (2), 356–376. Saunders, J.G., 2007. Sterculiaceae of Paraguay. II. Waltheria. Bonplandia 16 (1-2), 143–180. Saunders, J.G., Pozner, R., 2007. A new penicillate-stigma species of Waltheria (Sterculiaceae, Hermannieae) endemic to Belize. Novon 17, 79–86. Sharma, B.D., 1970. Studies of Indian pollen grains in relation to plant taxonomy— Sterculiaceae. Proc. Natl. Inst. Sci. India 35 (4), 320–359.