The convergent evolution of exine shields in angiosperm pollen

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1 Grana ISSN: (Print) (Online) Journal homepage: The convergent evolution of exine shields in angiosperm pollen Heidemarie Halbritter & Michael Hesse To cite this article: Heidemarie Halbritter & Michael Hesse (1995) The convergent evolution of exine shields in angiosperm pollen, Grana, 34:2, , DOI: / To link to this article: Published online: 01 Sep Submit your article to this journal Article views: 220 Citing articles: 10 View citing articles Full Terms & Conditions of access and use can be found at

2 Grana 34: , 1995 The convergent evolution of exine shields in angiosperm pollen HEIDEMARIE HALBRITTER and MICHAEL HESSE Halbritter, 1. & Hesse. hl The convergent evolution of exine shields in angiosperm pollen. - Gnna 34: ISSN The appearance of the pollen grains (or tetrads) of some taxa of only distantly related angiosperm families Berberidaceae (hfnnhoiiin), Bignoniaceae (Catalpa), Euphorbiaceae (Phy//nntkifs), Iridaceae (Iris sect. Jirrio), hfalpighiaceae (hfalpighia, Bariisteriopsis, Heteropferis) and hlartyniaceae (Ibicella, Probosciden) is very similar. For this pollen type \ve propose thc term clypeate (= covered with shields), i.e. pollen grains with large, isolated. polygonal or circular (s)exinous shields, separated by grooves. Despite their similar appearance both their aperture condition and wall stratification are different. PIi~llai~thus x elongotiis and the hfolpighin species investigated have (panto-)porate pollen grains, and local intine thickenings are present beneath the pores, which are located between the exine shields in a regular (P/iyl/aiirAirs) or more irregular manner (dfdpighia). This pantoporate condition differs markedly from the aperture condition which is present in all of the other taxa investigated and can best be described as inaperturate. Heideinarie HalDritter arid hfichael Hesse, liistitirte of Botany and Botaiiicnl Garden of the University of Vienna. Rennweg 14, A-I030 Wieii, Austria. (hfaiiiiscript accepted 25 Noveinber 1994) The pollen grains (or tetrads in Catalpa) of several taxa in the distantly related angiosperm families of Berberidaceae (Malioiiia aqiiifoliiint), Bignoniaceae (Catalpa ovatn, C. biriigei, C. bignonioides), Euphorbiaceae (Plijllantltiis x elongariis), Iridaceae sect. Jiiiio (Iris Aucheri, I. bucharica, I. caiicrrsicn, I. kopctdagensis, I. orchioides, I.persica, I. plariifolio = I. alata), Malpighiaceae (Bariisteriopsis canrpcstris, Heteropteris clriserrii, Malpigliia coccifern, M. glabrn, hf. gimdalajnrerzsis, M. piinicifolia) and Martyniaceae (Ibicclln hiten. Proboscidea fragrnns, P. loirisinnica, P. paniji ora) are characterized by f isolated, polygonal or rounded exine shields, irrespective of the aperture condition and the sculpturing of these shields. Some authors have referred to this peculiar, evidently convergent pollen type. but a precise term is lacking. In Mahonia pollen, Blackmore & Heath (1984) and Furness (1985) refer to exine plates divided by furrows. Schulze (1971a,b) describes the pollen grain of most members of Iris sect. Jiiito as having Exine Schildchen (small exine shields). Sometimes areolate has been used to describe the pollen type or pollen ornamentation by Gentry & Tomb (1979, for some Bignoniaceae), by Punt (e.g. 1980, 1986, 1987, for some fli~llarttliiispecies, Euphorbiaceae), and by Bretting & Nilsson (1988, for some Martyniaceae), but, unfortunately, this term is ambiguous and unsuitable. Xi & Zhou (1992) report this pollen type for some- Malpighiaceae but avoid giving it a special term. We have investigated this pollen type with respect to pollen ornamentation, wall configuration, and aperture condition. We propose clypeate and exine shields in describing this peculiar pollen type. MATERIAL AND METHODS The present study is based on the investigation of fresh material from the Botanical Garden of the University of Vienna (HBV), dry material from the herbaria WU and \V. Till, and material fixed by P. Bretting, and provided by S. Nilsson. Berberidaceae: hfahoriin aqrrifoliitm (Pursh) Nutt. (HBV, Fig. 1); Bignoniaceae: Catnlpn Digrioiiioides Waldst. (HBV), C. briiigei C.A. hley. (HBV, Fig. 2C-E), C. ointa G. Don (HBV, Fig. 2A,B); Euphorbiaceae: PhjI/utithus x eloiigattrs (Jacqu.) Steud. (HBV, Fig. 3); a putative hybrid between P. epipliyllnnthirs L. and P. arbiisciila (SW.) Gmelin (Gndy L. Webster and pers. comm.) Iridaceae: Iris L. sect. Jitno [Tntt.] Benth. [syn. subgenus Scorpiris Spach]: nomenclature according to Wendelbo & hhthew (1975). I. Aticheri (Baker) Sealey (WU, Fig. 5C), I. biichnricn Foster (HBV, Fig. 4), I. caitcasica Hoffm. (WU), I. kopetdqensis (Vved.) hlathew & Wendelbo (HBV); I. orchioides Cam. (WU) [non I. orchioides Dykes!] (WU, Fig. 5A.W I. persica L. (WU), I. plntiifolin (hliller) Fiori & Paol. = I. olnta Poir (WU, Fig. 5D.E); hfalpighiaceae: Bnnisteriopsis cainpestrir (Adr. Juss.) Little (\V. Till). Heteropteris dirseiiii Nied. (\V.TiW. hfalpighiu coccifero L. (WU). hf. gbbra L. (HVB, WU, Fig. 6A-E), hf. girndn/ajarensis Rose (LVu). hf. pitnicifolia L. (WU. Fig. 60; Grona Scandinavian University Press. ISSN

3 Exine shields iii Crtzgiosperrn pollen 109 hlarty niaceae: Ibiccllo Iirfeo (Lindley) Eselt. (HBV, Fig. 7A-C). Proboscitlenfro~mris (hliller) Thell. (HBV, Fig. 7D,E), P. louisiorzica (hliller) Thell. ssp. frogroris (Lindley) Bretting (P. Brctting). Pr. loiainnicn (hliller) Thell. ssp. Ioitisianicn (Lindley) Bretting (P. Bretting, Fig. 7F), Pr. porvijloro (Wooton) Wooton & Stanley ssp. porviflorn Bretting (P. Bretting). For Lhl observations living pollen grains were investigated in water. In mature pollen grains, germinated on sugar-agar, the emergence point of pollen tubes was checked. For SEhI investigations either untreated air dried pollen grains were mounted on aluminium stubs, or hydrated and living pollen grains were fixed in 25% GA (or were based unfixed), dehydrated in an acetone series, critical point dried and then mounted on stubs. All specimens were subsequently sputter-coated with gold. For TEhl investigations the pollen grains were fixed in Sorensen- or sodium cacodylate-buffered 25% glutaraidehq.de, and some spccimens were also postfixed in 2% 0.0,. After dehydration in an ethanol series the specimens were embedded in Spuds resin. The ultrathin sections were poststained with uranyl acetate and lead citrate. We did not acetolyse the material because. in most cases. the pollen grains disintegrate. RESULTS General aspects In all of the taxa investigated the pollen shape and diameter vary conspicuously, depending on the actual hydration stage. In the dry state the pollen grains are sometimes collapsed and irregularly shaped, and normally covered by a thin surface coating (Figs. 1 A, 2A, 3A, 4A, 6A, 7A). This surface coating is removed (partially or totally) during preparation. - Generally, the pollen type or class is clypeate, i.e. the surface of the pollen grains is characterized by more or less equally distributed exine shields. The exine sculpturing - more precisely: the sculpturing of the respective exine shields - may be reticulate, perforate, etc. and is not dependant on this special pollen type. The circular or polygonal shields are separated by grooves, which are more or less narrow in the dry state. In the hydrated state the shields become separated from each other and since the exine is markedly reduced in the grooves, it ruptures (Figs. la,b; 2A.B; 3A.B; 4A,B; 6A.B; 7A.B). The dimension of the grooves depends on the degree of hydration, while the diameter of the shields generally remains unchanged. There are restrictions to the degree to which the grooves can expand in the pantoporate examples (Phjllnntliirs x rlotzgatirs and hfdpighin ssp.; Figs. 3 and 6) because of the relatively thick endexine beneath the grooves. Rupturing does not occur and so the expansion during hydration is limited. In all the other species with clypeate pollen the exine ruptures in these regions, and the intine becomes pariially uncovered. Apertrire cotidition. -The pollen tube can arise practically anywhere (except in hfdpighin ssp. and Plijllnnthiis x elorzgotiis pollen with pores) and forces its way through by pushing aside the shields (Fig. 7D). From this standpoint there is no aperture region s.str., and the pollen grains Grono 31 (1995)

4 110 H. HaIDritter arid M. Hesse Fig. 1. dfdtoriin qiiijohn. (A) Pollen in dry state, with thin surface coating. (B) Pollen in hydrated state. (C) Pollen wall stratification in and near the grooves (e = exine, i = intine). (D) Pollen wall stratification in clypeate regions, and - left - near a groove (ec = ectexine, en = endexine, i = intine). Scale bars: 10 Iim in A. B, 1 pm in C, D. therefore resemble inaperturate or omniaperturate pollen (Table 1). Berberidaceae Ahhonia aqrrifalir~m: the average pollen diameter varies from about 30 pm (in the dry state) to 45 pi (hydrated). The c. 4-6 large shields, separated by grooves, have a relatively smooth surface, but exhibit small granules near the grooves (Fig. IA,B). Pollen nnll stratification. - The shields have a massive, smooth, sparsely perforated exine, with a homogeneous, non-differentiated ectexine (with small cavities without columellae and/or foot-layer) and an inner layer of different structure and electron-density, which can be described as a fibrous or granular endexine; the ectexine becomes thinner toward the grooves, and is missing in the actual groove region. The fibrous granular endexine covers (in dry state) the grooves. The intine is of uniform thickness (Fig. 1C.D). Gram 34 (1995)

5 Eririe shields in nnigiosperni pollen 1 I 1 Fig. 2. A, B. Crrtdpn 0wk7. C-E. C. brrngei. (A) Pollcn tetrad in dry state, with thin surface coating. (B) Pollen tetrad in hydrated state. (C) Pollen tetrad in cross section. (D) Pollen wall stratification in clypeate regions (ec = ectexine, EIZ = endexinc, i = intine). (E) Pollen wall stratification in P groove, note ruptured endexine (en = endexine, i = intine). Scale bars: 10 pm in AX, I pm in D, E. Rignoniaceae Cnfcrlpcr (permanent tetrads): the tetrad diameter varies from c. 35 pm in the dry state up to 50 pi in the hydrated state. In the dry state the pollen tetrads appear irregular collapsed (Fig. 2A) while in the hydrated state the tetrads appear more spherical, and the four constituting pollen grains can be more easily identified (Fig. 23). The (c. 10 distally located, single grain) polygonal exine shields are finely reticulate. Pnllrri ndl stmtificcrtion (Cntolpci biingci). - The exine Gram 34 (1995)

6 1 12 H. Hnlbritter nid Hesse Fig. 3. PItyllorrrkrrs x rlor~gorrrs. (A) Pollen in dry state, with thin surface coating. (B) Pollen in hydrated State. (C) Close-up of shields and grooves: pores in thc points of coalescence of three grooves (~ITTOII.~). (D) Pollen grain in cross Section. (E) Detail of (D), pollen wall stratification. Thickened, sfratified intinc bcncath the pores (cc = cctcxinc, en = endcxine, i = inline). SCnk bars: 10 Itm in A, B, D. 1 pm in c, E. cmmlrl3-1 (I553

7 Exine shields in angiosperm pollen 1 13 Fig. 4. Iris birchnricn. (A) Pollen in dry state, with thin surface coating. (B) Pollen in hydrated state. (C) Detail of (B): grooves with exine lunips (arrows). (D) Pollen in cross section. (E) Detail of (I)): pollen \Val1 stratification in clypeate regions (e = exine, it = endintine, ir = ecfintine). Scale bars: 10 Irm. Crana 3-1 (1995)

8 I14 H. Halbritter and M. Hesse Fig. 5. A, B. Iris orchioides. C. I. arrcheri. D, E. I. planifolia. All from herbarium material. Scale bars: 10 prn. Note two distinct.ornamentations of the exine shields within sect. Jirno. shields consist of ectexine with rectum and columellne, n thin (sometimes discontinuous) foot-layer, and a relatively thick endexine. In the grooves the ectexine is generally reduced. The distal \Val1 area of a monad is comparable with that of n single pollen grain. The intine is single-layered and of uniform thickness (Fig. 2C-E). Groria 34 (1995)

9 Exitre shields in angiosperm pollen 115 Fig. 6. A-E. Alnlpighin glnbm. F. U. pwricifolin. (A) Pollen in dry state, with thin surfacc coating. (B) Pollen in hydrated state. Grooves (arrows) expanded. (C) Detail of (B): porus. (D) Pollen wall stratification in ponte regions (ec = ectexine. en = endsxine, i = intine). (E) Pollen wall stratification in and near the grooves (cc = ectexine, ert = endexine, i = intine). (F) Pollen from herbarium material. The shields are linked with exine bridges. Scale bars: 10 pm. Grnrin 34 (1995)

10 1 16 H. Halbritter and M. Hesse Fig. 7. A-C. Ibicello hrm. D. E. Proboscirlecr~rc~rar2s. F. P. loirisionica ssp. loirisiorticn. (A) Pollen in dry state, with thin surface coating. (B) In hydrated state; the shields are widely separated by grooves. (C) Pollen wall stratification of clypcate region (ec = ectexine, en = endexine, i = intine). (D) Germinated pollen grain (light microscope, fresh pollen material). (E) Detail of a hydrated pollen grain; ectexinous granules (arrows) on the expanded intine of the grooves. (F) Collapsed pollen grain; ectexinous granules on the surface of the (unexposed) groove. Scale bars: 10 prn in A, B, D. E. 1 prn in C, F.

11 Euphorbiaceae Pliyllnnthus x elongntris: The average pollen diameter vanes from 12 to 15 pn in the dry state to Itm in the hydrated state. The reticulum of the somewhat numerous, polygonal (c. 20) shields is separated by a clear-cut mum from the grooves (Fig. 3B,C). Pores are located at the intersections of the grooves (Fig. 3,C). Therefore the pollen grains are pantoporate. Polleri irnll stratijicntiorz (Fig. 30, E). - The exine shield consists of ectexine with columellae and tectum, an extremely thin foot-layer, and a thick endexine. There is neither foot-layer nor endexine in the pores. The intine is thin and single-layered, only beneath the pores (not in the grooves) it is stratified and thickened. Iridaceae Iris members of sect. Jiirio: The average pollen diameter in the dry state is about 70 [mi, and in the hydrated state rim. The number of large, polygonal shields in I. Aircheri, I. brrchorica, I. orchioides and I. plariifolia (and also in I. caircasica, 1. kopefdagerisis and I. persicn, not shown), is about 8 (Figs. 4A,B; 5A,C.D). The shields are reticulate (except I. plnnifolia), the muri are often discontinuous and, within the lumina, isolated processes are found (Figs. 4B; 5A,B). Pollen nrall stratification (Iris 6i~lzaricn). - In the shields we find a massive tectum, columellae and a discontinuous thin foot-layer. In the grooves the exine is reduced to a fragile, discontinuous foot-layer. An endexine is generally Erine shields in angiosperm pollen 117 branched columellae, a thin foot-layer, a massive endexine, and a very thin intine. The intine is thickened beneath the pores where it is covered by a faint exine-layer (Fig. 6D). hlartyniaceae Probosciden and 16icella: the average pollen diameter ranges from c. 40 pm (in the dry state) to 55 Itm (when hydrated), (Fig. 7A,B). The circular exine shields (c. 20) are heterobrochate: the lumina are much smaller in the center of the shields than at the edge. Pollen ninll stratificofiori (I. Irrtea; Fig. 7C) [ond P. loirisinnica ssp. fragrarts]. - The shields consists of ectexine (with tectum and columellae, the latter sometimes fusing bas-ally, forming a very inconsistent foot-layer) and endexine (with small cavities and a tendency towards disruption). In the grooves the ectexine consists of granules, while the endexine is of the same thickness as in the shields. The single-layered intine does not show any thickenings. DISCUSSION Terminology Many terms for sexine patterns focus on sriinll areas and describe the arrangement of the sculpturing elements, e.g. perforate, striate, rugulate, reticulate. but also areolate (in the first sense, see below!). In contrast other terms describe the gross or macro-ornamentation of pollen grains focusing on the arrangement of lnrge exine elements (eg. lophate or fenestrate). Our pollen type shows a very typical amngelacking. The intine is thick and bi-layered throughout, the rnent of large exine plates or shields, for which we failed to outer layer is tubular, and the inner layer homogeneous (Fig. 4D,E). The pollen sculpturing of I. plariifolia differs from that of the other members of sect. Jirno investigated: the large, polygonal, isolated shields are pilate-granulate (Fig. 5D, E). RIalpighiaceae The average pollen diameter of all the Mnlpighin and Heteropteris species investigated varies from c. 30 prn (when dry) to ca. 50 [tm (in the hydrated state). The unusual appearance of this pollen type results from the irregularly formed exine shields with exine bridges and - at least in our investigated hlnlpigkin and Heteropteris species - the irregular distribution of the pores; in Banisteriopsis canipestris the shields are more regularly formed. The c. 8 shields - often of different size - are smooth at their periphery, but show a vermiculate pattern in the central region (Fig. 6). The shields of 6lnlpighia and Heferopteris pollen are not completely isolated, but linked at their ends by exine bridges, so that the grooves often do not totally surround the shields (Fig. 6F). Pores (c. 8) are located in the grooves. Pollen 1c.011 strntijicatiori (hfnlpigliin gia6rn). - The shields show a thick, finely perforated, smooth tectum, low. find an unequivocal (english) term. Although suitable descriptions exist in German literature. As early as 1837 Fritzsche used sechseckige Exine Felder (hexagonal exine areas) for Martyzia (= Ibicelln) and hfnhonin. Schulze (1971 a,h) used Exine Schildchen (small shields) for most members of Iris sect. Jirno: the pollen type of Jirrio members differs greatly from that of the other Iris subgenera in having exine shields. From this we propose cljpente = with exirze shields (srrrrorrnded by groows)). The latin word clypeirs (clipeiu) means (rounded) shield, medallion, and the adjective clypeate therefore can be applied to a pollen surface covered with f isolated (s)exine shields. Our term helps to avoid areole/areolate (with its different meaning) which is sometimes used for this pollen type or class. The use of areolate in describing pollen sculpturing is ambiguous. Areolate was originally applied to pollen grains with small areas (areolae) separated by small grooves (fossulae) forming a negative reticulum (Erdtman 1947) or with small sexinous, usually circular or polygonal areas separated by grooves f forming a negative reticulum (Erdtman 1952). Both definitions. coming from the LO- Analysis, can be interpreted in several ways and. in fact, were used for ~iru dr@erent femrres. (I) An exine is called areolate if small (s)exine regions have a typical form of Cruttn 31 (1995)

12 1 18 H. Hrrlbritter arid AI. Hesse sculpturing, i.e. a so-called rzcgntiw reticulum, seen by LM (eg Erdtman 1952: p. 21, Fig. 153B; Guinet & Ferguson 1989, Goldblatt & Le Thomas 1992,or Salgado-Labouriau et al. 1993a); a special case are the areolate lens-shaped areas of some grains of the 16-grain polyad of Zopoteca (Miniosoideae) (Guinet & Hernandez 1989). (2) Polle/i grains are sometimes termed areolate when rather large (mostly), reticulate, isolated (s)exine areas are distributed across the wliole pollen grain (Gentry & Tomb 1979, Lobreau-Callen et al. 1988, Punt 1980, 1986, 1987, Punt et al. 1993, Bretting & Nilsson 1988). Distribution of the clypeate pollen type and evolutionary considerations We have found clypeate pollen in six angiosperm families (dicots and monocots): Berberidaceae, Bignoniaceae, Euphorbiaceae, Iridaceae, Malphighiaceae, Martyniaceae. A search through relevant literature has shown that doubtless clypeate pollen also exists (but unrecognized) in Gentianaceae (the so-called Speciosrrs-type in Lisiarzthrrs, found in some Crrlolisianthru and Lisiarithzrs species, Nilsson 1970), in Xanthorroeaceae (the unipantocolpate - sic! - pollen of Baxteria arrstralis, Chanda & Ghosh 1976), and probably also in Fumariaceae (Platjcclprzos, cf. Erdtman 1952, Valdes et al. 1987). Martin Cacao & Femandez (1987) depict some Iridaceae pollen, and Xi & Zhou (1992) some other Malpighiaceae. Additional examples may also be known. In Afahoizirr aqziifoliirirz (Berberidaceae) Blackmore & Heath (1984) and Furness (1985) refer to the aperture condition (irregularly pantocolpatekyncolpate) and - simultaneously - to the specific pollen type: the apertures divide the exine into 4-8 plates, separated by fused colpi. This demonstrates that, naturally, the aperture configuration can hardly be treated in isolation from the sculpturing type, which becomes evident when looking at the question of evolution. Clypeate pollen can be derived morphologically from either 3-colpate to pantocolporate/syncolporate types within the genus Plijlkantlzrrs (cf. Lobreau-Callen et al. 1988, Meewis & Punt 1983, Punt 1972, 1980, 1986, 1987, and pers. conim.), or from sulculate types (Chanda & Ghosh.1976). Normally, the exine shields are completely isolated; but the (irregularly shaped) shields in those members of the Malpighiaceae investigated are generally anastomosing. We also call this pollen type clypeate, although it is similar to the pantocoipate/pantocoiporate types. Of course, severd pollen types exist (e.g. with some exine shields in addition to other surface elements), which cannot easily be delineated from the genuine clypeate pollen type: e.g., in PassiJlora caerirlca there are three (reticulate) large opercula called pontopercules by Spirlet (1965), which resemble exine shields separated by grooves and a syncolpate aperture condition is present. Aizcrizoize regirza (Ranunculaceae) pollen also shows some sort of exine shields (Huynh 1970, Fig. 21, and Fumess 1985) and similar examples may exist. Pollen grains with exine shields have evolved inde- pendently at the family level (at least within six angiosperm families), but also independently in several tribes of the Bignoniaceae itself: c.f. especially Gentry & Tomb (1979 for some members of Bignonieae and Tecomeae; N.B.: introducing on p. 762, in addition to areolate, camporeticulate for this pollen type, a term otherwise unknown), but also Ferguson & Santisuk (1973, for some Stereospernzrinz species), and Bove (1993, for Afarrsoa). The diverse occurrence of the clypeate pollen type represents a further, good example of convergent evolution -quite similar to the class of spiraperturate pollen grains (Furness 1985), in which form follows function. There is some evidence that, analogously to spiraperturate pollen, clypeate pollen can also be interpreted as a particular functional syndrome: a widely identical harmomegathic effect and a peculiar mode of pollen germination can be expected. Apertural condition and formation of pollen tubes The increase in pollen volume during hydration is achieved mainly by the broadening of the grooves, while the shields remain largely unchanged in their respective dimensions. This is because the variation in width of the grooves depends on the normal harmomegathic effect i.e. the degree of hydration (except in pantoporate species, see below), and not to interspecific variation as described by Bretting & Nilsson (1988). Note: Unhydrated pollen show only very narrow grooves; therefore some characters remain hidden in dry pollen grains, as is the case in dry/hydrated sulcate pollen (Halbritter & Hesse 1993). In those Iris, Crrtcllpa, IbicellalProboscicIea, and Afahonia species studied there are definitely no (morphological) apertures, and the pollen grains, therefore, resemble inaperturate or omniaperturate pollen types; Ludlow-Wiechers & Roldin- Ramos (1984) have already considered Proboscidea fragrrrrzs and Mrrrtjrzia ~liiiiiia pollen as inaperturate. The reduced exine in the grooves could function as apertures, as has already been suggested by Bretting & Nilsson (1988) for their Afarljrzia, lbicella and Proboscidea examples. Germinating pollen enlarge conspicuously, the pollen tube can arise practically everywhere and forces its way through by pushing aside the exine shields (Fig. 7D). In contrast, in various Malpighiaceae spp. (Xi & Zhou 1992, and our investigated species) and - from a functional/ontogenetic point of view - also in PIijllorirhi~~ x elorigatirs there are pores, and we face a pantoporate condition. CONCLUSION In the clypeate pollen from six angiosperm families studied here there are interesting differences between the pollen wall stratification and the apcrtural condition of Plzjllarithrrs x elongcitrrs and hlfllpiglzirr glrrbra on the one hand, and the Ibicella, Proboscidea, Iris and Cutalpa species studied on the other hand, which can be seen only by TEhl and only by Grario 3-1 (I 995)

13 Exiric sliields in nrigiospcrttr yollcti 119 comparing dry and hydrated material; in SEM-investigations of unexpanded, dry pollen the morphological differences are only slight, if visible at all. ACKNOWLEDGEMENTS We are very grateful to Dr. Gndy L. Webster for the correct nomenclature of our P/~jl/nnthcrs material (Webster, pen. comm.), and to Dr. S. Nilsson for providing us with hlartyniaceae material. \Ve also thank Dr. A. Le Thomas, Dr. I.K. Ferguson, Dr. S. Nilsson, Dr. \V. Punt and some anonymous reviewers who provided us with important comments on earlier drafts and information about rclevant literature. Thanks are also due to hlag. h1.g. Schlag for checking the English. REFERENCES Blackmore. S. & Heath, G.L.A Berberidaceae. - In: The Northwest European Pollen Flora IV (ed. \V. Punt & G.C.S. Clarke). pp Elsevier, Amsterdam. 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