Abstract
• Background and Aims Raphides are ubiquitous in the palms (Arecaceae), where they are found in roots, stems, leaves, flowers and fruits. Their occasional presence in embryos, first noticed over 100 years ago, has gone largely unexamined.
• Methods Embryos from 148 taxa of palms, the largest survey of palm embryos to date, were examined using light microscopy of squashed preparations under non‐polarized and crossed polarized light.
• Key Results Raphides were found in embryos of species from the three subfamilies Coryphoideae, Ceroxyloideae and Arecoideae. Raphides were not observed in the embryos of species of Calamoideae or Phytelephantoideae. The remaining subfamily, the monospecific Nypoideae, was not available for study.
• Conclusions Within the Coryphoideae and Ceroxyloideae, embryos with raphides were rare, but within the Arecoideae, they were a common feature of the tribes Areceae and Caryoteae.
Key words: Anatomy, Arecaceae, calcium oxalate, embryology, embryos, Palmae, raphides
INTRODUCTION
Raphides, bundles of needle‐shaped crystals of calcium oxalate monohydrate in specialized cells, are found throughout the Monocotyledonae (Franceschi and Horner, 1980; Prychid and Rudall, 1999) and are ubiquitous in the palms (Arecaceae). They are known from roots, stems, leaves, flowers and fruits (Uhl and Dransfield, 1987; Tomlinson, 1961, 1990) but occasionally turn up in unusual sites, such as anther tissue (Henderson and Rodríguez, 1999), the pseudopedicel epidermis (Barfod and Uhl, 2001) and epidermal trichome ‘sacs’ (Robertson, 1978). They were first noticed in palm embryos by Micheels (1891).
Examining material supplied by M. Treub from the Buitenzorg (now Bogor) Botanic Garden, Micheels (1891) reported the presence of raphides in the embryos of Caryota sp. and Archontophoenix alexandrae (F. Muell.) H. Wendl. (as Ptychosperma alexandra). Osenbrüg (1894) surveyed 35 taxa from throughout the family and found raphides in the embryos of 11 species. More recent embryological accounts pay little attention to raphides. Their absence was not specifically mentioned in accounts on the embryos of coconut (Haccius and Philip, 1979), Chamaerops humilis L. (Guignard, 1961), Elaeis guineensis Jacq. (Vallade, 1966) and Livistona chinensis (Jacq.) R. Br. ex Mart. (Kulkarni and Mahabale, 1974). Rao (1955) made no mention of raphides in the embryo of Areca catechu L. Shirke and Mahabale (1972) illustrated raphides in their drawings of Caryota urens L. embryos but did not remark on their significance.
Nevertheless, examining Osenbrüg’s work (Osenbrüg, 1894), one finds that the presence or absence of raphides in embryos seems to follow certain presumed evolutionary lineages within the palm family. The purpose of this survey was to determine if raphides are confined to certain lineages and if they could be used as additional evidence of phylogenetic relationships.
METHODS
Palm seeds were harvested from fruits that were mature and ready to be dispersed. Most material for this study was collected as fresh seeds from Fairchild Tropical Garden (FTG). Some seeds were collected from wild or cultivated plants elsewhere in the Miami area (see Table 1); vouchers for these samples are deposited in the herbarium of FTG. Two samples of African rattans were supplied as living seeds for propagation by Dr T. Sunderland from wild plants in Cameroon (voucher at K, the herbarium of the Royal Botanic Gardens, Kew). Some samples were collected as mature seeds preserved in glycerine–alcohol (in the spirit collection at FTG); the appearance of raphides in pickled and fresh materials was the same.
Table 1.
Raphides in palm embryos, arranged by tribe (Uhl and Dransfield, 1999)
| Taxon, accession or voucher | Raphides in embryo |
| Calameae | |
| Calamus asperrimus Blume, Osenbrüg (1894) | Absent |
| Calamus paspalanthus Becc., Chee et al. FRIM47366 | Absent |
| Calamus sp., Osenbrüg (1894) | Absent |
| Daemonorops geniculata Mart., Chee et al. FRIM47312 | Absent |
| Eremospatha wendlandiana Dammer ex Becc., Sunderland 3009 (at K) | Absent |
| Lacosperma opacum (H. Wendl. & Mann) Drude, Sunderland 3005 (at K) | Absent |
| Raphia farinifera (Gaertner) Hylander, 77222L | Absent |
| Raphia taedigera (Mrt.) Mart., Osenbrüg (1894) | Absent |
| Lepidocaryeae | |
| Lepidocaryum tenue Mart. var. gracile (Mart.) Henderson (as L. gracile Mart.), Osenbrüg (1894) | Absent |
| Mauritia sp., Osenbrüg (1894) | Absent |
| Corypheae | |
| Acoelorraphe wrightii (Griseb. & H. Wendl.) H. Wendl. ex Becc., P2313F | Absent |
| Brahea armata S. Wats., Zona s.n. | Absent |
| Chamaerops humilis L., Osenbrüg (1894) | Absent |
| Chuniophoenix nana Burret, 8178E | Absent |
| Coccothrinax argentea (Lodd. ex Schultes) Sarg. ex Becc. (as Thrinax argentea >Mart.), Osenbrüg (1894) | Absent |
| Coccothrinax borhidiana Muñiz, 85195B | Absent |
| Coccothrinax crinita Becc. ssp. brevicrinis Borhidi & Muñiz, 88659A | Absent |
| Coccothrinax cupularis (León) Muñiz & Borhidi, 87226B | Absent |
| Copernicia macroglossa H. Wendl., P1894 | Absent |
| Cryosophila stauracantha (Heynhold) R. Evans, 8089A | Absent |
| Cryosophila warscewiczii (H. Wendl.) Bartlett, RM1338D | Absent |
| Licuala ferruginea Becc., Chee et al. FRIM47356 | Absent |
| Licuala grandis H. Wendl., 79536B | Absent |
| Licuala lauterbachii Damm. & K. Schum., 93290E | Absent |
| Licuala paludosa Griff. (as L. amplifrons Miq.), Osenbrüg (1894) | Absent |
| Licuala paludosa Griff., 72522A | Absent |
| Licuala peltata Roxb. ex Buch.‐Ham. var. sumawongii L. G. Saw, 70320D | Absent |
| Licuala rumphii Blume, FG4456A | Absent |
| Livistona chinensis (Jacq.) R. Br. ex Mart., 94345B | Absent |
| Livistona endauensis J. Dransf. & K. M. Wong, Chee et al. FRIM47327 | Absent |
| Livistona muelleri F. M. Bailey, FG66762B | Absent |
| Livistona robinsoniana Becc., 8310A | Absent |
| Livistona saribus (Lour.) Merr. ex Chev., FG4220A | Present, abundant |
| Livistona woodfordii Ridl., FG591026A | Absent |
| Livistona sp., Osenbrüg (1894) | Absent |
| Pritchardia sp., 93691C | Absent |
| Rhapis excelsa (Thunb.) Henry, P682A | Present |
| Sabal minor (Jacq.) Pers. (as S. adansonii Guers.), Osenbrüg (1894) | Absent |
| Sabal minor (Jacq.) Pers., RM765C | Absent |
| Sabal yapa Wright ex Becc., 57690C | Absent |
| Schippia concolor Burret, 5759A | Absent |
| Serenoa repens (Bartram) Small, P244A | Absent |
| Serenoa repens (Bartram) Small, Zona s.n. | Absent |
| Thrinax excelsa Lodd. ex Mart., 83347E | Absent |
| Thrinax morrisii H. Wendl., RM2185E | Absent |
| Thrinax morrisii H. Wendl., 93985C | Absent |
| Thrinax radiata Lodd. ex J. A. & J. H. Schultes, P2059A | Absent |
| Trachycarpus fortunei (W. J. Hooker) H. Wendl. (as T. excelsa Wendl.), Osenbrüg (1894) | Absent |
| Washingtonia robusta H. Wendl., P339B | Absent |
| Zombia antillarum (Descourt ex Jackson) L. H. Bailey, P2665B | Sparse |
| Phoeniceae | |
| Phoenix canariensis hort. ex Chabaud, 672010C | Absent |
| Phoenix dactylifera L., Osenbrüg (1894) | Absent |
| Phoenix roebelinii O’Brien, 89800A | Absent |
| Borasseae | |
| Bismarckia nobilis Hildebrandt & H. Wend., 92251D | Absent |
| Latania lontaroides (Gaertner) H. E. Moore × L. loddigesii Mart., 75611B | Absent |
| Medemia argun (Mart.) Wuerttemberg ex H. Wendl., Osenbrüg (1894) | Absent |
| Cyclospatheae | |
| Pseudopheonix lediniana R. W. Read, 74595A | Absent |
| Pseudopheonix sargentii H. Wendl. ex Sarg., 5880A | Absent |
| Pseudopheonix sargentii H. Wendl. ex Sarg., 97608A | Absent |
| Ceroxyleae | |
| Ceroxylon alpinum Bonpl. (as C. andicola H.B.K.), Osenbrüg (1894) | Absent |
| Hyophorbeae | |
| Chamaedorea seifrizii Burret, FG4137 | Absent |
| Chamaedorea seifrizii Burret, RM1556B | Absent |
| Chamaedorea schiedeana Mart., 931048J | Absent |
| Chamaedorea tepejilote Liebm. ex Mart., 80709C | Present, abundant |
| Gaussia attenuata Becc., 79558C | Absent |
| Gaussia maya (O. F. Cook) R. Read & Quero, 7563A | Absent |
| Hyophorbe lagenicaulis (L. H. Bailey) H. E. Moore, 80509A | Absent |
| Caryoteae | |
| Arenga caudata (Lour.) H. E. Moore, 87536A | Absent |
| Arenga engleri Becc., 89925A | Absent |
| Arenga microcarpa Becc., 81598C | Absent |
| Arenga obtusifolia Mart., 591101C | Absent |
| Arenga pinnata (Wurmb) Merrill (as A. saccharifera Lab.), Osenbrüg (1894) | Absent |
| Arenga tremula Becc., DF126A | Sparse |
| Arenga tremula Becc., P802D | Sparse |
| Arenga westerhoutii Griff., Chee et al. FRIM47322 | Absent |
| Arenga sp., 80349D | Present |
| Caryota cumingii Lodd. ex Mart., 77961C | Present, abundant |
| Caryota mitis Loureiro, X‐9–34 | Present, abundant |
| Caryota mitis Loureiro (as C. sobolifera Mart.), Osenbrüg (1894) | Present |
| Caryota urens L, Osenbrüg (1894) | Present |
| Wallichia porpyrocarpa Mart., Osenbrüg (1894) | Present |
| Wallichia sp., S‐82–97 | Present, abundant |
| Areceae | |
| Adonidia merrillii (Becc.) Becc., P1163F | Sparse |
| Archontophoenix alexandrae (F. Muell.) H. Wendl. (as Ptychosperma alexandrae F. Muell.), Osenbrüg (1894) | Present |
| Areca catechu L., Osenbrüg (1894) | Present |
| Areca glandiformis Lamarck, Osenbrüg (1894) | Present |
| Areca guppyana Becc., 93294 | Present, abundant |
| Bentinckia nicobarica (Kurz) Becc., 83150C | Absent |
| Brassiophoenix drymophloeoides Burret, FG3172J | Sparse |
| Calyptrocalyx cf. hollrungii (Becc.) J. L. Dowe & M. D. Ferrero, 64140 | Present |
| Calyptrocalyx cf. polyphyllus Becc., 86331A | Sparse |
| Calyptrocalyx spicatus (Lamarck) Blume, Osenbrüg (1894) | Present |
| Carpentaria acuminata (H. Wendl. & Drude) Becc., 89213B | Absent |
| Cyrtostachys renda Blume, Chee et al. FRIM 47370 | Present, abundant |
| Clinostigma samoense H. Wendl., 79218G | Absent |
| Dictysperma album (Bory) H. Wendl. & Drude ex Scheff. var. aureum Balf. f., 88373A | Absent |
| Drymophloeus pachycladus (Burret) H. E. Moore, 65982F | Present, abundant |
| Drymophloeus subdistichus (H. E. Moore) H. E. Moore, 76396A | Present |
| Dypsis decaryi (Jum.) Beentje & J. Dransf., 92253A | Present |
| Dypsis leptocheilos (Hodel) Beentje & J. Dransf., 93324B | Present, abundant |
| Dypsis lutescens (H. Wendl.) Beentje & J. Dransf., FG4016 | Present, abundant |
| Euterpe edulis Mart., Osenbrüg (1894) | Sparse |
| Euterpe oleracea Mart., Osenbrüg (1894) | Present, abundant |
| Heterospathe elata Scheffer, 79252B | Absent |
| Heterospathe phillipsii D. Fuller & J. L. Dowe, Phillips s.n. | Absent |
| Hydriastele rostrata Burret, FG3175N | Sparse |
| Iguanura sp., 87269 | Present, abundant |
| Kentiopsis oliviformis (Brongn. & Gris.) Brongn., 77146C | Absent |
| Normanbya normanbyi (W. Hill) L.H. Bailey, 66656B | Present, abundant |
| Oenocarpus bacaba Mart., Osenbrüg (1894) | Present |
| Oenocarpus bataua Mart., Osenbrüg (1894) | Present |
| Oncosperma horridum Scheff., Chee et al. FRIM47323 | Present, abundant |
| Oncosperma tigillarium (Jack) Ridl., 6472D | Sparse |
| Orania sylvicola (Griff.) H. E. Moore (as O. macrocladus Mart.), Osenbrüg (1894) | Absent |
| Orania sp., 82434 | Absent |
| Pinanga coronata Blume, 6227G | Present, abundant |
| Pinanga coronata Blume (as P. kuhlii Blume), Osenbrüg (1894) | Present |
| Pinanga negrosensis Becc., 92160 | Present |
| Pinanga malaiana Scheff., Chee et al. FRIM47306 | Present, abundant |
| Pinanga malaiana Scheff., Chee et al. FRIM47351 | Present, abundant |
| Pinanga scortechini Becc., Chee et al. FRIM47348 | Present, abundant |
| Prestoea acuminata (Willd.) H. E. Moore var. montana (Graham) Henderson & Galeano (as Euterpe montana Grah.), Osenbrüg (1894) | Sparse |
| Ptychosperma burretiana Essig, 81592D | Absent |
| Ptychosperma caryotoides Ridl., 74263D | Sparse |
| Ptychosperma salomonense Burret, 951015E | Absent |
| Ptychosperma sp., FG4786A | Sparse |
| Rhopalostylis sapida H. Wendl. & Drude, Boyer s.n. | Absent |
| Roystonea borinquena O. F. Cook, Zona & Salzman 481 | Absent |
| Roystonea lenis León, Zona et al. 390 | Absent |
| Roystonea oleracea (Jacq.) O. F. Cook (as Oreodoxa oleracea Mart.), Osenbrüg (1894) | Absent |
| Roystonea oleracea (Jacq.) O. F. Cook, 96538A | Absent |
| Roystonea princeps Burret, 64563A | Absent |
| Roystonea regia (Kunth) O. F. Cook, 94‐1090A | Absent |
| Satakentia likiuensis (Hatusima) H. E. Moore, 68317J | Absent |
| Siphokentia beguinii Burret, 82538C | Absent |
| Solfia samoensis Rechinger, Tipama’a 1 | Sparse |
| Veitchia arecina Becc., 74116F | Absent |
| Veitchia spiralis H. Wendl., 961336G | Sparse |
| Veitchia winin H. E. Moore, Zona & Lewis 881 | Present |
| Veitchia winin H. E. Moore, 79208A | Sparse |
| Wodyetia bifurcata A. K. Irvine, 8234E | Absent |
| Wodyetia bifurcata A. K. Irvine, 93320A | Absent |
| Cocoeae | |
| Acrocomia aculeata (Jacq.) Lodd. ex Mart. (as A. sclerocarpa Mart.), Osenbrüg (1894) | Absent |
| Aiphanes minima (Gaertner) Burret, FG4040 | Absent |
| Allagoptera arenaria (Gomes) Kuntze, 64817R | Absent |
| Astrocaryum mexicanum Liebm. ex Mart., 931041 | Absent |
| Attalea sp. (as ‘Attalea guichire’), Osenbrüg (1894) | Absent |
| Bactris major Jacq., RM1583A | Absent |
| Butia capitata (Mart.) Becc. (as Cocos australis hort.), Osenbrüg (1894) | Absent |
| Cocos nucifera L., 952146A | Absent |
| Desmoncus orthocanthos Mart., 62153A | Sparse |
| Elaeis guineensis Jacq., Osenbrüg (1894) | Absent |
| Elaeis guineensis Jacq., P1959B | Absent |
| Gastrococos crispa (Kunth) H. E. Moore, 661023B | Absent |
| Syagrus amara (Jacq.) Mart., 80296D | Absent |
| Syagrus × costae Glassman, 62232D | Absent |
| Syagrus orinocensis (Spruce) Burret, 61208C | Absent |
| Syagrus schizophylla (Mart.) Glassman, 961125A | Absent |
| Syagrus sp., 59894E | Absent |
| Geonomeae | |
| Calyptronoma rivalis (O. F. Cook) L. H. Bailey, 81292A | Absent |
| Geonoma interrupta (Ruiz & Pav.) Mart. var. interrupta, Zona et al. 944 | Sparse |
| Phytelephantoideae | |
| Phytelephas macrocarpa Ruiz & Pav., Osenbrüg (1894) | Absent |
Taxa positive for raphides are in boldface type.
All accession numbers refer to plants growing at Fairchild Tropical Garden. Collector and collection number refer to voucher or spirit collections at FTG, unless otherwise indicated.
Raphide presence was qualified as ‘sparse’ when fewer than ten raphide bundles were seen in the embryo or ‘abundant’ when more than 25 bundles were observed.
Material studied by Osenbrüg (1894) is included; nomenclature for his material is updated to follow Moore (1963), Uhl and Dransfield (1987), Henderson (1995) and Henderson et al. (1995).
Seeds were dissected, and the embryos were removed. The embryo usually came away from the endosperm with little effort. It was placed in a drop of toluidine blue, squashed under a coverslip, and examined in polarized and non‐polarized light at ×100 and ×250. Raphides, when present, were usually visible in normal light, but became highly visible in polarized light. Images were captured electronically using a Pixera Professional® (Los Gatos, California) digital image capture system, using polarized light. Post‐capture processing consisted of converting RGB colour mode to greyscale (half‐tone), increasing resolution to 300 dpi, and enhancing contrast.
Raphide presence was further qualified as ‘sparse’ when fewer than ten raphide bundles were seen in the embryo. It was called ‘abundant’ when more than 25 bundles were observed in the embryo preparation. No other mineral inclusions, such as silica bodies or styloids, were observed in palm embryos.
RESULTS
The results of this survey of 148 taxa are presented in Table 1, organized by taxonomic tribes (Uhl and Dransfield, 1999), which are more likely to represent monophyletic groups. To present a more complete picture, the 35 observations of Osenbrüg (1894) are also included in Table 1, alongside 127 new observations. The new observations overlap with those of Osenbrüg for only six taxa, but in each case the new results agree with those presented by Osenbrüg (1894). Eight taxa were sampled from two different accessions, but in each case the results were concordant.
Raphides were observed in the embryos of palms from only six tribes, the Corypheae, Hyophorbeae, Caryoteae, Areceae, Cocoeae and Geonomeae. These tribes are presently classified in three subfamilies: Coryphoideae, Ceroxyloideae and Arecoideae (Uhl and Dransfield, 1999). Raphides were not observed in the following groups: Calameae and Lepidocaryeae (which together comprise the Calamoideae), Phoeniceae, Borasseae, Cyclospatheae, Ceroxyleae and Phytelephantoideae. Embryos of the Nypoideae, Iriarteeae and Podococeae were not available for examination.
Within the Corypheae, Rhapis excelsa, one species of Livistona and the monotypic genus Zombia were observed to have raphides present in their embryos. Many species of Livistona remain to be sampled, as do several recognized species of Rhapis, but the data show the trait of raphides in embryos to be variable within the genus Livistona.
Only one member of the Hyophorbeae, Chamaedorea tepejilote, was found to have raphides in its embryos, although two other species were lacking raphides.
The three genera of the Caryoteae, well known for their raphide‐rich fruits, were surprisingly diverse in the raphide content of their embryos. Most species of Arenga lacked raphides, while Caryota and Wallichia had raphides in their embryos.
Raphides were present in the embryos of 19 genera from the Areceae, including members of the subtribes Ptychospermatinae (Fig. 1), Dypsidinae, Linospadicineae, Arecinae (Fig. 2), Euterpeinae, Cyrtostachydinae, Onco spermatinae and Archontophoenicinae. One noteworthy sample from among these genera is Solfia samoensis, the sole member of its genus. Although the sizes of raphide bundles were not recorded, Solfia was unique in having small bundles, less than half the size of bundles in other palms and difficult to see without polarized light.
Fig. 1. Embryo squash of Drymophloeus pachycladus (65982F) in crossed polarized light, showing abundant raphides of calcium oxalate. Scale bar = 0·1 mm.
Fig. 2. Embryo squash of Pinanga negrosensis (92160) in crossed polarized light, showing abundant raphides of calcium oxalate. Scale bar = 0·1 mm.
Only one sample of Cocoeae was observed to have raphides in its embryo: Desmoncus orthocanthos (as D. chinatlensis Leibm. ex Mart) had approximately five raphide bundles in the embryo examined. Likewise, of the two Geonomeae sampled, one, Geonoma interrupta var. interrupta, was found to have raphides in its embryos.
DISCUSSION
Within the palm family, the systematic value of calcium oxalate crystals was thought to be nil, as raphides are ubiquitous in the vegetative organs of palms (Tomlinson, 1961, 1990); however, Osenbrüg (1894) demonstrated that raphide presence in embryos is a variable trait. The results presented here give a clearer picture of the systematic distribution of raphides in palm embryos, which are neither randomly nor evenly distributed among the 148 taxa surveyed.
Raphides were observed in three out of 42 Coryphoideae samples, an occurrence rate of 7 %. Osenbrüg (1894) did not report raphides among any of the eight genera from the Coryphoideae examined by him. Among the three tribes that comprise the Coryphoideae, only the Corypheae has members with raphides in their embryos. The three samples in which raphides were observed are from three genera – Livistona, Rhapis and Zombia – that are not believed to be closely related (Hahn, 2002).
The subfamily Ceroxyloideae—comprising the Ceroxyleae, Hyophorbeae and Cyclospatheae—is surely not monophyletic; however, its three tribes taken individually show evidence of monophyly (Lewis and Doyle, 2001; Hahn, 2002). No raphides were observed by Osenbrüg in one member of the Ceroxyleae, nor did this study find raphides in the embryos of Cyclospatheae. Raphides were found in one Chamaedorea of the Hyophorbeae; however, as the genus Chamaedorea is large and diverse additional sampling is needed before one can draw meaningful conclusions about the rate of occurrence of raphides in embryos in this Hyophorbeae.
The subfamily Arecoideae comprises six tribes, four of which were sampled in the present study. In the tribe Caryoteae, which comprises three genera, only Caryota and Wallichia appear to have abundant raphides in their embryos, and this character may help set these two genera apart from the remaining genus, Arenga. The presence of raphides in the embryos of Arenga tremula, however, is a caution flag to an otherwise clear distinction between Arenga and the other genera.
The tribe Areceae has the most species with raphides in their embryos. Of 55 taxa of Areceae reported in Table 1, 34 were found to have at least some raphides in their embryos, which is an occurrence rate of 62 %. Only two genera, Ptychosperma and Veitchia, both members of the Ptychospermatinae, had species with and without raphides in their embryos, an observation suggesting that the raphide presence might be taxonomically useful at the species level. The tribe Cocoeae, as reported by Osenbrüg (1894), lacked raphides in the embryos of its species. However, this survey found one member, Desmoncus orthocanthos, that has raphides in its embryos, albeit sparsely. One species out of 19 Cocoeae reported in Table 1 is an occurrence rate of 5 %. Only two species of Geonomeae were sampled, and one species of Geonoma was found to have raphides in its embryos. Additional sampling is desired.
There is no correlation between the presence of raphides in anthers (Henderson and Rodríguez, 1999) and raphides in embryos (this study). Thirty‐three taxa were sampled in both studies. The results of a Fisher Exact Probability test show no positive or negative association for the two conditions (P one‐tailed = 0·36773; P two‐tailed = 0·69422).
The function of raphides in embryos has not been addressed experimentally; however, there are several hypotheses for the function of raphides in plant cells, and these provide a useful framework in which to address the question in regard to palm embryos. Raphides may serve as a calcium storage depot, to be tapped as needed (Franceschi, 1989; Ilarsan et al., 1996; Ilarsan et al., 2001), and certainly, rapid seedling development and growth require an adequate supply of calcium. Raphides may also serve as a source of oxalate, which some plants secrete from their root tips to detoxify aluminium ions in the soil (Ma and Miyasaka, 1998; Ma et al., 2001). Oxalate can be broken down by oxalate oxidase into carbon dioxide and hydrogen peroxide; the latter is critical in cross‐linking cell wall polymers during cell wall extension (Lane, 1994), another process that is common in rapidly growing and developing seedlings. Raphides may also play a role in sequestering excess calcium (Franceschi, 1989; Fink, 1991; Webb, 1999), although one imagines that an embryo would be an unlikely sink for excess minerals. Finally, raphides have a known function as an anti‐herbivory defence (Ward et al., 1997; Finley, 1999; Molano‐Flores, 2001). As herbivores are deterred only after they initiate chewing and as embryos are very small and could be fatally damaged by even minor herbivory, investing embryos with raphides hardly seems like an effective defensive strategy. Palm seeds are, in general, well protected by fibres, tannins, raphides and silica bodies in the tissues of the ovary (Uhl and Moore, 1973). The most likely—but as yet untested—function for raphides in palm embryos is as storage depots for calcium, oxalate and/or hydrogen peroxide.
Regarding those embryos lacking raphides, a question that remains unanswered by this survey is, when do seedlings acquire raphides in their vegetative tissues? At some point, presumably during or immediately after germination, raphides are formed in the seedlings, where they have a presumed defensive function (Tomlinson, 1990).
Given their scattered occurrence in the family, raphides in embryos must have evolved and/or been lost many times. The presence of raphides in palm embryos follows certain phylogenetic trends and most likely has some utility as an indicator of relationships at the subtribe level or below. In certain genera within which the trait is polymorphic, such as Livistona and Ptychosperma, the presence of raphides may be a useful taxonomic character as well. The function of raphides in embryos is a topic deserving further study.
ACKNOWLEDGEMENTS
I am grateful to Angela Rosario for assistance in translating the original Osenbrüg paper. Jean‐Christophe Pintaud kindly supplied a copy of the Micheels reference. I thank Dr Jack Fisher, Dr Sawsan Khuri, Dr C. Prychid and an anonymous reviewer for their helpful comments on this paper. Dr Carl Lewis assisted in collected many of the samples and made useful comments on an early draft of this paper.
Supplementary Material
Received: 25 July 2003; Returned for revision: 25 November 2003; Accepted: 18 December 2003; Published electronically: 23 February 2004
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