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Brief Communications |
2Freie Universität Berlin, Fachbereich Geowissenschaften, Fachrichtung Paläontologie Malteser Str. 74-100, Haus D, D-122 49 Berlin, Germany; 3Freie Universität Berlin, Institut für Biologie—Systematische Botanik und Pflanzengeographie Altensteinstraße 6, D-141 95 Berlin, Germany
Received for publication October 22, 2002. Accepted for publication January 16, 2003.
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ABSTRACT |
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 TOP ABSTRACT LITERATURE CITED |
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Key Words: Boraginales • Ehretia • Europe • London Clay flora • Scanning electron microscopy (SEM) • Tertiary • transfer cells • ultrastructure
Transfer cells are specialized plant cells that improve the short-distance transport of soluble substances, especially across tissue boundaries. Transport is optimized by a surface extension of the plasma membrane on numerous cell wall ingrowths in the form of pins or nets or spongiose structures, which are easy to identify under scanning electron microscopy (SEM) or transmission electron microscopy (TEM) and are even visible under high resolution light microscopy (LM). Transfer cells are highly polar in their organization; the characteristic ingrowths usually develop on one face of the cell only.
Since their first general description (Gunning and Pate, 1969
), transfer cells have been observed in various tissues of embryophytes, e.g., the xylem and phloem, in haustoria of parasitic plants, in seeds, and in the placental region connecting the gametophyte and the sporophyte of embryophytes (Graham et al., 2000
; Frey et al., 2001
; Thompson et al., 2001
). The widespread presence of transfer cells underscores the importance of these cells in numerous structural and physiological contexts. Graham et al. (2000)
consider the development of placental transfer cells as apomorphy of embryophytes. However, the occurrence of transfer cells in different organs suggests their development several times independently in various plant groups.
Based on the occurrence of transfer cells in basal land plants such as mosses and ferns, they may have arisen early in plant evolution (almost in the Silurian: Frey et al., 2001
), possibly as a structure to bridge tissue boundaries required by living in a nonaqueous environment. However, no fossil evidence of those cells had been reported for any plant group.
Transfer cells form the seed coat and are also found along the funicle in Cordiaceae, Ehretiaceae, Heliotropiaceae, and some Hydrophyllaceae sensu strictu (Figs. 3, 5), but not in Boraginaceae sensu strictu. In the course of a larger survey on Boraginales (Diane et al., 2002
) we also investigated Ehretia fossils (endocarp remnants, Figs. 1, 2, 4) from the London Clay flora deposited in the Natural History Museum (London, UK). We clearly observed transfer cells in the fossilized seed coat of Ehretia clausentia Chandler (NHM PV.34572; Ehretiaceae, Boraginales; Chandler, 1961
, 1964
) from the Lower Eocene (about 50 million years old) of southern England (Figs. 6, 7). In position and structure, they are indistinguishable from those of extant species (e.g., Ehretia macrophylla Wall., Fig. 8).
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, 1964
; Collinson, 1983
). While the typical London Clay flora is mainly pyritic, the material from Nursling (locality of the investigated fossils of Ehretia clausentia) is almost entirely carbonaceous (Collinson, 1983
). The importance of the London Clay flora is underscored by the evidence presented here on the exceptional preservation of intracellular detail. In this particular case, the excellent condition is probably also due in part to the hard endocarp of Ehretia clausentia, which protected the seed coat before and during fossilization.
Precise systematic placement of fossils (especially plant fossils) is often difficult, or impossible, especially when unambiguous cellular characters are lost during fossilization. In this case, however, presence, position, and structure of the fossil transfer cells lend additional support to the systematic position of Ehretia clausentia in the Ehretia II subclade (Gottschling et al., 2002
). The fossil transfer cells together with the formerly multilayered, sclerenchymatic endocarp suggest that Ehretia clausentia is a representative of the Primarily Woody Boraginales (Cordiaceae, Ehretiaceae, Heliotropiaceae, also including parasitic Lennoaceae) that are closely allied based on both morphological (Diane et al., 2002
) and molecular data (Gottschling et al., 2001
).
The possible functions of transfer cells have been discussed recently (Frey et al., 2001
; Thompson et al., 2001
; Diane et al., 2002
). Nutrient transfer, herbivore deterrence, and water uptake and transfer have all been proposed. In the Primarily Woody Boraginales, the seeds are protected by a hard and multilayered endocarp (Figs. 3, 5). The seed coat of transfer cells may act here as a wick, ensuring rapid water uptake via the funicle (Diane et al., 2002
): the swelling seed coat and embryo then rupture the hard endocarp along preformed lines of structural weakness.
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FOOTNOTES |
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4 Author for reprint requests (hahilger{at}zedat.fu-berlin.de
)
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LITERATURE CITED |
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TOPABSTRACTLITERATURE CITED |
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Chandler M. E. J. 1964 The Lower Tertiary Floras of southern England 4: a summary and survey of findings in the light of recent botanical observations. British Museum, London, UK
Collinson M. E. 1983 Fossil plants of the London Clay. Palaeontological Association, London, UK
Diane N. H. H. Hilger M. Gottschling 2002 Transfer cells in the seeds of Boraginales. Botanical Journal of the Linnean Society 140: 155-164[CrossRef]
Frey W. M. Hofmann H. H. Hilger 2001 The gametophyte–sporophyte junction: unequivocal hints for two evolutionary lines of archegoniate land plants. Flora 196: 431-445[ISI]
Gottschling M. H. H. Hilger M. Wolf N. Diane 2001 Secondary structure of the ITS1 transcript and its application in a reconstruction of the phylogeny of Boraginales. Plant Biology 3: 629-636[CrossRef][ISI]
Gottschling M. D. H. Mai H. H. Hilger 2002 The systematic position of Ehretia fossils (Ehretiaceae, Boraginales) from the European Tertiary and implications for character evolution. Review of Palaeobotany and Palynology 121: 149-156[CrossRef]
Graham L. E. M. E. Cook J. S. Busse 2000 The origin of plants: body plan changes contributing to a major evolutionary radiation. Proceedings of the National Academy of Science 97: 4535-4540
Gunning B. E. S. J. S. Pate 1969 ‘Transfer cells’. Plant cells with wall ingrowths, specialized in relation to short distance transport of solutes—Their occurrence, structure, and development. . Protoplasma 68: 107-133[CrossRef][ISI]
Thompson R. D. G. Hueros H. A. Becker M. Maitz 2001 Development and functions of seed transfer cells. Plant Science 160: 775-783
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