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Your request to send this item has been completed. APA 6th ed. Note: Citations are based on reference standards. However, formatting rules can vary widely between applications and fields of interest or study. The specific requirements or preferences of your reviewing publisher, classroom teacher, institution or organization should be applied. The E-mail Address es field is required. Please enter recipient e-mail address es. The E-mail Address es you entered is are not in a valid format.
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Oceanography -- Periodicals. View all subjects More like this Similar Items Search this publication for other articles with the following words:. With the aim of gaining insights about this condition, we investigated the placenta in two species collected during a recent expedition to New Zealand, Treubia lacunosa Col. Prosk and Monoclea forsteri Hook. Blepharoplast morphology supports an isolated position of this group and also emphasizes important affinities with Haplomitrium Carothers and Rushing, Molecular analysis, based on a plastid DNA intron, has shown that Treubia and Apotreubia do not cluster with the jungermanniopsid or the marchantiopsid line but form a distinct clade referred to as the Treubiopsida Stech et al.
The genus Monoclea includes only two species, M. The samples were immediately transferred to the laboratory and processed for electron microscopy. Only the youngest sporophytes with unelongated setae were used. At this stage the capsules contained either dividing sporocytes or just formed tetrads in both species. In both Monoclea and Treubia the sporophyte arises subapically and has a massive seta with a relatively small foot of bulbous shape Fig. The transition between the foot and seta in Monoclea is slightly narrowed, suggesting the presence of a rudimentary collar Fig.
In both species there are several layers of cells with labyrinthine walls transfer cells on both the sporophytic and gametophytic side of the placenta Fig. In Monoclea , healthy transfer cells of sporophytic and gametophytic origin are close to each other Figs 1 C and 2A. In Treubia the two generations are separated by a wide space, including several layers of dead collapsed cells of gametophytic origin Figs 1 D and 2B. In Monoclea the foot epidermal cells have highly branched and coarse wall ingrowths that form a wall labyrinth, closely similar in complexity and morphology to the extensive wall labyrinth typical of the sporophyte in marchantialean liverworts Fig.
The wall ingrowths in sporophytic transfer cells in Treubia are relatively thin and the extension of the wall labyrinth varies from cell to cell Fig. The cytoplasm associated with wall ingrowths in Treubia contains elements of rough endoplasmic reticulum ER Fig. In both species the gametophytic cells of the placenta generally have less prominent wall labyrinths than sporophytic cells.
As is typical of placental cells in most bryophytes, the plastids are scarcely differentiated and lack Fig. The plastids in gametophyte placental cells in Treubia are always segregated in cytoplasmic areas far from the wall labyrinth not shown. A high concentration of plasmodesmata is visible in the inner tangential walls of sporophytic transfer cells as well as in the walls of more internal cells of the foot.
The plasmodesmata in Monoclea are often branched Fig. Adjacent swollen plasmodesmata or plasmodesmal branches may either merge together, thus producing prominent cavities in the inside of the walls Fig. A highly distinctive feature of gametophytic transfer cells in Monoclea is the presence of conspicuous arrays of tubular smooth ER Figs 2 A and 4A. The cytoplasm contains a great number of vesicles ranging from less than one to several micrometres in diameter.
The bigger vesicles, of irregular shape, are often seen to merge with the plasmalemma outlining the wall ingrowths Fig. The cells contain relatively few dictyosomes and abundant coated vesicles associated with partially coated reticulum Fig. The discovery of a typical marchantialean placenta in M. A marchantialean placenta also occurs in Blasia among the metzgerialean liverworts Ligrone et al. On the other hand, the placental morphology in Treubia , characterized by the presence of transfer cells with thin wall ingrowths in both generations and several layers of collapsed gametophyte cells, is strongly reminiscent of the placenta in Fossombronia and Haplomitrium Ligrone et al.
Our observations on Treubia lacunosa contrast with the report of transfer cells being restricted to the sporophyte in the allied species Apotreubia hortonae Frey and Hilger, In line with molecular evidence Stech et al. It is noteworthy that three of them, Haplomitrium , Monoclea and Blasia , share monoplastidic meiosis, a character considered to be a symplesiomorphy shared primitive feature of embryophytes and coleochaetalean algae Renzaglia et al.
In a cladistic analysis based on male gametogenesis, Treubia forms a clade with Haplomitrium Garbary et al. Other features of Monoclea , viz. The fact that all the six taxa considered above, otherwise widely divergent for a number of morphological, developmental and molecular characters, share the same basic type of placenta with transfer cells in both generations, strongly suggests that this condition has been inherited from a common ancestor and is a plesiomorphy in liverworts. It is suggested that, with the only known exception of Riccia , a highly derived genus Wheeler, , this type of placenta has been retained in the whole marchantioid lineage Ligrone et al.
In contrast, it is suggested that all other types of placenta in liverworts have arisen by reduction, i. The extreme variability of placental morphology in simple thalloid liverworts mirrors a similar diversity in sperm morphology Renzaglia and Duckett, and might suggest polyphyly, were it not for the fact that here even closely allied genera such as Riccardia and Aneura may exhibit different placental morphologies. The available evidence indicates that a different type of placenta, with transfer cells in the sporophyte only, is the plesiomorphic condition in mosses Ligrone et al.
The conclusion that liverworts and mosses do not share a plesiomorphic type of placenta is compatible with phyletic trees where these are resolved as sister groups see, for example, Garbary and Renzaglia, ; Hedderson et al. In contrast, the highly distinctive type of placenta present in the anthocerotes viz.
Figure 5 illustrates the distribution and possible interrelationships of four basic types of placenta in the major bryophyte groups. Based on this assumption, the anthocerotes are placed in a basal position, a situation consistent with other recent analyses based on morphological, biochemical and molecular data, not including placental morphology Garbary and Renzaglia, ; Hedderson et al. Frey et al. Goffinet, Types 1, 3 and 4, which occur in the Metz geriidae, are also considered to be derived, directly or indirectly, from type 2.
The clustering of the Tetraphidales viz. Tetraphis , Buxbaumia and Diphyscium with the Bryopsida, instead of the Polytrichales cf. Hedderson et al. However, the hypobasal origin of the foot in this group Roth, , contrasting with the epibasal origin in the other mosses, calls into question the homology of the sphagnalean placenta with other types of placenta Ligrone et al.
A hypobasal origin is also assumed by some authors for example, Schuster, for the sporophyte foot and seta in the Sphaerocarpales and Marchantiales. Whether functional differences result from the morphological variability of the placenta in bryophytes is not known. A placenta with transfer cells in both generations occurs both in taxa with an elongated seta, such as Haplomitrium , Blasia and Monoclea , and in those with a short seta and sporophytes on archegoniophores, i.
Reduction of the sporophyte apparently involved the disappearance of a foot and placental transfer cells in Riccia Ligrone et al. On the other hand, transfer cells are also lacking in certain metzgerialean liverworts e. Pellia and Cryptothallus with exceedingly large sporophytes Ligrone et al. We can conclude that the lack of transfer cells from one or both sides of the placenta does not necessarily imply a lower efficiency in terms of nutrient translocation; rather this suggests that hitherto unrecognized adjustments in the mechanism of nutrient translocation might have compensated for, or even triggered, the disappearance of transfer cells in certain lineages.
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Further analysis based on comparative data on fern placenta is presented in Duckett and Ligrone It is pertinent to note that the development of a wall labyrinth in cultured cells of maize was found to involve no major change in wall composition DeWitt et al. The presence of labyrinthine walls in collapsed gametophyte cells in Treubia indicates that these are placental transfer cells crushed down by the expanding foot.
Collapsed gametophyte cells occurring in the placental space is a typical feature of the placenta in bryophytes, although not present in certain taxa, including Monoclea , and it was suggested to be a major character distinguishing the bryophyte lineage from tracheophytes Frey et al. In addition to Treubia , sporophytic mitochondria larger than their gametophytic counterpart have been reported in Calobryum blumei Nees [syn.
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Haplomitrium blumii Nees Schuster] Ligrone et al. Branched plasmodesmata are common in higher plants, notably in vascular tissues Lucas et al. Intercisternal proteinaceous bridges are a normal feature of the Golgi system both in plant and animal cells and have been shown to be responsible for the stability of dictyosomal stacks Cluett and Brown, This study confirms placental morphology as a valuable character for defining the phylogeny and interrelationships of bryophytes and lower tracheophytes.
Arrows indicate a rudimentary collar in the foot in Monoclea. C and D, Details of the placental region showing cells with labyrinthine walls arrows in both the sporophyte S and the gametophyte G.
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Several layers of collapsed gametophyte cells asterisk are interposed between the foot and living gametophyte placental cells. Transmission electron microscope images of the placenta in Monoclea A and Treubia B , showing transfer cells in the sporophyte S and gametophyte G. Arrows in A point to arrays of smooth endoplasmic reticulum. The wall ingrowths in Treubia are much thinner than those in Monoclea. A—D, Transmission electron microscope images of placental transfer cells in Monoclea and Treubia. A, Elements of endoplasmic reticulum arrows are visible in the narrow cytoplasmic channels between wall ingrowths in sporophyte cells in Treubia.
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B, Plastid P in sporophyte cell in Monoclea. C, Plastids P and mitochondria M in sporophyte cell in Treubia. D, Mitochondria M in gametophyte cell in Treubia.