The placenta in Monoclea forsteri Hook. and Treubia lacunosa (Col.) Prosk: insights into placental evolution in liverworts.

Placental morphology is remarkably diverse between major bryophyte groups, especially with regard to the presence and distribution of transfer cells in the sporophyte and gametophyte. In contrast, with the exception of metzgerialean liverworts, placental morphology is highly conserved within major bryophyte groups. Here we examine the ultrastructure of the placenta in Monoclea forsteri and Treubia lacunosa, basal members of the marchantialean and metzgerialean liverwort lineages, respectively. In both species several layers of transfer cells are found on both sides of the placenta, with sporophytic transfer cells exhibiting prominent wall labyrinths. Consistent with previous reports of a similar placenta in other putatively basal and isolated liverwort genera such as Fossombronia, Haplomitrium, Blasia and Sphaerocarpos, this finding suggests that this type of placenta represents the plesiomorphic (primitive) condition in liverworts. Distinctive ultrastructural features of placental cells in Monoclea include branched plasmodesmata in the sporophyte and prominent arrays of smooth endoplasmic reticulum, seemingly active in secretion in the gametophyte. These arrays contain a core of narrow tubules interconnected by electron-opaque rods, structures with no precedent in plants. Analysis of the distribution of different types of placenta in major bryophyte groups provides valuable insights into their inter-relationships and possible phylogeny.

Conducting tissues and phyletic relationships of bryophytes.

Internal specialized conducting tissues, if present, are restricted to the gametophytic generation in liverworts while they may occur in both generations in mosses. Conducting tissues are unknown in the anthocerotes. Water-conducting cells (WCCs) with walls perforated by plasmodesma-derived pores occur in the Calobryales and Pallaviciniaceae (Metzgeriales among liverworts and in Takakia among mosses. Imperforate WCCs (hydroids) are present in bryoid mosses. A polarized cytoplasmic organization and a distinctive axial system of microtubules is present in the highly specialized food-conducting cells of polytrichaceous mosses (leptoids) and in less specialized parenchyma cells of the leafy stem and seta in other mosses including Sphagnumn. A similar organization, suggested to reflect specialization in long-distance symplasmic transport of nutrients, also occurs in other parts of the plant in mosses, including rhizoids and caulonemata, and may be observed in thallus parenchyma cells of liverworts. Perforate WCCs in the Calobryales, Metzgeriales and Takakia, and hydroids in bryoid mosses, probably evolved independently Because of fundamental differences in developmental design, homology of any of these cells with tracheids is highly unlikely. Likewise, putative food-conducting of bryophytes present highly distinctive characteristics and cannot be considered homologous with the sieve cells of tracheophytes.

The sporophyte-gametophyte junction in the hornwort, Dendroceros tubercularis Hatt (Anthocerotophyta).

The placenta of the anthocerote, Dendroceros tubercularis Hatt., consists of long and branched haustorial cells, that arise from the foot and gametophyte transfer cells. Both cell types contain electron-dense vacuolar deposits that were digested by pronase and therefore are assumed to be protein. These deposits were negative to the PATAg test for carbohydrates. Protein bodies were also found in the parenchyma cells of the foot and younger meristematic cells at the base of the capsule. Vacuolar deposits of osmiophilic material in the gametophyte cells external to the placenta were stained non-specifically with PATAg method and were not affected by pronase. The haustorial cells have pleomorphic plastids lacking starch and a thylakoid system, whereas the transfer cells have well developed chloroplasts. No pronase-sensitive material was detected in the apo plastic space separating gametophyte and sporophyte cells. These results suggest that protein is synthesized in the haustorial cells, perhaps from precursors provided by transfer cells, and is then transferred, via plasmodesmata, to the parenchyma cells of the foot and eventually to the cells of the growing capsule.

Cytology and development of a mycorrhiza-like infection in the gametophyte of Conocephalum conicum (L.) Dum. (Marchantiales, Hepatophyta).

An aseptate fungus associated with the gametophyte of the hepatic Conocephalum conicum (L.) Dum. was studied by light and electron microscopy. The fungus forms a highly branched mycelium external to the plant. Fungal hyphae colonize the smooth-walled rhizoids, through which they pass into the gametophyte parenchyma of the midrib. The fungus in the parenchyma is entirely intracellular. Vesicles are found in rhizoids and a few ventral layers of parenchyma cells. Prominent arbuscules develop in more internal cells from lateral branches of infecting hyphae that spread from cell to cell. Infected host cells show cytoplasmic proliferation, especially of ribosomes, Plastids and mitochondria, The arbuscules eventually degenerate leaving clumps of collapsed hyphae. Reinfection of cells with degenerate arbuscules was not observed. The ultrastructural and developmental characteristics of the fungus and its high cytological compatibility with the host are indicative of a well integrated symbiotic association similar to the vesicular-arbuscular mycorrhizas of higher plants.

The ultrastructure of the placenta in Sphagnum.

The placenta of two Sphagnum species was examined by electron microscopy. In contrast to all mosses so far investigated, neither sporophyte nor gametophyte placental cells of Sphagnum develop wall ingrowths. The sporophyte cells are highly vacuolate and the gametophyte cells close to them degenerate to produce a system of spaces filled with mucilage. Whether this type of placenta represents a primitive or derived condition in mosses is discussed.

The development of the placenta in the anthocerote Phaeoceros laevis (L.) Prosk.

The development of the placenta in the anthocerote Phaeoceros laevis (L.) Prosk. was studied by transmission electron microscopy. By the time the sporophyte emerges from the involucre, a conspicuous placental region is formed by the intrusive growth of sporophyte foot haustorial cells into the adjacent gametophyte vaginula tissue. The separation of gametophyte cells by haustorial cells and their incorporation into the placenta are preceded by the loosening and swelling of their walls and the formation of a periplasmic space. This process causes the disruption of the plasmodesmata, and may eventually result in the complete isolation and consequent degeneration of the cells. Crystals are commonly observed in the vacuoles of gametophyte placental cells. Crystals become more abundant during cytoplasmic degeneration, and are released in the placental lacunae that result from the complete dissolution of gametophyte cells. During the subsequent phase of capsule elongation, the gametophyte placental cells that retain the symplastic connection with the adjoining gametophyte parenchyma develop a wall labyrinth typical of transfer cells. Obliteration of the wall labyrinth by deposition of lightly staining wall material is observed later in sporophyte development, in concomitance with capsule dehiscence. Crystals are negative to the periodic acid/thiocarbohydrazide/silver proteinate test for carbohydrates whilst they are completely digested by pepsin or protease, denoting protein composition.

Ultrastructure of the sporophyte foot-gametophyte vaginula complex inTimmiella barbuloides (Brid.) Moenk.

The sporophyte foot of the mossTimmiella barbuloides consists of an unistratose epidermis of transfer cells, a parenchymatous cortex, and a small central strand consisting only of hydroids. The parenchymatous tissue of the vaginula develops one layer of transfer cells opposite the foot, whose lower extremity extends into the gametophyte stem's central strand. From the bottom to the top of the foot the ultrastructure of the sporophyte transfer cells shows some gradual changes that appear related to a functional specialization of these cells. According to a centripetal gradient, the quantity of plastid starch progressively lessens in both vaginula parenchyma and foot cortex. the observed morphological patterns suggest that in the foot-vaginula complex nutrients are translocated radially up to the sporophyte central strand.