Early evolution of life cycles in embryophytes: A focus on the fossil evidence of gametophyte/sporophyte size and morphological complexity

Abstract Embryophytes (land plants) are distinguished from their green algal ancestors by diplobiontic life cycles, that is, alternation of multicellular gametophytic and sporophytic generations. The bryophyte sporophyte is small and matrotrophic on the dominant gametophyte; extant vascular plants have an independent, dominant sporophyte and a reduced gametophyte. The elaboration of the diplobiontic life cycle in embryophytes has been thoroughly discussed within the context of the Antithetic and the Homologous Theories. The Antithetic Theory proposes a green algal ancestor with a gametophyte‐dominant haplobiontic life cycle. The Homologous Theory suggests a green algal ancestor with alternation of isomorphic generations. The shifts that led from haplobiontic to diplobiontic life cycles and from gametophytic to sporophytic dominance are most probably related with terrestrial habitats. Cladistic studies strongly support the Antithetic Theory in repeatedly identifying charophycean green algae as the closest relatives of land plants. In recent years, exceptionally well‐preserved axial gametophytes have been described from the Rhynie chert (Lower Devonian, 410 Ma), and the complete life cycle of several Rhynie chert plants has been reconstructed. All show an alternation of more or less isomorphic generations, which is currently accepted as the plesiomorphic condition among all early polysporangiophytes, including basal tracheophytes. Here we review the existing evidence for early embryophyte gametophytes. We also discuss some recently discovered plants preserved as compression fossils and interpreted as gametophytes. All the fossil evidence supports the Antithetic Theory and indicates that the gametophytic generation/sporophytic generation size and complexity ratios show a gradual decrease along the land plant phylogenetic tree.     

Early evolution of life cycles in embryophytes:A focus on the fossil evidence of gametophyte/sporophyte size and morphological complexity

Embryophytes (land plants) are distinguished from their green algal ancestors by diplobiontic life cycles,that is,alternation of multicellular gametophytic and sporophytic generations.The bryophyte sporophyte is small and matrotrophic on the dominant gametophyte; extant vascular plants have an independent,dominant sporophyte and a reduced gametophyte.The elaboration of the diplobiontic life cycle in embryophytes has been thoroughly discussed within the context of the Antithetic and the Homologous Theories.The Antithetic Theory proposes a green algal ancestor with a gametophyte-dominant haplobiontic life cycle.The Homologous Theory suggests a green algal ancestor with alternation of isomorphic generations.The shifts that led from haplobiontic to diplobiontic life cycles and from gametophytic to sporophytic dominance are most probably related with terrestrial habitats.Cladistic studies strongly support the Antithetic Theory in repeatedly identifying charophycean green algae as the closest relatives of land plants.In recent years,exceptionally well-preserved axial gametophytes have been described from the Rhynie chert (Lower Devonian,410 Ma),and the complete life cycle of several Rhynie chert plants has been reconstructed.All show an alternation of more or less isomorphic generations,which is currently accepted as the plesiomorphic condition among all early polysporangiophytes,including basal tracheophytes.Here we review the existing evidence for early embryophyte gametophytes.We also discuss some recently discovered plants preserved as compression fossils and interpreted as gametophytes.All the fossil evidence supports the Antithetic Theory and indicates that the gametophytic generation/sporophytic generation size and complexity ratios show a gradual decrease along the land plant phylogenetic tree.     

The origin of alternation of generations in land plants: a focus on matrotrophy and hexose transport

A life history involving alternation of two developmentally associated, multicellular generations (sporophyte and gametophyte) is an autapomorphy of embryophytes (bryophytesphytes + vascular plants). Microfossil data indicate that Mid Late Ordovician land plants possessed such a life cycle, and that the origin of alternation of generations preceded this date. Molecular phylogenetic data unambiguously relate charophycean green algae to the ancestry of monophyletic embryophytes, and identify bryophytes as early-divergent land plants. Comparison of reproduction in charophyceans and bryophytes suggests that the following stages occurred during evolutionary origin of embryophytic alternation of generations: (i) origin of oogamy; (ii) retention of eggs and zygotes on the parental thallus; (iii) origin of matrotrophy (regulated transfer of nutritional and morphogenetic solutes from parental cells to the next generation); (iv) origin of a multicellular sporophyte generation; and (v) origin of non-flagellate, walled spores. Oogamy, egg/zygote retention and matrotrophy characterize at least some modern charophvceans, and are postulated to represent pre-adaptative features inherited by embryophytes from ancestral charophyceans. Matrotrophy is hypothesized to have preceded origin of the multicellular sporophytes of' plants, and to represent a critical innovation. Molecular approaches to the study of the origins of matrotrophy include assessment of hexose transporter genes and protein family members and their expression patterns. The occurrence in modern charophyceans and bryophytes of chemically resistant tissues that exhibit distinctive morphology correlated with matrotrophy suggests that Early-Mid Ordovician or older microfossils relevant to the origin of land plant alternation of generations may be found.     

Evolution of land plants: insights from molecular studies on basal lineages

The invasion of the land by plants, or terrestrialization, was one of the most critical events in the history of the Earth. The evolution of land plants included significant transformations in body plans: the emergence of a multicellular diploid sporophyte, transition from gametophyte-dominant to sporophyte-dominant life histories, and development of many specialized tissues and organs, such as stomata, vascular tissues, roots, leaves, seeds, and flowers. Recent advances in molecular genetics in two model basal plants, bryophytes Physcomitrella patens and Marchantia polymorpha, have begun to provide answers to several key questions regarding land plant evolution. This paper discusses the evolution of the genes and regulatory mechanisms that helped drive such significant morphological innovations among land-based plants.     

Homologous Versus Antithetic Alternation of Generations and the Origin of Sporophytes

The late-nineteenth/early-twentieth century debate over homologous versus antithetic alternation of generations is reviewed. Supporters of both theories, at first, used Coleochaete as a model for the origin of land-plant life cycles. The early debate focused on the morphological interpretation of the sporophyte and on whether vascular cryptogams had bryophyte-like ancestors. The terms of the debate shifted after the discovery that the alternation of morphological generations was accompanied by an alternation of chromosome number. Supporters of homologous alternation now promoted a model in which land plants had been derived from an algal ancestor with an isomorphic alternation of haploid and diploid generations whereas supporters of antithetic alternation favored a model in which land plants were derived from a haploid algal ancestor with zygotic meiosis. Modern evidence that embryophytes are derived from charophycean green algae is more compatible with an updated version of the antithetic theory.     

ALTERNATION OF GENERATIONS IN LAND PLANTS: NEW PHYLOGENETIC AND PALAEOBOTANICAL EVIDENCE

Current ideas on the evolution of alternation of generations in land plants are reviewed in the context of important recent advances in plant systematics and the discovery of remarkable new palaeobotanical evidence on early embryophyte life cycles. An overview of relationships in major groups of green plants is presented together with a brief review of the early fossil record as a prelude to discussing hypotheses of life cycle evolution. Recent discoveries of life cycles in the early fossil record are described and assessed. The newly discovered gametophyte and sporophyte associations are based on exceptionally well-preserved material from the Rhynie Chert, Scotland (Middle Devonian: 380–408 Myr) and compression fossils from other Devonian localities. These data document diplobiontic life cycles in plants at the ‘protracheophyte’ and early tracheophyte level of organization. Furthermore, the early fossils have a more or less isomorphic alternation of generations, a striking departure from life cycles in extant embryophytes. This unexpected similarity between gametophyte and sporophyte calls for a cautious approach in identifying ploidy level in early groups. Viewed in a systematic context, the neontological and palaeontological data contribute towards the formulation of a coherent hypothesis of life cycle evolution in major, early embryophyte groups. Evidence from extant groups strongly supports a single direct origin of the diplobiontic life cycles of land plants from haploid, haplobiontic life cycles in ancestral ‘charophycean algae’. The interest of the new palaeobotanical data lies in its relevance to life cycle evolution at the restricted level of vascular plants rather than at the more general level of embryophytes (vascular plants plus ‘bryophytes’). The occurrence of morphologically complex, axial gametophytes in early vascular plants is consistent with the moss sister-group proposed in some cladistic analyses. Similarities of moss gametophytes to fossils in the vascular plant stem-group are discussed, and it is argued that the late appearance of mosses in the macrofossil record may be due to the problem of recognizing stem-group taxa. The new palaeobotanical evidence conflicts with previous hypotheses based on extant groups that interpret morphological simplicity as the plesiomorphic condition in the gametophytes of vascular plants. These new data indicate that a significant elaboration of both gametophyte and sporophyte occurred early in the tracheophyte lineage, and that the gametophytes of extant ‘pteridophytes’ are highly reduced compared to those of some of the earliest ‘protracheophytes’. Vestiges of this early morphological complexity may remain in the gametophytes of some extant groups such as Lycopodiaceae.     

Alternation of generations – unravelling the underlying molecular mechanism of a 165‐year‐old botanical observation

Characteristically, land plants exhibit a life cycle with an ‘alternation of generations’ and thus alternate between a haploid gametophyte and a diploid sporophyte. At meiosis and fertilisation the transitions between these two ontogenies take place in distinct single stem cells. The evolutionary invention of an embryo, and thus an upright multicellular sporophyte, in the ancestor of land plants formed the basis for the evolution of increasingly complex plant morphologies shaping Earth's ecosystems. Recent research employing the moss Physcomitrella patens revealed the homeotic gene BELL1 as a master regulator of the gametophyte‐to‐sporophyte transition. Here, we discuss these findings in the context of classical botanical observations.     

Ultrastructural studies of plasmodesmatal and vascular translocation of tobacco rattle virus (TRV) in tobacco and potato

The studies focus on an ultrastructural analysis of the phenomenon of intercellular and systemic (vascular) transport of tobacco rattle virus (TRV) in tissues of the infected plants. TRV is a dangerous pathogen of cultivated and ornamental plants due to its wide range of plant hosts and continuous transmission by vectors—ectoparasitic nematodes. Two weeks after infection with the PSG strain of TRV, tobacco plants of the Samsun variety and potato plants of the Glada variety responded with spot surface necroses on inoculated leaf blades. Four weeks after the infection a typical systemic response was observed on tobacco and potato leaves, necroses on stems and lesions referred to as corky ringspot. Ultrastructural analysis revealed the presence of two types of TRV virions: capsidated and non-capsidated forms in tobacco and potato tissues. In the protoplast area, viral particles either occurred in a dispersed form or they formed organised inclusions of virions. We demonstrated for the first time the presence of non-capsidated-type TRV in the vicinity of and inside plasmodesmata. Capsidated particles of TRV were observed in intercellular spaces of the tissues of aboveground and underground organs. Expanded apoplast area was noted at the cell wall, with numerous dispersed non-capsidated-type TRV particles. These phenomena suggest active intercellular transport. Our ultrastructure studies showed for the first time that xylem can be a possible route of TRV systemic transport. We demonstrated that both capsidated and non-capsidated virions, of varied length, participate in long-distance transport. TRV virions were more often documented in xylem (tracheary elements and parenchyma) than in phloem. Non-capsidated TRV particles were observed inside tracheary elements in a dispersed form and in regular arrangements in potato and tobacco xylem. The presence of TRV virions inside the bordered pits was demonstrated in aboveground organs and in the root of the tested plants. We documented that both forms of TRV virions can be transported systemically via tracheary elements of xylem.     

A fundamental plant evolutionary problem: the origin of land-plant sporophyte; is a new hypothesis possible?

The origin of the sporophyte in land plants represents a fundamental phase in the plant evolution. Today this subject is controversial and, in my opinion, scarcely considered in our textbooks and journals of botany, in spite of its importance. There are two conflicting theories concerning the origin of the alternating generations in land plants: the "antithetic" and the "homologous" theory. These have never been fully resolved. The antithetic theory maintains that the sporophyte and gametophyte generations are fundamentally dissimilar and that the sporophyte originated in an ancestor organism with haplontic cycle by the zygote dividing mitotically rather than meiotically, and with a developmental pattern not copying the developmental events of the gametophyte. The sporophyte generation was an innovation of critical significance for the land-plant evolution. By contrast, the homologous theory simply stated that a mass of cells forming mitotically from the zygote adopted the same developmental plan of the gametophyte, but giving origin to a diploid sporophyte. In this context, a very important question concerns the possible ancestor or ancestors of the land plants. Considerable evidences at morphological, cytological, ultrastructural, biochemical and, especially, molecular level, strongly suggest that the land plants or Embryophyta (both vascular and non-vascular) evolved from green algal ancestor(s), similar to those belonging to the genus Coleochaete, Chara and Nitella, living today. Their organism is haploid for most of their life cycle, and diploid only in the zygote phase (haplontic cycle). On the contrary, the land plants are characterized by a diplo-haplontic life cycle. Several questions are implied in these theories, and numerous problems remain to be solved, such as, for example, the morphological difference between gametophyte and sporophyte (heteromorphism, already present in the first land plants, the bryophytes), and the strong gap existing between these last with a sporophyte dependent on the gametophyte, and the pteridophytes having the gametophyte and sporophyte generations independent. On the ground of all of the evidences on the ancestors of the land plants, the antithetic theory is considered more plausible than the homologous theory. Unfortunately, no phylogenetic relationship exists between some green algae with diplontic life cycle and the land plants. Otherwise, perhaps, it should be possible to hypothesize another scenario in which to place the origin of the alternating generations of the land plants. In this case, could the gametophyte be formed by gametes produced from the sporophyte, through their mitoses or a delayed fertilization process?     

Endophytic cyanobacteria in a 400-million-yr-old land plant: A scenario for the origin of a symbiosis?

Direct evidence for the origin and evolution of land plant/cyanobacterial symbioses is virtually absent from the fossil record. Here we report on rare occurrences of prostrate mycorrhizal axes of the Early Devonian land plant Aglaophyton major that host a filamentous cyanobacterium, which enters the plant through the stomata and colonizes the substomatal chambers and intercellular spaces in the outer cortex. In dead ends of the intercellular system, the filaments form loops and continue growth in reverse direction. Some filaments penetrate parenchyma cells close to and within the mycorrhizal arbuscule-zone and form intracellular coils. This discovery represents the earliest direct evidence for cyanobacteria growing inside land plants, and offers a model for the types of associations that may have preceded the evolution of mutualistic land plant/cyanobacterial symbioses.     

Identification of high levels of phytochelatins, glutathione and cadmium in the phloem sap of Brassica napus. A role for thiol-peptides in the long-distance transport of cadmium and the effect of cadmium on iron translocation

Phytochelatins (PCs) are glutathione-derived peptides that function in heavy metal detoxification in plants and certain fungi. Recent research in Arabidopsis has shown that PCs undergo long-distance transport between roots and shoots. However, it remains unknown which tissues or vascular systems, xylem or phloem, mediate PC translocation and whether PC transport contributes to physiologically relevant long-distance transport of cadmium (Cd) between shoots and roots. To address these questions, xylem and phloem sap were obtained from Brassica napus to quantitatively analyze which thiol species are present in response to Cd exposure. High levels of PCs were identified in the phloem sap within 24 h of Cd exposure using combined mass spectrometry and fluorescence HPLC analyses. Unexpectedly, the concentration of Cd was more than four-fold higher in phloem sap compared to xylem sap. Cadmium exposure dramatically decreased iron levels in xylem and phloem sap whereas other essential heavy metals such as zinc and manganese remained unchanged. Data suggest that Cd inhibits vascular loading of iron but not nicotianamine. The high ratios [PCs]/[Cd] and [glutathione]/[Cd] in the phloem sap suggest that PCs and glutathione (GSH) can function as long-distance carriers of Cd. In contrast, only traces of PCs were detected in xylem sap. Our results suggest that, in addition to directional xylem Cd transport, the phloem is a major vascular system for long-distance source to sink transport of Cd as PC–Cd and glutathione–Cd complexes.     

A cladistic approach to the phylogeny of the “Bryophytes”

The importance of a cladistic approach in reconstructing the phylogeny of bryophytes is discussed and illustrated by an analysis of the major groups of bryophytes with respect to the tracheophytes and the green algae. The cladistic analysis, using 51 characters taken from the literature, gives the following tentative results: (1) the embryophytes as a whole are monophyletic; (2) the bryophytes (sensu lato) are paraphyletic; (3) the mosses share a more recent common ancestor with the tracheophytes than do the liverworts or hornworts; (4) the hornworts appear to share a more recent common ancestor with the moss-tracheophyte lineage than with the liverworts; however, the existence of several homoplasies makes this placement more problematical; (5) the origin of alternation of generations in the embryophytes, based on out-group comparison with their oogamous, haplontic, algal sister groups, was by progressive elaboration of the primitively epiphytic sporophyte generation; and (6) the presence of vascular tissue (xylem and phloem) can best be interpreted as a synapomorphy of the moss-tracheophyte clade, and tracheids (xylem with ornamented walls) as a synapomorphy of the tracheophytes; therefore, the prevailing designation of “vascular plants” for the tracheophytes alone is inaccurate.     

Phylogenetic analyses of morphological evolution in the gametophyte and sporophyte generations of the moss order Hookeriales (Bryopsida)

Morphological characters from the gametophyte and sporophyte generations have been used in land plants to infer relationships and construct classifications, but sporophytes provide the vast majority of data for the systematics of vascular plants. In bryophytes both generations are well developed and characters from both are commonly used to classify these organisms. However, because morphological traits of gametophytes and sporophytes can have different genetic bases and experience different selective pressures, taxonomic emphasis on one generation or the other may yield incongruent classifications. The moss order Hookeriales has a controversial taxonomic history because previous classifications have focused almost exclusively on either gametophytes or sporophytes. The Hookeriales provide a model for comparing morphological evolution in gametophytes and sporophytes, and its impact on alternative classification systems. In this study we reconstruct relationships among mosses that are or have been included in the Hookeriales based on sequences from five gene regions, and reconstruct morphological evolution of six sporophyte and gametophyte traits that have been used to differentiate families and genera. We found that the Hookeriales, as currently circumscribed, are monophyletic and that both sporophyte and gametophyte characters are labile. We documented parallel changes and reversals in traits from both generations. This study addresses the general issue of morphological reversals to ancestral states, and resolves novel relationships in the Hookeriales.     

Vegetative and reproductive innovations of early land plants: implications for a unified phylogeny

As the oldest extant lineages of land plants, bryophytes provide a living laboratory in which to evaluate morphological adaptations associated with early land existence. In this paper we examine reproductive and structural innovations in the gametophyte and sporophyte generations of hornworts, liverworts, mosses and basal pteridophytes. Reproductive features relating to spermatogenesis and the architecture of motile male gametes are overviewed and evaluated from an evolutionary perspective. Phylogenetic analyses of a data set derived from spermatogenesis and one derived from comprehensive morphogenetic data are compared with a molecular analysis of nuclear and mitochondrial small subunit rDNA sequences. Although relatively small because of a reliance on water for sexual reproduction, gametophytes of bryophytes are the most elaborate of those produced by any land plant. Phenotypic variability in gametophytic habit ranges from leafy to thalloid forms with the greatest diversity exhibited by hepatics. Appendages, including leaves, slime papillae and hairs, predominate in liverworts and mosses, while hornwort gametophytes are strictly thalloid with no organized external structures. Internalization of reproductive and vegetative structures within mucilage-filled spaces is an adaptive strategy exhibited by hornworts. The formative stages of gametangial development are similar in the three bryophyte groups, with the exception that in mosses apical growth is intercalated into early organogenesis, a feature echoed in moss sporophyte ontogeny. A monosporangiate, unbranched sporophyte typifies bryophytes, but developmental and structural innovations suggest the three bryophyte groups diverged prior to elaboration of this generation. Sporophyte morphogenesis in hornworts involves non-synchronized sporogenesis and the continued elongation of the single sporangium, features unique among archegoniates. In hepatics, elongation of the sporophyte seta and archegoniophore is rapid and requires instantaneous wall expandability and hydrostatic support. Unicellular, spiralled elaters and capsule dehiscence through the formation of four regular valves are autapomorphies of liverworts. Sporophytic sophistications in the moss clade include conducting tissue, stomata, an assimilative layer and an elaborate peristome for extended spore dispersal. Characters such as stomata and conducting cells that are shared among sporophvtes of mosses, hornworts and pteridophytes are interpreted as parallelisms and not homologies. Our phylogenetic analysis of three different data sets is the most comprehensive to date and points to a single phylogenetic solution for the evolution of basal embryophytes. Hornworts are supported as the earliest divergent embryophyte clade with a moss/liverwort clade sister to tracheophytes. Among pteridophytes, lycophytes are monophyletic and an assemblage containing ferns, Equisetum and psilophytes is sister to seed plants. Congruence between morphological and molecular hypotheses indicates that these data sets are tracking the same phylogenetic signal and reinforces our phylogenetic conclusions. It appears that total evidence approaches are valuable in resolving ancient radiations such as those characterizing the evolution of early embryophytes. More information on land plant phylogeny can be found at: http: //www.science.siu.edu/ landplants/index.html.     

Major transitions in the evolution of early land plants: a bryological perspective

Background Molecular phylogeny has resolved the liverworts as the earliest-divergent clade of land plants and mosses as the sister group to hornworts plus tracheophytes, with alternative topologies resolving the hornworts as sister to mosses plus tracheophytes less well supported. The tracheophytes plus fossil plants putatively lacking lignified vascular tissue form the polysporangiophyte clade. Scope This paper reviews phylogenetic, developmental, anatomical, genetic and paleontological data with the aim of reconstructing the succession of events that shaped major land plant lineages. Conclusions Fundamental land plant characters primarily evolved in the bryophyte grade, and hence the key to a better understanding of the early evolution of land plants is in bryophytes. The last common ancestor of land plants was probably a leafless axial gametophyte bearing simple unisporangiate sporophytes. Water-conducting tissue, if present, was restricted to the gametophyte and presumably consisted of perforate cells similar to those in the early-divergent bryophytes Haplomitrium and Takakia. Stomata were a sporophyte innovation with the possible ancestral functions of producing a transpiration-driven flow of water and solutes from the parental gametophyte and facilitating spore separation before release. Stomata in mosses, hornworts and polysporangiophytes are viewed as homologous, and hence these three lineages are collectively referred to as the 'stomatophytes'. An indeterminate sporophyte body (the sporophyte shoot) developing from an apical meristem was the key innovation in polysporangiophytes. Poikilohydry is the ancestral condition in land plants; homoiohydry evolved in the sporophyte of polysporangiophytes. Fungal symbiotic associations ancestral to modern arbuscular mycorrhizas evolved in the gametophytic generation before the separation of major present-living lineages. Hydroids are imperforate water-conducting cells specific to advanced mosses. Xylem vascular cells in polysporangiophytes arose either from perforate cells or de novo. Food-conducting cells were a very early innovation in land plant evolution. The inferences presented here await testing by molecular genetics.     

轮藻和陆地植物系统发育及其进化

Charophytic algae and land plants together make up a monophyletic group, streptophytes, which represents one of the main lineages of multicellular eukaryotes and has contributed greatly to the change of the environment on earth in the Phanerozoic Eon. Significant progress has been made to understand phylogenetic relationships among members of this group by phylogenetic studies of morphological and molecular data over the last twenty-five years. Mesostigma viride is now regarded as among the earliest diverging unicellular organisms in streptophytes. Characeae are the sister group to land plants. Liverworts represent the first diverging lineage of land plants. Hornworts and lycophytes are extant representatives of bryophytes and vascular plants, respectively, when early land plants changed from gametophyte to sporophyte as the dominant generation in the life cycle. Equisetum, Psilotaceae, and ferns constitute the monophyletic group of monilophytes, which are sister to seed plants. Gnetales are related to conifers, not to angiosperms as previously thought. Amborella, Nymphaeales, Hydatellaceae, Illiciales, Trimeniaceae, and Austrobaileya represent the earliest diverging lineages of extant angiosperms. These phylogenetic results, together with recent progress on elucidating genetic and developmental aspects of the plant life cycle, multicellularity, and gravitropism, will facilitate evolutionary developmental studies of these key traits, which will help us to gain mechanistic understanding on how plants adapted to environmental challenges when they colonized the land during one of the major transitions in evolution of life.     

Unresolved problems on the origin and early evolution of land plants.

The origin of land plants or embryophytes from the Charophyceae is generally accepted today by the botanists. In fact, numerous morphological, cytological, ultrastructural, biochemical and molecular characters are shared in these organisms. A fundamental problem is still constituted by the evolution of the sporophyte, i.e. the appearance of two different phase cycles (gametophyte/sporophyte alternance), although two theories ("antithetic" and "homologous") try to explain this evolutionary event.However, another phylogenetic dilemma is represented, in my opinion, either by the formation of bryophytes or by the transition from these first land plants to the pteridophytes, considering them at whole organism level.The bryophyte gametophyte is the most elaborate of the land plants. It presents several complex characters, principally the growth developmental form, the appearance of multicellular sex organs, antheridia and archegonia. Also the sporophyte shows a complicated structure that is not found in the other land plants or tracheophytes. The sporangium, in particular, exhibits some intricate morphological traits such as the peristome of true mosses for spore dispersion, the elaters of liverworts and the indeterminate growth in the hornworts.The pteridophytes are represented especially by their dominant sporophyte. This latter has the capacity to produce multiple sporangia and, in many cases, two kinds of spores which develop in male and female gametophyte (heterosporous pteridophytes). Another important characteristic of this sporophyte is its ability to become independent of the gametophyte. However, one of the most innovative character is the formation of true vascular elements (xylem and phloem).All these very large evolutionary jumps are discussed on the basis of the phyletic gradualistic neo-Darwinian theory and the punctuated equilibrium theory of Eldredge and Gould. In this context other genetic evolutionary mechanisms are also considered.Nevertheless, the origin of bryophytes and pteridophytes remain, at the moment, a mystery.     

Two theories of origin of the land-plant sporophyte: Which is left standing?

Questions concerning the two competing theories of the development of alternating generations in land plants, the homologous theory and the antithetic theory, have never been fully resolved. In the majority of recent accounts there appears to have been increasing de facto support (if one considers the ontogenetic processes and phylogenetic consequences discussed) for the antithetic theory. However, this preference is usually not plainly stated (as such) in these discussions, and some support has also continued for the homologous theory. The crux of both theories (homologous and antithetic) centers upon how the sporophyte may have originated in the life cycle. One problem with the homologous theory is that it is not made explicit how the development of a dependent sporophyte could have occurred in the life cycle (when the precedent organisms are considered to have had free-living, putatively similar, gametophytes and sporophytes). The antithetic theory, by contrast, offers a definite ontogenetic mechanism or process (retention of the zygote on the gametophyte, delay of zygotic meiosis, with zygotic mitoses occurring first) by which a dependent sporophyte might have originated and persisted, in the context of a life cycle formerly lacking a sporophyte generation. Also, a review of a variety of evidence (morphological, cytological, biochemical, etc.) would appear to lend more support to the antithetic theory than to the homologous theory. In discussing types of algae now known to be most clearly related to land plants (i.e., charophytes, particularly advanced forms), the type of life cycle exhibited by these particular algae (haplontic, with zygotic meiosis; no sporophyte present) suggests that only an antithetic origin of the sporophyte in land plants is actually feasible.     

Anatomy of the vegetative organs and secretory structures of Rhaponticum carthamoides (Asteraceae)

Roots, stems, rhizomes and leaves of Rhaponticum carthamoides (Willd.) Iljin (a Siberian adaptogenic plant, originating from the Altai and Saian Mountains) of different ages were investigated by means of light and electron microscopy. Schizogenous secretory reservoirs occurred in every organ, and were located within the secondary xylem (adventitious roots and rhizome of young plants), at the interface of endodermis/cortical parenchyma (roots and hypocotyl), along phloem and primary xylem (older rhizome), around the vascular bundles (inflorescence stem, petiole and leaf midrib veins) and along phloem (cotyledonary and leaf veins). At the interface of endodermis/inner parenchyma, secretion accumulated in the intercellular spaces prior to the formation of proper epithelial cells. The secretion as observed by transmission electron microscopy comprised three components: soluble (i.e. absent from sections; probably phenolic), insoluble and strongly osmiophilic (probably phenolic) and insoluble, moderately osmiophilic (probably lipidic). Numerous osmiophilic oil droplets, similar to the lipidic secretion inside the reservoirs, local proliferation of rough endoplasmic reticulum and numerous multivesicular bodies characterized epithelial cells in all organs. Leucoplasts (in subterranean organs) with osmiophilic inclusions and peroxisomes with crystalloid inclusions were specific for parenchyma cells. Peltate glandular hairs were formed on leaf blades.  © 2004 The Linnean Society of London, Botanical Journal of the Linnean Society , 2004, 144 , 207–233.