Towards an ontogenetic understanding of inflorescence diversity

Backgrounds and Aims

Conceptual and terminological conflicts in inflorescence morphology indicate a lack of understanding of the phenotypic diversity of inflorescences. In this study, an ontogeny-based inflorescence concept is presented considering different meristem types and developmental pathways. By going back to the ontogenetic origin, diversity is reduced to a limited number of types and terms.

Methods

Species from 105 genera in 52 angiosperm families are investigated to identify their specific reproductive meristems and developmental pathways. Based on these studies, long-term experience with inflorescences and literature research, a conceptual framework for the understanding of inflorescences is presented.

Key Results

Ontogeny reveals that reproductive systems traditionally called inflorescences fall into three groups, i.e. ‘flowering shoot systems’ (FSS), ‘inflorescences’ sensu stricto and ‘floral units’ (FUs). Our concept is, first, based on the identification of reproductive meristem position and developmental potential. The FSS, defined as a seasonal growth unit, is used as a reference framework. As the FSS is a leafy shoot system bearing reproductive units, foliage and flowering sequence play an important role. Second, the identification of two different flower-producing meristems is essential. While ‘inflorescence meristems’ (IMs) share acropetal primordia production with vegetative meristems, ‘floral unit meristems’ (FUMs) resemble flower meristems in being indeterminate. IMs produce the basic inflorescence types, i.e. compound and simple racemes, panicles and botryoids. FUMs give rise to dense, often flower-like units (e.g. heads). They occur solitarily at the FSS or occupy flower positions in inflorescences, rendering the latter thyrses in the case of cymose branching.

Conclusions

The ontogenetic concept differs from all existing inflorescence concepts in being based on meristems and developmental processes. It includes clear terms and allows homology statements. Transitional forms are an explicit part of the concept, illustrating the ontogenetic potential for character transformation in evolution.     

A hydrothermally precipitated catalytic iron sulphide membrane as a first step toward life

We propose that life emerged from growing aggregates of iron sulphide bubbles containing alkaline and highly reduced hydrothermal solution. These bubbles were inflated hydrostatically at sulphidic submarine hot springs sited some distance from oceanic spreading centers four billion years ago. The membrane enclosing the bubbles was precipitated in response to contact between the spring waters and the mildly oxidized, acidic and iron-bearing Hadean ocean water. As the gelatinous sulphide bubbles aged and were inflated beyond their strength they budded, producing contiguous daughter bubbles by the precipitation of new membrane. [Fe₂S₂]^+/0 or [Fe₄S₄]^2+/+ clusters, possibly bonded by hydrothermal thiolate ligands as proferredoxins, could have catalyzed oxidation of thiolates to disulphides, thereby modifying membrane properties.We envisage the earliest iron sulphide bubbles (pro botryoids) first growing by hydrostatic inflation with hydrothermal fluid, but evolving to grow mainly by osmosis (the protocellular stage), driven by (1) catabolism of hydrothermal abiogenic organics trapped on the inner walls of the membrane, catalyzed by the iron sulphide clusters; and (2) cleavage of hydrophobic compounds dissolved in the membrane to hydrophilic moieties which were translocated, by the proton motive force inherent in the acidic Hadean ocean, to the alkaline interior of the protocell. The organics were generated first within the hydrothermal convective system feeding the hot springs operating in the oceanic crust and later in the pyritizing mound developing on the sea floor, as a consequence of the reduction of CO, CO₂, and formaldehyde by Fe²⁺- and S^2–-bearing minerals.We imagine the physicochemical interactions in and on the membrane to have been sufficiently complex to have engendered auto- and cross-catalytic replication. The membrane may have been constructed in such a way that a successful parent could have informed the daughters of membrane characteristics functional for the then-current level of evolution.Correspondence to: M. J. RussellGlossary: Hollow pyrite botryoids: hollow hemispheres of cryptocrystalline pyrite (FeS₂) 0.1–1 mm across. Fischer-Tropsch syntheses: the highly exothermic catalytic hydrogenation of CO to hydrocarbons and aliphatic oxygenated compounds using finely divided iron. Greigite (Fe₃S₄): metastable iron sulphide precipitated from aqueous solution in a gel at 100°C and containing two-thirds of its iron as the high-spin ferric ion. Haber-Bosch process: the exothermic catalytic hydrogenation of nitrogen to yield ammonia. Probotryoid: a hydrostatically inflated colloidal iron monosulphide bubble; precursor to hollow botryoids and the progenitor to protocells. Proferredoxins: [Fe₂S₂] and [Fe₃MS₄] clusters (M = Fe, Mo, W, Ni, etc.) ligated by abiogenic thiols and thiolates. Protocell: a cell comprised mainly of abiogenic organics including thiols with subordinate iron sulphides, partly as proferredoxins; growth results from catabolism and osmotic pressure