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The Basic Flower
Review ARCHAIC FLORAL GROUND PLAN - MODIFIED SHOOT
Terms associated with a 'typical', modern flower:
'COLLECTIVE' TERMS DEALING WITH THE FOUR FLORAL WHORLS:
Gynoecium - applied to the uppermost
set - the carpels or a single carpel
Also, terms for fusion or joining of floral appendages:
connate - fusion of appendages from
a single whorl
While we are concerned with the flower as a structural entity and the terminology used by botanists to 'map' variation in floral structure, it is useful to place this unique feature of the angiosperms into a functional context.
Modern classification systems assume that variation at all taxonomic ranks is the result of phyletic or evolutionary change. This process of gradual modification through time via natural (or human mediated) selection requires genetic variation as a 'raw material' and meiotic recombination is a primary source or producer of genetic variation. Thus, sexual reproduction and associated meiotic events of gamete production is of fundamental importance. Flowering plants - relative to other vascular plants - show unique, specialized attributes in this area.
A mature fern represents the diploid (two genomes or '2n') or sporophytic phase of the fern life cycle. Meiosis occurs in fern sporangia to yield haploid (1 genome or 'n') spores which, in most ferns, germinate and produce a free living (separate from the sporophyte, often photosynthetic) plant (gametophyte) that eventually produces gametes (sperm and egg) that merge (syngamy) to form the first diploid cell (zygote) which then develops to produce the embryo and next sporophytic (diploid) turn of the cycle.
The same 'alternation of generations' process occurs in flowering plants, but with significant changes. These relate to enhanced protection for critical elements - both haploid and diploid by way of structural reduction, simplification and - more or less - a more 'streamlined' process. When you look at an angiosperm in flower you are looking at both diploid (sporophytic) and haploid (gametophytic) phases of the life cycle. The angiosperm gametophytes - individual plants that are genetically distinct from the parent sporophyte - develop within sporangia on the parent sporophyte. Two types of gametophytes are produced by flowering plants. They result from meiotic events at two sporangial centers within the flower, each relating to the two terminal reproductive whorls (gynoecium and androecium):
Available data from anatomical study of extant angiosperms and the fossil record indicate that the angiosperm carpel evolved from an ovule-bearing leaf that had the ovules arranged along the margin, a megasporophyll. This ovules, like modern gymnosperms ('gymnosperm' = 'naked seed) were exposed to herbivores and ambient environmental conditions. As indicated in the figure below, genetic variation in these ancient 'pre-angiosperms' included a tendency toward folding of these reproductive leaves and this was evidently favored by natural selection. Given possible selective advantages for ovule protection, this selective 'trend' proceeded to the point that the margins of this leaf made contact and eventually fused. This resulted in the production, over many generations of variation and selection, of a container for the ovules ('angiosperm' = 'vessel seed'). This modified megasporophyll, conduplicated (folded) to enclose the ovules, represents the basic unit of the angiosperm gynoecium - the carpel (the 'folding' selective scenario is known as the 'conduplicate carpel theory')
fossil record indicates that the first flowering plant gynoecia were part
of a shoot-like flower, i.e., many separate carpels (apocarpous)
arranged along an elongate floral axis or receptacle. Floral
evolution over the past 100,000 years has featured reduction in the number
of carpels on this extended receptacle, shortening of the receptacle, and connation
of carpels to produce a syncarpous gynoecium, as depicted below:
Patterns of structural variation among gynoecia of extant flowering plants reflect an evolutionary mosaic; lineages have followed different paths of development at different rates. However, the theoretical constructs of megasporophyll conduplication and subsequent reduction and fusion provide a foundation for placing observed variation with a logical and structured (therefore predictive) context. The ovules, for instance, tend to show specific, often linear, arrangements along the wall of the ovary and these are categorized as placentation types. A single carpel will show a single line of placentation and a single locule and this is known as marginal placentation (photo). Connation of carpels to produce a syncarpous gynoecium or compound pistil, as depicted above, yields a pattern of septa, locules, and lines of ovules that is known as axile placentation. When more than one row of ovules is attached to the pericarp (ovary wall), this is called parietal placentation (photo). Other common placentation patterns are depicted below.
Archaic elements of the Magnoliophyta
(flowering plants), both monocots (Liliopsida) and dicots (Magnoliopsida)
tend to show apocarpous gynoecia (photo).
A single flower will carry many simple pistils, each showing marginal
placentation if more than one ovule is present (photo).
More advanced taxa tend toward syncarpy . The multicarpellate
gynoecium will be represented by a single, compound pistil.
As depicted below (right) multiple basic elements (carpels) of a syncarpous
gynoecium are signaled by 1) lobes of the stigma or multiple styles,
2) locules (sometimes referred to as 'cells') of a cross-section ovary,
or 3) lines of ovules or placentation type.