It is not an easy task to capture floral diversity by floral diagrams. To be fully comprehensive, several volumes would have to be written, comprising several hundreds of drawings. However, characters on floral diagrams are clear enough to reflect where a taxon belongs and can be used for identification at least to family level. Floral diagrams are increasingly important as a principal means in understanding the complexity of flowers and leading to hitherto unexplored floral characters. While the phylogeny of angiosperms is being progressively refined by worldwide collaborative research (APG III will be published before this book will be out), the challenge for studying flowers and their hidden secrets becomes increasingly important. While writing this book, I made several new observations of flower structures by study of fresh flowers, while the information from the literature was either too basic, or restricted to obscure nineteenth-century work. While Eichler had broader access to a wide botanical knowledge, this knowledge has become progressively eroded during the twentieth century because of emphasis on new exciting areas of botany. Nowadays, we know much more about the genetic structure of plant groups than about their floral structure. The scope of morphological research is immense, especially in tropical families for which only a fraction of the diversity is currently known. In the present biodiversity crisis, such studies are a race against time, with the certainty that pertinent scientific knowledge and aesthetic models are lost forever, before even being discovered.

Floral diagrams can describe flowers with a high degree of detail. Although it is difficult to represent specific three-dimensional depth, the amount of information provided is considerable and may include developmental and anatomical evidence (see p. 37).

Floral diagrams support recognition of plant species, as major groups can be identified by their floral diagram, but are also a reflection of the evolution of flowers in angiosperms. Major changes in the floral Bauplan can be stressed by representing floral diagrams in the context of the phylogenetic tree of angiosperms. Floral diagrams make a comparison between divergent characters possible by stressing the positional relationships of floral structures.

Floral diagrams are not rigidly fixed in time, but are an expression of the developmental plasticity in flowers. Important morphological changes are often the result of subtle shifts in the primordial body during development, and this is also reflected at the genetic level. One aspect of morphological observations is that changes are often gradual, without clear-cut boundaries in characters between different clades. Several plant groups are characterized by ‘tendential features’ or ‘apomorphic tendencies’ (see Endress and Matthews, 2006a), namely characters that may not be generalized in a clade, but occur on a much more frequent basis than in any related clades. I call these characters ‘cryptic apomorphies’. These characters can either be synapomorphic (present in a clade but not in all members), non-synapomorphic (a predisposition to evolve a character in a clade) or represent several independently derived features (autapomorphies) in a clade (Endress and Matthews, 2006a).

The early diverging eudicots represent a transitional grade between basal angiosperms and core eudicots (Fig. 7.1). Ranunculales is the basal order of eudicots with the highest floral diversity (Ronse De Craene, Soltis and Soltis,2003; Soltis et al., 2005). Other intermediate orders (e.g. Proteales, Buxales, Trochodendrales) generally have much reduced, dimerous flowers and the link between Ranunculales and core eudicots remains unclear on a floral morphological basis (e.g. Ronse De Craene, 2004; Wanntorp and Ronse De Craene, 2005).

Ranunculales

The order is highly diverse in terms of floral structure. As such it occupies a transitional position between basal angiosperms and core eudicots (Ronse De Craene, Soltis and Soltis, 2003). Several derived characters tend to be concentrated in Menispermaceae, Ranunculaceae and Papaveraceae, such as median or transversal monosymmetry, sepal and petal differentiation and fusion, petal appendages in the shape of spurs, syncarpy and (pentamerous) cyclic flowers. Unisexual flowers with synandry have evolved in Menispermaceae and Lardizabalaceae (Endress, 1995b).

The androecium is highly variable, ranging from numerous spirally arranged stamens to a single stamen. All core Ranunculales share nectariferous petals that are of probably staminodial origin (Erbar, Kusma and Leins, 1998; Walker-Larsen and Harder, 2000; Ronse De Craene, Soltis and Soltis, 2003).

Figure 6.1 gives one recent phylogeny of monocots based on Chase et al. (2005). There is much uncertainty about the closest sister group of the monocots, which lies within the basal Angiosperms. Options, such as Ceratophyllaceae and Chloranthaceae, were discussed in Soltis et al. (2005) but are not resolved. Moore et al. (1997) suggested a sister group relationship of monocots and eudicots, including Ceratophyllaceae, but this was questioned by Endress and Doyle (2009), who associated monocots with magnoliids. The trimerous monocot floral formula (P3+3A3+3G3) is found in a number of basal angiosperms, but there is no certaintly of any morphological links and basal monocots have a more variable floral Bauplan.

Monocots consist of three major units: a basal Acorales–Alismatales grade, a lilioid grade and a higher commelinid clade.

The basal monocots: Acorales and Alismatales

The Alismatales contains 14 families and is the most diverse order of the monocots in organ number and floral morphs. While the formula P3+3A3+3G3 fits well with most other monocots, the number and development of organs is much more variable in the Alismatales. The link of Alismatales with Acorus is strong and the separation of an order Acorales is arbitrary, as some molecular studies place Acorus within Alismatales (see Buzgo et al., 2006).

A bract tends to be present (sometimes included in the flower: Acorus) or is obviously reduced or absent (Araceae, Aponogetonaceae).

Definition and significance of floral diagrams

A floral diagram is a schematic cross-section, preferably through a young flower in which all the individual organs or elements of the flower are projected into one plane. A floral diagram is the best way to show the number and topological properties of floral organs. Different parts of the flower are represented by clear symbols, and the spaces between organs are an approximate reflection of the distance between the organs. Fusions between different parts are shown by connecting lines, and the orientation of the flower by placing reference points in relation to the axis of the inflorescence and the subtending bracts. Floral diagrams have two major attributes: (1) the information that can be retrieved from a good floral diagram is immense and replaces extensive descriptions or even drawings, and (2) they facilitate a whole-scale comparison of floral structures across the angiosperms. An inconvenience is that it is concentrated on a limited number of characters at the expense of others that do not fit on the diagrams. Endress (2008a) discussed the benefits of accurate drawings over detailed descriptions. A floral diagram is a clearer way to convey information, as a synthesis of the details of flowers.

There are different ways to create floral diagrams. These range from a simplified drawing of the position of organs in the flower in relation to each other, to a very complex representation of details that cannot always be seen without a lens.

This book is not exhaustive, as the total number of families has not been covered, although it is comprehensive in reflecting floral diversity of the majority of angiosperms. Major groups of angiosperms have been covered to reflect important floral evolutionary patterns. Sections below summarize most important floral attributes of major clades of angiosperms.

Basal angiosperms, monocots and early diverging eudicots

Floral evolution of basal angiosperms has been studied in a phylogenetic context by several authors (e.g. Doyle and Endress, 2000; Ronse De Craene, Soltis and Soltis, 2003; Zanis et al., 2003; Endress and Doyle, 2009). Mapping of floral characters on phylogenetic trees implies that cyclic flowers have evolved rapidly from a helical flower with relatively few stamens and carpels as in Amborella and the ANA-grade (Austrobaileyales, Nymphaeales, Amborellales), with frequent reversals to a spiral phyllotaxis in different orders. The most recent addition in Nymphaeales, Hydatellaceae, raises questions about the flowers of the earliest angiosperms (see Rudall et al., 2007; Endress and Doyle, 2009).

Flowers of basal angiosperms have a low synorganization with high plasticity in organ number and position (Endress, 1990, 2001, 2008c). The transition between vegetative shoot and flower is often gradual with a progressive change from leaves to bracts and tepals within the inflorescence or at the base of the flower (Endress, 2003b; Remizowa and Sokoloff, 2003; Buzgo et al., 2007). Several taxa, also in early diverging eudicots and basal monocots, have an unclear distinction between bracts and perianth.