Perspective

The primary vascular system extends throughout the root system, the stem and its lateral branches, and appendages of the stem such as leaves, flowers, and fruits. The basic pattern of the primary vascular system is established initially by the arrangement of provascular tissue in the embryo. As development of the young plant proceeds, the provascular tissue becomes restricted to the shoot apex and to the root tip proximal to the root cap. Differentiation in the provascular tissue leads to the development of mature, functional primary xylem and primary phloem (Fig. 6.1). In primitive plants with central columns of primary vascular tissue (protosteles) (many pteridophytes as well as the roots of most plants), phloem surrounds the xylem (Fig. 6.1a). In those with tubular vascular systems (siphonosteles) this is usually also true, but in some taxa phloem may bound the xylem on the interior as well as on the exterior (Fig. 6.1b). In seed plants in which the primary vascular systems consist of discrete, or relatively discrete, vascular bundles (eusteles) (Fig. 6.1c, d), the spatial relationship of primary xylem and primary phloem varies according to the bundle type, i.e., whether collateral, bicollateral, amphicribral, or amphivasal. In collateral bundles, the primary xylem comprises the part of the bundle toward the inside of the stem and the primary phloem comprises the outer part (Figs 6.1, 6.2, 6.4) whereas in bicollateral bundles phloem occurs both to the inside and to the outside of the primary xylem.

Perspective

A large part of this book, thus far, has dealt with the typical condition in stems of gymnosperms and dicotyledons. This chapter will present interesting and important information about stem growth in monocotyledons as well as development and patterns of organization in lianas (vines) and other plants usually characterized as having “anomalous” structure. Unlike gymnosperms and dicotyledons, monocotyledons, even the largest taxa among the palms, do not produce a typical vascular cambium. Although most are characterized solely by primary growth, some palms, some members of the Liliaceae and Agavaceae, and a few other monocotyledons increase in size by secondary growth. The tissues derived from the secondary meristem are strikingly different from the secondary xylem and phloem of the gymnosperms and other angiosperms.

Primary peripheral thickening meristem

As in other plants, the activity of apical meristems of monocotyledons results primarily in an increase in length of the stems. The diameter of a palm stem does not vary greatly from the base to the most distal leaf-bearing region; thus considerable diametric growth must occur in the internodes just beneath the apical meristem, and this is accomplished by activity of the primary peripheral thickening meristem. This meristem is a rather diffuse region located in the periphery of the broad region of the stem immediately below the apical meristem. Its longitudinal extent varies in different species.

Perspective: leaf traces

Vegetative shoots consist of stems bearing leaves. In order to develop, and to synthesize various necessary compounds required by the plant, leaves must have access to a source of water and essential minerals which are transported into them from the stem through the primary xylem. Photosynthate and other compounds synthesized in the leaves are, in turn, transported through the primary phloem into the stem and root system for storage and/or use. This transport of substances takes place in primary vascular connections between the stem vascular system and the base of leaves called leaf traces. Traces may diverge from the stem vascular system some distance below, or very near, the nodes (sites of attachment of leaves to stems) at which they enter the leaves. Leaf traces are composed of protoxylem, metaxylem, protophloem, and metaphloem, and typically contain transfer cells in both primary xylem and primary phloem. In seed plants, leaf traces are often larger and contain more tracheary cells than the vascular bundles from which they diverge, and they may increase in size distally. A leaf may be vascularized by only one or by several to many leaf traces.

In order to understand the morphology of nodal regions of shoots we must observe both transverse and median longitudinal sections through these regions. Remember that stems bear many leaves in various spatial distributions.

Perspective

It is difficult to overemphasize the importance of the vascular cambium which produces secondary xylem and secondary phloem. In the following two chapters we shall discuss in detail the structure, functions, and the importance to the plant of these tissues which also have great significance for mankind. Wood (i.e., secondary xylem) is a material of which the buildings in which we live and work are constructed. It is the source of the paper on which we write, on which newspapers, magazines, and books are printed, and of many synthetic fabrics such as rayon and nylon of which our clothes are made, to name only a few of its many uses. The phloem is of the utmost importance as the tissue through which photosynthate is transported from the leaves to sites of utilization or storage in the plant. It is the availability of photosynthate which makes possible the development of nutritious, edible parts of plants, such as fruits, nuts and grains, bulbs, tubers, other edible roots, and leaves, etc., the source of so much of the food supply of humans and other organisms. It is important, therefore, that we know more about the detailed structure and activity of the vascular cambium, a lateral meristem of such great significance.

Structure of the vascular cambium

It is generally agreed that the vascular cambium is composed of a layer of cells only one cell thick, and that all of these cells are meristematic cambial initials from which cells of the secondary xylem and secondary phloem are derived.

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).