Morphology

Abstract

Most species of Diplusodon are subshrubs or shrubs, with only three taxa that are small trees (Fig. 3.1E): D. virgatus Pohl var. virgatus, D. ciliatiflorus T. B. Cavalc., and D. heringeri Lourteig. The trees grow to 2–4 m tall, with a slender unbranched trunk and a well-defined, much-branched crown. These taxa grow on the margins of gallery forests and cerrado woodlands. All other species in the genus are subshrubs or shrubs that can be sparsely to much-branched, with a single main stem.

3.1 Habit and Life Forms

Most species of Diplusodon are subshrubs or shrubs, with only three taxa that are small trees (Fig. 3.1E): D. virgatus Pohl var. virgatus, D. ciliatiflorus T.B.Cavalc., and D. heringeri Lourteig. The trees grow to 2–4 m tall, with a slender unbranched trunk and a well-defined, much-branched crown. These taxa grow on the margins of gallery forests and cerrado woodlands. All other species in the genus are subshrubs or shrubs that can be sparsely to much-branched, with a single main stem.

Fig. 3.1

Habit in Diplusodon. A. Unbranched subshrub. B. Caespitose subshrub. C. Branched subshrub. D. Branched shrub. E. Small tree. F–H. Xylopodia

In subshrubs (Figs. 3.1A–C), wood is formed from the base to the middle or distal position of the plant. The young upper branches are not woody, hence the designation of “semishrub,” which some authors, such as Eiten (1968), apply to describe this condition. A few species, for example, Diplusodon cordifolius Lourteig, D. helianthemifolius Mart. ex DC. var. pemphoides (DC.) Koehne, and D. parvifolius Mart. ex DC., are considered shrubs (Fig. 3.1D) but are very branched from the base with secondary growth reaching the upper branches. The shrubby or subshrubby species vary greatly in size, from 7 cm to 3 m tall.

The subshrubby habit in Diplusodon is commonly associated with a well-developed, woody, subterranean, perennial xylopodium. The xylopodia vary in size and form, and they generally produce a single aerial stem with lateral branches (Fig. 3.1F–H). Following seasonal fires, many buds develop a number of stems synchronously, at the same level from the xylopodia, forming a caespitose subshrub (Fig. 3.1B). Raunkiaer (1934) classified these plants as perennial hemicryptophytes. The xylopodia provide good protection for plants against fire, which is a common occurrence in the cerrados. Xylopodia are found in many plants of the cerrados.

It is common to find species of Diplusodon that have both an erect, well-developed subshrubby habit and a caespitose form, the latter of which flowers precociously and is strongly associated with resprouting following a fire. The occurrence of caespitose forms that are quite distinct from the common form of a species sometimes creates difficulties for delimitation of taxa (see the commentary for D. ciliiflorus Koehne).

The stem, beginning at the region where branches are initiated, generally possesses a rhytidome that splits into thin delicate threads, making it difficult to interpret the type of indumentum and the form of the stem in this region. The stem can, at times, display fistulas, as in Diplusodon marginatus Pohl, but ants have not been observed inside of these stems. In some species of Diplusodon, such as D. buxifolius (Cham. & Schltdl.) DC., D. lanceolatus Pohl, D. leucocalycinus Lourteig, D. orbicularis Koehne, D. paraisoensis Lourteig, D. strigosus Pohl, and D. uninervius Koehne, thickened parts of the stem and lateral branches were confirmed to be galls. The occurrence of galls in Diplusodon was recorded by several authors, such as Coelho et al. (2013) for D. buxifolius, Silva et al. (2018), and Maia and Fernandes (2004) for D. virgatus, as the first record of galls in this species. Marini-Filho and Fernandes (2012) reported that stem galls drain nutrients and decrease shoot performance in D. orbicularis and that they also reduce toxic aluminum concentration in the plant tissues.

The upper lateral branches are typically decussate, and the indumentum, if present, is exceedingly variable. The branches can be cylindrical, subquadrangular (when the laterals are slightly flattened), or distinctly quadrangular. Furthermore, the four angles can develop small to prominent wings up to 2 mm wide, as in Diplusodon alatus T.B.Cavalc., or wings can be absent. The upper branches also vary in coloration, for example, brownish in D. bradei, pale yellow in D. astictus, or wine-colored to red in D. glaucescens DC. and D. sordidus Koehne.

3.2 Indumentum, Trichomes, and Colleters

3.2.1 Indumentum

After Koehne’s studies, little attention was given to the value of indumentum and trichome types as diagnostic characters in Lythraceae treatment. Recently, more emphasis has again been placed on the great variability in indumentum types as well as trichome morphology. Trichomes have been of considerable importance in taxonomic analyses generally in Lythraceae and especially in Lagerstroemia L. and Cuphea P. Browne (Schoemberg and Hofmeister 1986; Amarasinghe et al. 1991). With respect to Diplusodon, Lourteig (1989) distinguished groups within the sections based solely on indumentum. The great diversity of indumentum and trichome types in Cuphea, however, has not been detected in Diplusodon, and indumentum and trichome types, although helpful in delimiting some taxa, vary widely in others.

Variation in the density and types of trichomes was observed within species and among populations. Of special note is the occurrence in Diplusodon leucocalycinus Lourteig of some individuals bearing two types of intermixed trichomes, simple and stellate, while other individuals only have one or the other. The variation in density of trichomes among populations has been the basis for recognition of varieties and forms in many species.

The following types of indumentum occur in the Diplusodon species (following nomenclature of Radford et al. 1974; Johnson 1975): canescent, ciliate, floccose, hirsute, hispid, papillose, pubescent, sericeous, stellate, strigose, velutinous, and villous. The indumentum can be composed of trichomes that are erect, appressed, oblique, or acroscopic or basiscopic (Uphof 1962).

Most Diplusodon species have indumentum on all parts that falls away with age. For this reason, most species are glabrescent, and some are conspicuously pubescent when young. For example, D. adpressipilus Lourteig is characterized by sericeous or silvery indumentum but can have entire branches with leaves that become green or nearly glabrous with age. Other species, such as D. ciliatiflorus, D. decussatus Gardner, D. orbicularis, D. sordidus, and D. speciosus (Kunth) DC., are pilose when seedlings but glabrous at maturity.

Some species, for example, Diplusodon ciliatiflorus and D. panniculatus Koehne, have trichomes only on the flowers. When present on flowers, indumentum generally occurs on the floral tube, sepals, and epicalyx. In a few species, such as D. leucocalycinus and D. appendiculosus Lourteig, trichomes are present on the apex of the ovary and persist on the capsule. In some individuals of D. petiolatus (Koehne) T.B.Cavalc., trichomes occur on the anthers.

3.2.2 Trichomes

There are five main types of trichomes in Diplusodon:

Type 1: Unicellular, non-glandular trichomes with the body elongated, conical, and narrow and the base bulbous (Fig. 3.2A, B, D, G, H, J). This type occurs frequently in species with villous, velutinous, hirsute, and sericeous indumentum on all parts of the plant, for example, Diplusodon argyrophyllus T.B.Cavalc., D. bradei, and D. incanus Gardner.

Fig. 3.2

Trichomes in Diplusodon. A. Type 1, D. candollei (Pereira et al. 790, BHCB). B. Type 1, D. paraisoensis Cavalcanti et al. 390, CEN. C. Type 2, D. epilobioides Hatschbach 41484, NY. D. Type 1, D. bradei (Zappi et al. [CFCR] 10601, SPF). E. Type 3, D. divaricatus (Rizzo 5113 & Barbosa 4362, UFG). F. Type 3, D. sessiliflorus (Philcox & Onishi 4782, UB). G. Type 1, D. strigosus (Cavalcanti et al. 1086, CEN). H. Type 1, D. adpressipilus (Cavalcanti et al. 400, SPF). I. Type 4, D. oblongus (Cavalcanti et al. 432, SPF). J. Type 1, D. villosus (Cavalcanti et al. 1265, CEN). K. Type 5, D. rosmarinifolius (Cavalcanti et al. 478, CEN). L. Type 5, D. helianthemifolius var. helianthemifolius (Cavalcanti et al. 252, CEN). M. Type 5, D. puberulus (Irwin et al. 22974, UB). N. Type 2, D. oblongus (Cavalcanti et al. 432, CEN). O. Colleter

Type 2: Unicellular, non-glandular trichomes forming short protuberances with thick walls and bulbous bases (Fig. 3.2C, N). This type is rare in the genus and occurs in species that are apparently glabrous or puberulous, such as Diplusodon epilobioides Mart. ex DC. and D. puberulus Koehne.

Type 3: Unicellular, glandular trichomes that are botuliform and narrow or obtuse at the apex (Fig. 3.2E, F). This type is frequently found on the sepals of Diplusodon hatschbachii Lourteig and D. sessiliflorus Koehne. In D. divaricatus Pohl, these trichomes occur on the sepals and also on the floral tube. Typically, these trichomes have a narrow base and a large body with a thick cell wall and wide lumen. The body of these trichomes may become elongated and slender in the direction of the apex forming a long neck, as can be seen, for example, in D. divaricatus and D. sessiliflorus. The base is enclosed externally by a layer of elongate epidermal cells, probably with a specialized function, which differ from the other epidermal cells in size and arrangement and are strongly united at the base. The basal cells accompany the trichome when it falls away. Uphof (1962) called these accessory cells and suggested that they may be responsible for the elevating the trichome, forming a “cushion.” These trichomes are filled with a deep orange substance that is apparently viscous or oily and not water soluble. Fahn (1989) described similar glandular trichomes that he called “stinging hairs,” but with the upper part narrow instead of broad (Figs. 3.2E, F).

Type 4: Unicellular, papillate glandular trichomes (Fig. 3.2I). This special trichome occurs on the inner margin of the sepals and is present in all species of Diplusodon, even though they are inconspicuous in most species. Trichomes of this type can be easily seen on the sepal margins of D. appendiculosus. Structurally, they possess a rounded or short, elongated, capitate apex with thick cell walls. In herbarium material of D. appendiculosus, there are hard, orange, translucent drops, like resin, secreted by these papillae along the sepal margins that can be seen on buds prior to anthesis.

Type 5: Multicellular, stellate, non-glandular trichomes that occur in tufts and are sessile or have a small stalk (Fig. 3.2K–M). In this type, a single terminal cell is replaced by a group of cells forming a stellate trichome (Uphof 1962). The apical cells are connected only at the base, and the free parts spread horizontally or, when in tufts, are ascending. In some species, a number of contiguous cells grouped around a central cell grow together over the center cell forming the pedestal of a stellate hair (Uphof 1962; Theobald et al. 1988). Stellate trichomes are found in Diplusodon helianthemifolius var. helianthemifolius, D. helianthemifolius var. pemphoides, D. macrodon Koehne, D. puberulus, and D. rosmarinifolius, among others. In D. helianthemifolius var. helianthemifolius, the stellate trichomes possess a stalk.

Lourteig (1989) referred to the trichomes of these species as “dendritic.” However, dendritic trichomes, according to a number of authors (Metcalfe and Chalk 1988; Radford et al. 1974; Uphof 1962) are elongated trichomes, in the form of a tree, with branches or extensions along their length. This is not the case in Diplusodon; therefore, this multicellular type should be called “stellate.”

All the morphological trichome types analyzed in this study have high taxonomic value for distinguishing species, especially the stellate and botuliform types, which are rare and characterize just a few species. The glandular papillose trichomes on the internal margin of the sepals, not recognized prior to this study, may be important at the generic level given that Diplusodon is defined in the family by a single autapomorphy, which is the presence of semi-lunate septal walls in the ovary (Graham et al. 1993). A more extensive survey should be made to check for the occurrence of this character in other genera of Lythraceae. Solereder (1899 apud Uphof 1962) referred to peculiar globose trichomes that occur in some Lythraceae, such as Adenaria Kunth, Pehria Sprague, and Woodfordia Salisb., but he did not mention their presence on sepals, as in Diplusodon. The surface ornamentation of trichomes in Diplusodon can vary from rugulate to striate.

3.2.3 Colleters

In Lythraceae, there are structures on the branches at the base of the leaves, bracts, and prophylls, which are referred to as stipulate structures or as stipules. Dahlgren and Thorne (1984) described them as characteristic of the order Myrtales and called them stipules, describing them as lateral or axillary, rudimentary, fingerlike projections, infrequently taking the form of long trichomes. They further added that in Diplusodon, the stipules are more advanced, being divided into small structures resembling axillary leaf trichomes.

In Diplusodon, these stipule-like structures are on the branches in the region of the leaf insertion near the bud, and they are arranged in a line. They are a type of multicellular trichome, without vascularization, actively secretory, that stores a dense orange to yellow substance that is possibly mucilaginous (Fig. 3.2O, 13.44I, 13.71.D). Metcalfe and Chalk (1988) recognize that stipules generally occur in pairs and can assume many forms. In general, stipules are vascularized by lateral traces or branches of lateral traces (Metcalfe and Chalk 1988). The term colleter was primarily used by Schumann (1891 apud Metcalfe and Chalk 1988) for secretory structures found on and among the stipules of Rubiaceae. Solereder (1908) described the same structures as “shaggy hairs” and applied the term colleter to trichomes that secret mucilage and are found on axillary buds. For Metcalfe and Chalk (1988), colleters are multicellular glandular structures usually composed of an elongate axis, with reduced epidermal cells that secrete mucilage, gum, or resin and are found on leaves and stipules of many eudicot families. For reasons related to their position and structure, these stipule-like structures in Diplusodon are here interpreted as colleters.

3.3 Leaves

3.3.1 General Aspects

The phyllotaxy in Diplusodon is invariably opposite and decussate. However, some species, such as D. uninervius Koehne, D. candollei Mart. ex DC., and D. puberulus, have liner leaves that are borne on branches with very short internodes, and the leaves appear to be verticillate or form fascicles. The external leaf morphology is a source of good characters whose states are variable among species and are therefore extensively used in the taxonomy of the genus. The leaves are always entire, and in most species they are coriaceous and deciduous. Leaf form can vary from linear to elliptic, lanceolate, ovate, or orbicular (Figs. 3.3, 3.4 and 3.5). Leaf size, presence or absence of a petiole, presence or absence of domatia, a revolute or plane margin, presence or absence of indumentum, and the type of indumentum can vary widely among the species; for this reason, these features are of taxonomic importance in the genus. Most species possess subsessile leaves, that is, with a petiole 0.5–2 mm long, while others have sessile leaves, and still fewer have long-petiolate leaves (e.g., D. bradei, D. lanceolatus, D. nitidus Mart. ex DC., and D. ramosissimus Pohl have petioles up to 2.5 cm long). The petioles are generally cylindrical and thin, appearing wide and compressed in a few species, such as D. leucocalycinus, D. appendiculosus, and D. paraisoensis.

Fig. 3.3

Patterns of venation in Diplusodon. A. Hyphodromous, D. myrsinites (Krieger 8621, SPF). B. Eucamptodromous, D. virgatus var. virgatus (Irwin et al. 17786, UB). C. Eucamptodromous, D. buxifolius (Zappi & Scatena [CFCR] 10922, SPF). D. Hyphodromous, D. hexander (Cavalcanti et al. 250, SPF). E. Eucamptodromous, D. ciliiflorus (Cavalcanti et al. [CFSC] 9630, SPF). F. Eucamptodromous-craspedrodromous/hyphodromous, D. alatus (Menezes 1208, SPF). G. Acrodromous-basal, D. punctatus var. punctatus (Burchell 7659, K). H. Eucamptodromous, D. virgatus var. virgatus (Badini s.n., 3727, OUPR). I. Acrodromous-basal-supranumerary, D. quintuplinervius (Irwin et al. 23070, K). J. Hyphodromous, D. uninervius (Cavalcanti et al. 189, SPF). K. Eucamptodromous-craspedrodromous/hyphodromous, D. alatus (Pirani et al. 1751, SPF). L. Hyphodromous, D. praetermissus (Cavalcanti et al. 401, CEN). M. Hyphodromous, D. hexander (Glaziou 19175, R). N. Eucamptodromous, D. ciliiflorus (Rossi et al. [CFCR] 3078, SPF). O. Hyphodromous, D. hexander (Souza et al. [CFCR] 8786, SPF). P. Eucamptodromous, D. ciliiflorus (Cavalcanti et al. [CFCR] 9488, SPF). Q. Hyphodromous, D. hexander (Cavalcanti et al. 201, SPF). X-ray images taken at the British Museum of Natural History, London, England

Fig. 3.4

Patterns of venation in Diplusodon. A. Eucamptodromous, D. ramosissimus (Hunt & Ramos 6303, UB). B, E. Eucamptodromous, D. oblongus (Ratter et al. 4075, UB). C. Eucamptodromous, D. stellatus (Menezes 1209, SPF). D. Eucamptodromous, D. incanus (Gardner 4138, K). F. Acrodromous-basal-supranumerary, D. paraisoensis (Gardner 3721, NY). G. Hyphodromous, D. puberulus (Irwin et al. 23692, UB). H. Eucamptodromous, D. hirsutus (Glaziou 14700, MO). I. Eucamptodromous, D. argyrophyllus (Harley et al. 27056, K). J. Hyphodromous, D. gracilis (Gardner 4139, K). K. Eucamptodromous, D. oblongus (Cavalcanti 87, SPF). L. Eucamptodromous, D. bahiensis (Harley et al 17006, SPF). M. Eucamptodromous, D. epilobioides (Martinelli et al. 11163, BHCB). N. Eucamptodromous, D. helianthemifolius var. helianthemifolius (Longhi-Wagner [CFCR] 9325, SPF). O. Eucamptodromous, D. nitidus (Mexia 5862, NY). P, Y. Eucamptodromous, D. rosmarinifolius (Ratter et al. 3133, K). Q. Eucamptodromous, D. hirsutus (Cavalcanti et al. [CFSC] 10099, SPF). R. Eucamptodromous, D. lanceolatus (Cavalcanti et al. 421, CEN). S. Eucamptodromous, D. microphyllus (Amaral et al. [CFSC] 8403, SPF). T. Acrodromous-basal-supranumerary, D. sigillatus (Irwin et al. 32190, NY). U. Eucamptodromous, D. helianthemifolius var. helianthemifolius (Menezes et al. [CFCR] 10677, SPF). V. Eucamptodromous, D. lanceolatus (Cavalcanti et al. 303, CEN). W. Acrodromous-basal-supranumerary, D. strigosus (Cavalcanti et al. 473, CEN); X. Eucamptodromous, D. parvifolius (Zappi & Kameyama [CFSC] 9636, SPF). Y. Eucamptodromous, D. hirsutus (Glaziou 14699, K). Z. Acrodromous-basal-supranumerary, D. helianthemifolius var. pemphoides (Cavalcanti et al. [CFSC] 9636, SPF). A¹. Eucamptodromous, D. leucocalycinus (Irwin et al. 24231, UB). B¹. Acrodromous-basal-supranumerary, D. villosus (Lima 58-2994, K). C¹. Acrodromous-basal-supranumerary, D. villosus (Prance & Silva 58184, UB). D¹. Acrodromous-basal-supranumerary, D. villosus (Glaziou 21419, K). E¹. Eucamptodromous, D. hirsutus (Cavalcanti et al. [CFSC] 9683, SPF). F¹. Eucamptodromous, D. bradei (Zappi et al. [CFCR] 10601, SPF). X-ray images taken at the British Museum of Natural History, London, England

Fig. 3.5

Patterns of venation in Diplusodon. A. Acrodromous-basal-supranumerary, D. decussatus (Burchell 8039, K). B. Acrodromous-basal-supranumerary, D. villosissimus (Warming s.n., K). C. Acrodromous-basal-supranumerary, D. panniculatus (Mori et al. 16947, CEN). D. Acrodromous-basal-supranumerary, D. paraisoensis (Cavalcanti et al. 390, CEN). E. Acrodromous-basal, D. orbicularis (Cavalcanti et al. [CFSC] 9619, SPF). F. Acrodromous-basal-supranumerary, D. speciosus (Gardner 3726, K). G. Acrodromous-basal-supranumerary, D. marginatus (Pereira 1253, IBGE). H. Acrodromous-basal, D. glaucescens (Scatena et al. [CFCR] 10477, SPF). I. Acrodromous-basal, D. sordidus (Cavalcanti et al. 403, CEN); J. Acrodromous-basal-supranumerary, D. speciosus (Cavalcanti et al. 86, SPF). K. Acrodromous-basal-supranumerary, D. sigillatus (Cavalcanti et al. 382, SPF). L. Acrodromous-basal, D. sordidus (Cavalcanti et al. 30, SPF). M. Acrodromous-basal, D. rotundifolius (Cavalcanti et al. 217, SPF). N. Acrodromous-basal, D. orbicularis (Cavalcanti et al. [CFSC] 10623, SPF). X-ray images taken at the British Museum of Natural History, London, England

Domatia are found in many species of Diplusodon and provide a significant character for distinguishing some species as D. cordifolius, D. leucocalycinus, D. ramosissimus, among others. The term domatia, as defined by Metcalfe and Chalk (1988), is applied to depressions, pockets, sacks, or tufts of trichomes that are localized in the axils of the midvein on the abaxial surface of the leaves. Frequently in Diplusodon, they are located in the axils of the midvein and the lateral veins, and they may or may not be accompanied by trichomes on the leaf blade. These domatia are classified as the “pocket” type (Metcalfe and Chalk 1988). The pocket-type domatium frequently takes the form of a flattened funnel with a broad distal opening and appears in a reduced form in Diplusodon, for example, in D. villosus and D. incanus. Based on their small size, these may be acarodomatia.

The leaves of Diplusodon are also notable for the presence of more than one basic pattern of venation. This feature was first noted by Candolle (1828), who described 35 species in the genus, separating them into 4 unnamed groups based on the following leaf venation characters: (1) univeined, linear leaves (Figs. 3.3D, L, M, O, Q and 3.4G); (2) leaves with 2–3 major veins departing from the base (Figs. 3.4F, I, U, X, B¹, D¹ and 3.5A–D, F, G, I–K, L, N); (3) pinnate-veined leaves (Figs. 3.3B, E, F, H, K, P and 3.4A, B, E, L, O, Q, R, T, V, A¹, C¹, G¹, H¹); and (4) palmately veined leaves (Fig. 3.5M). Koehne (1877, 1903) also highlighted patterns of leaf venation, which he used to establish four sections: Subuninerves, Penninerves, Palmatinerves, and Palmatipenninerves. This classification was followed until recently when Lourteig (1989) synonymized section Palmatipenninerves with Palmatinerves and correctly changed the name of section Subuninerves to section Diplusodon.

In this study, I analyzed the external morphology and venation patterns of the leaves in great detail, in addition to anatomical study of the leaves. The study of the external morphology of the venation patterns, evaluated by X-ray, shows the patterns recognized by Koehne (Figs. 3.3, 3.4 and 3.5).

To use venation patterns as a taxonomic character, it is necessary to have a sound understanding of Koehne’s terminology (1877, 1903), in addition to the dicotyledonous leaf architecture classification system of Hickey (1973, 1988). Hickey proposed six basic types of venation, some of which included subtypes. In Diplusodon, adapting the terminology of Hickey (1973, 1988), three basic patterns are recognized: (1) pinnate-hyphodromous, (2) pinnate-camptodromous/eucamptodromous, and (3) acrodromous.

• 1. Pinnate-hyphodromous (=hyphodromous) – Pattern in which all veins except the primary vein are absent or are present but rudimentary or hidden within a coriaceous or fleshy mesophyll (Hickey 1973, 1988). This pattern occurs in some species of Diplusodon, but not in all the species that Koehne and Lourteig considered members of section Subuninerves (=sect. Diplusodon). The pinnate-hyphodromous pattern will be referred from this point on and in the descriptions only as hyphodromous.

X-ray analysis of the leaves, corroborated by anatomical study of the cross-section of the mesophyll, indicates that some of the species considered hyphodromous by Koehne (1877, 1903) and Lourteig (1989), for example, Diplusodon hexander Mart. ex DC. and D. puberulus, have a prominent primary vein and secondary lateral veins in which the vascular bundles are so poorly developed that it is sometimes difficult to find them in the mesophyll. In these leaves, viewed macroscopically, the secondary veins are not seen, and the leaves appear to bear only a midvein. In other cases, as in D. candollei, the lateral veins are not seen in microscopic cross-sections of the leaf. The leaves of all such species are described here as hyphodromous (Figs. 3.3D, J, M, O and 3.4G).

In other species included in the section Diplusodon by Koehne (1877, 1903) and Lourteig (1989), such as D. appendiculosus, D. astictus, and D. virgatus var. virgatus, the anatomy of the leaf mesophyll includes secondary lateral veins with well-developed vascular bundles, minus the sclerenchymatous xylem sheath or with the sheath poorly developed. In this cases though the have lateral are inconspicuous is considered as eucamptodromous pattern.

• 2. Pinnate-camptodromous/eucamptodromous (=eucamptodromous) – Pattern in which the secondary lateral veins separate from the primary vein, diminishing apically toward the margin, and are connected to the supradjacent secondaries by a series of cross veins (Figs. 3.3B, E, H, K, M and 3.4A, B, L, N). This pattern is found in the species of section Penninerves (Koehne 1877, 1903) and will be cited here on only as eucamptodromous. In the leaves of Diplusodon bradei and D. decussatus (Figs. 3.3H and 3.4A), some of the secondary veins appear to form arcs, connecting to supradjacent veins, thus showing a pinnate-camptodromous/brochidodromous pattern; however, the connections to the tertiary veins are weak.

• 3. Acrodromous – Pattern in which two or more primary veins or strongly developed secondary veins extend in convergent arches toward the apex of the leaf. The arches are not recurved at the base. This pattern is common in Diplusodon, and Koehne included taxa with this pattern in sections Palmatinerves and Palmatipenninerves (=sect. Palmatinerves of Lourteig 1989). Koehne defined section Palmatinerves as possessing conspicuous lateral veins, all of which departed from a single point at the base of the primary central vein. This subtype 1 pattern is here referred to as the acrodromous-basal type. Koehne (1877, 1903) included only Diplusodon glaucescens, D. orbicularis, and D. rotundifolius (Fig. 3.5E, H, M, N) in this section. The section Palmatipenninerves was defined by Koehne (1877, 1903) as possessing two or more pairs of veins separating from the base of the central vein and two or more additional veins diverging along the length of the central vein. This section includes the majority of species of Diplusodon. Almost all the species included in the acrodromous-basal pattern, beyond having the central veins diverging from each side at the base of the central vein, also have supradjacent veins separating along the length of the central vein (Fig. 3.5A, F, L, N). Such veins can be opposite (Fig. 3.5J, K) or alternate (e.g., Fig. 3.5A, F, L, N) and form different angles, from 20° to 60°, in relation to the primary vein. In this cases, this acrodromous pattern, with various primary veins diverging from a single point at the base of the central vein in addition to other veins diverging from the central vein at different points above the base, is considered here as subtype 2, a new pattern, the acrodromous-basal-supranumerary pattern.

Diplusodon also has intersecondary veins (Fig. 3.4H¹ and 3.5C), referred to by Hickey (1988) as intermediary veins, among the secondary and tertiary veins that occur most commonly between the secondary veins in pinnate leaves. In Diplusodon marginatus, a fibrous marginal vein composed of the tertiary, quaternary, and lower-order veins forms a single vein that runs close to the leaf margin (Fig. 3.5G).

Leaf venation characters have long been used as an important source of taxonomic information in Diplusodon. To utilize such characters, however, it is necessary to determine how species fit into the patterns defined here. This requires detailed observations based not only on the external aspects of the leaves but also on their anatomy. Sometimes the leaf venation is very difficult to define, and for this reason, it is sometimes difficult to assign species to section. For example, D. punctatus Pohl in section Diplusodon (Koehne 1877, 1903; Lourteig 1989) has acrodromous leaf venation, D. astictus in section Diplusodon (Lourteig 1989) has eucamptodromous leaf venation, and D. panniculatus in section Penninerves (Koehne 1877, 1903; Lourteig 1989) has acrodromous leaf venation.

The Hickey classification of dicotyledonous leaf architecture (1973, 1988) applied to Diplusodon does not lead to obvious evolutionary interpretations, nor is there any other reference in which the Hickey types can be considered derived from one another. As noted, some patterns described in Diplusodon are classified differently because they occur in leaves with different forms. The venation patterns parallelodromous versus campylodromous versus acrodromous can be cited as an example.

The definitions are based fundamentally on two or more primary veins departing from a single point at or near the base of the leaf and all converging at a single point at or near the leaf apex. When the leaves themselves are narrow, the veins are almost parallel from the base to the apex. Similarly when the leaves are ovate with a strongly cordate base, the veins follow the shape of the blade before converging at the apex, thus forming arcs from the cordate leaf base. Therefore, variation in leaf shape sometimes can lead to complications in the interpretation of venation patterns.

3.4 Leaf Anatomy

Most species in Diplusodon are heliophytes and mostly inhabit areas subject to high temperatures and water stress. Consequently, they exhibit a series of xeromorphic features such as congested foliage with reduced leaves, coriaceous blades that are frequently concave with a revolute margin, and blades that are often covered by dense indumentum. Their leaf anatomy also reflects adaptation to xeromorphic habitats in some species (Table 3.1).

Table 3.1 List of anatomical characteristics for 22 species of Diplusodon

In cross-section, the Diplusodon leaf blade presents different outlines. They can be plane or almost plane, as in D. oblongus, D. glaucescens, D. marginatus, and D. rotundifolius (Fig. 3.6H–J, L); they can have a series of arcs, with furrows on the adaxial surface and protuberances on the abaxial surface as in D. heringeri, D. stellatus T.B.Cavalc., and D. minasensis Lourteig (Figs. 3.6F, G, K); they can be concave (Fig. 3.7B, C), as in D. candollei, D. hexander, and D. puberulus; or they can be conduplicate as in D. conduplicatus T.B.Cavalc. (Fig. 13.26D).

Fig. 3.6

Leaf anatomy in Diplusodon. A. Cross section, D. myrsinites (Krieger 8621, SPF). B. Cross section, D. candollei (Pereira et al. 932, BHCB). C, J. Cross section, D. marginatus (Pereira 1253, IBGE). D. Paradermal view, D. panniculatus (Mori et al. 16947, K). E, K. Cross section, D. minasensis (Cavalcanti et al. 220, SPF). F. Cross section, D. stellatus (Menezes 1209, SPF). G. Cross section, D. heringeri (Pedralli et al. 3366, CEN). H. Cross section, D. rotundifolius (Cavalcanti et al. 217, SPF). I. Cross section, D. glaucescens (Scatena et al. [CFCR] 10477, SPF). L. Cross section, D. oblongus (Cavalcanti et al. 87, SPF). A–C; F–L the same scale. Images made at Jodrell Laboratory, Royal Botanic Gardens, Kew, London, England

Fig. 3.7

Leaf anatomy in Diplusodon. A. Cross section, D. stellatus (Menezes 1209, SPF). B. Cross section, D. candollei (Pereira et al. 932, BHCB). C. Cross section, D. puberulus (Irwin et al. 22974, UB). A–C the same scale. Images made at Jodrell Laboratory, Royal Botanic Gardens, Kew, London, England

The leaf cuticle can vary from very thick and sinuous, as in Diplusodon minasensis and D. puberulus (Fig. 3.7C), or thick and almost straight, as in D. oblongus and D. candollei (Fig. 3.7B), to thin, as in D. glaucescens (Fig. 3.6I), D. helianthemifolius var. helianthemifolius, D. marginatus, D. myrsinites Mart. ex DC., and D. epilobioides. Eberlein (1904) considered D. epilobioides to have a striate cuticular surface, which was also observed in this study.

The epidermis is formed by one layer of generally isodiametric medium- to large-sized cells with thick walls and the anticlinal walls frequently curved. Interspersed among the normal epidermal cells, and more or less surrounded by them, are conspicuous mucilaginous cells usually larger than epidermal cells (Figs. 3.6E and 3.7B, C). In some cases, the mucilaginous cells are similar in size to normal epidermal cells and difficult to distinguish. Eberlein (1904) studied the foliar anatomy of ten species of Diplusodon and described the epidermis as one- or two-layered. He noted that mucilaginous epidermal cells occurred in all species examined and that, in some species, the mucilaginous cells could be recognized by their large size and their strong protrusion into the mesophyll. The presence of a second epidermal cell layer (hypodermis), referred to by Eberlein, was not observed in the species analyzed in this study. However some epidermal cells were seen to have divided periclinally with the innermost cell in the mesophyll becoming mucilaginous (Fig. 3.6E). The presence of mucilaginous cells is common in Lythraceae, and they are frequently mentioned as present in the leaves. Fahn (1989) refers to them as large, epidermal, cellular idioblasts containing mucilage. These cells have also been described in Lafoensia Vand. (Lourteig 1986), Koehneria S.Graham (1986), and Lourtella S.Graham (1987). Mucilaginous cells are most commonly found in the adaxial leaf epidermis, but they are also found in the abaxial leaf epidermis, as well as in the mesophyll or in the veins (Gregory and Baas 1989).

In leaf cross-sections of some Diplusodon species, it is possible to see mucilaginous cells in the adaxial epidermis (Fig. 3.7A–C) or on both adaxial and abaxial layers and also penetrating deeply into the mesophyll (Fig. 3.6G). In Diplusodon paraisoensis, in the interior of the mesophyll, a cavity surrounded by trichomes was observed. Possibly this is the secretory mucilage cavity referred to by Gregory and Baas (1989) as a mucilage cavity or canal originating by breakdown of groups of mucilage cells or mucilage idioblasts.

Mucilage cells in Diplusodon are frequently contracted, possibly the result of the xeric environment in which they grow. The idea that mucilage can serve to store water is defended by several authors (e.g., Solereder 1908; Haberlandt 1914; Gibson 1977; Gregory and Baas 1989). Others believe that the mucilage can reduce transpiration by acting as a protective layer on the leaf (Esdorn and Schanze 1954; Lyshede 1977, apud Gregory and Baas 1989).

The stomata in Diplusodon are most frequently seen on the abaxial epidermis; however, they also occur on the adaxial surface. They can be at the epidermal surface level, as in D. myrsinites (Fig. 3.6A), D. microphyllus Pohl, and D. virgatus (Eberlein 1904), or they can be sunken in small cavities, as in D. candollei (Fig. 3.6B), in which case they may or may not be protected by a dense covering of trichomes. The stomata pattern in Diplusodon is sufficiently variable that it does not constitute a good taxonomic character at the species level. The pattern can vary from paracytic, tricytic, or tetracytic to anomocytic on a single leaf. For example, the epidermis of D. panniculatus has tricytic, tetracytic, and actinocytic patterns, all within a small region of the epidermis (Fig. 3.6D). Eberlein (1904) described two, three, or rarely four subsidiary cells surrounding the stomata.

The mesophyll is generally composed of one or two layers of palisade-like parenchyma on the adaxial side, spongy parenchyma restricted to the central portion of the mesophyll, and one layer of palisade-like parenchyma on the abaxial surface. It is considered isobilateral (Figs. 3.6E, G and 3.7A).

Another pattern of isobilateral mesophyll is that found for Diplusodon oblongus where in cross-section the general condition is a uniform epidermal layer on both surfaces, resembling spongy parenchyma. On close examination, however, it is evident that the cells of the adaxial epidermal are vertically arranged like palisade parenchyma cells, while the cells of the abaxial surface are horizontally arranged (Fig. 3.6H, I, L). The leaves are interpreted as having an isobilateral mesophyll with a tendency toward dorsiventral arrangement. Leaves with clearly dorsiventral mesophyll are exemplified by D. candollei (Fig. 3.7C). Eberlein (1904) considered leaves in the majority of species to be dorsiventral, although the mesophyll sometimes shows a transition toward the isobilateral condition.

The species of Diplusodon examined in this study indicate that the structure of the leaf vascular bundles can be diagnostic for the species. The xylem region of the midvein is rich in mechanical elements, assuming a round shape (Figs. 3.6F–I and 3.7A) or an open arc (Fig. 3.6J), and it is accompanied by poorly developed internal phloem and well-developed external phloem. Sometimes the internal phloem is surrounded by a sheath of sclerenchymatous fibers (Fig. 3.6H). In other species, such as D. candollei, D. minasensis, and D. oblongus, the xylem is poorly developed internally and externally and has few mechanical elements (Fig. 3.6K–L).

As previously mentioned, the leaf venation of Diplusodon can be prominent or inconspicuous. Initially I thought that the prominence might be due to the presence of a well-developed sclerenchymatous bundle sheath. However, I found that this is the result of a proliferation of parenchymatous cells abaxial to the vascular bundles. This is demonstrated in leaves of D. minasensis, in which the veins are very prominent macroscopically, but the vascular bundle is poorly developed (Fig. 3.6K).

Other features recorded from leaves of Diplusodon are mucilage cells in the parenchyma of the veins and, frequently, calcium oxalate crystals (druses) in the mesophyll.

3.5 Inflorescences

Inflorescences of all Diplusodon species were dissected and observed using a standard stereoscopic microscope. Observations were made mainly on herbarium material, although living plants were also examined when available. Inflorescence descriptions and terminology are based on the typological system proposed by Troll (1964, 1969; see also Weberling 1965, 1988, 1989; Weberling et al. 1993).

3.5.1 Typology

In Diplusodon, the synflorescences are polytelic. Two types of inflorescences are found: simple synflorescences with the axis of the inflorescence unbranched (Fig. 3.8A) and compound synflorescences with the arrangement of the individual flowers of the inflorescence simple and repeated in the branching of the inflorescence (Fig. 3.8B–E).

Fig. 3.8

Inflorescences in Diplusodon. A. Botryum. B. Diplobotryum. C. Triplobotryum. D. Diplothyrse. E. Diplothyrsoid. F. Dichasial cyme. AcB Accessory branch, Pc 1 Paraclade of first order, Pc 2 Paraclade of second order, MF Main florescence, PF Parcial florescence, TF Terminal flower

In compound synflorescences, paraclades (hereafter “Pc”) may or may not develop below the main florescence (MF). Leaves of Diplusodon are opposite, verticillate, and fasciculate, and normally paraclades arise from both axils at each node.

Troll (1964) subdivided the synflorescence into the following parts: zone of enrichment, for the area that produces paraclades; zone of inhibition, for the area that precedes the zone of enrichment where the growth of paraclades is inhibited; and zone of innovation, exclusive of perennial plants, for the basal portion of the stem in which axillary buds do not develop within the same season, but produce new branches in the next season. The three zones together form the hypotagma. According to Weberling (1988), in woody plants, especially in those that grow under environmental stress, the inflorescence can be reduced, or the zonation of the flowering system can be extremely altered. In some species of Diplusodon, flowering can occur at a stage in which the vegetative part of the plant is practically reduced to an underground system, that is, plants of 3–10 cm can produce many flowers (paedomorphic flowering). Such paedomorphic flowering is the result of an earlier initiation of the inflorescence development in comparison with related species, and the extent to which the onset of the reproductive phase is initiated seems to be controlled by environmental disturbances such as fire. Good examples of this are D. ciliiflorus Koehne and D. pygmaeus T.B.Cavalc., found in the campos rupestres of Diamantina (state of Minas Gerais, Brazil) and Chapada dos Veadeiros (state of Goiás, Brazil), respectively, two regions subject to frequent fires, where populations were found in full flowering and with complete reduction in the inhibition zone. A similar behavior has been observed in other plants of the campos rupestres (rocky outcrops) and cerrados, such as species of Euphorbia (Cordeiro 1986) and Camarea (Mamede 1988).

The variation in the relative sizes of the different zones of the flowering systems is great, and it is responsible for the great diversity in inflorescences. With respect to this variation in Diplusodon, Troll (1970) mentions that in D. thymifolius Mart. ex DC. and D. virgatus var. virgatus, the main florescence (terminal on the main axis) remains very small in relation to the paracladial zone, while in other species, such as D. villosissimus Pohl and D. villosus, the principal florescence predominates even though the paracladial zone can also be well developed.

On the basis of the degree of branching, the synflorescences can be classified on how many times the paraclades are repeated. A double raceme, an inflorescence simply branched once with paraclades of the first order, is called a diplobotryum (Fig. 3.8B), a triple raceme with paraclades of the first and second order is a triplobotryum (Fig. 3.8C), and those with multiple branching orders (more than three) are called, in the case of racemes, pleiobotrya. In addition to paraclades, in synflorescences the inflorescence of the main axis, or main florescence, is also distinguished. It displays the characteristic features of the inflorescence type, and it is used as the basis for defining and classifying the synflorescence as a whole. The principal florescence is situated above the paraclades, in the distal part of the inflorescence axis (Fig. 3.8B–D). When the flowers of an inflorescence are provided with two prophylls, as in the case of Diplusodon, additional flowers can appear in the axils of these prophylls, producing what Troll calls a partial inflorescence, morphologically defined and referred to as a cyme.

Inflorescences of Diplusodon are generally polytelic with individual flowers organized in botrya (or racemes). In the species with synflorescences, the principal florescence is, with rare exceptions, also a botryum (Fig. 3.8B, C). All the polytelic species of Diplusodon that have synflorescences having diplo- or triplobotrya bear a main florescence, which for this reason are called heterothetic compound racemes, in contrast to the polytelic synflorescences that are composed only of lateral racemes that lack a principal florescence; the latter are called homothetic compound racemes. Homothetic compound racemes do not occur in Diplusodon. Florescences of the synflorescence, which often appear in Diplusodon, can be spike-like botrya when the flowers are nearly sessile (ca. 1 mm peduncle) or true spikes when the flowers are sessile. Florescence anthesis occurs from the base to the apex (acropetally).

In many groups of plants, it is difficult to make a distinction between the region where the inflorescence begins and the vegetative region ends. There are some cases in which the flowering region is distinct from the vegetative region, and in this case, the inflorescence is said to be bracteose. The foliage can also show a gradual reduction, then forming a frondose-bracteose inflorescence, or it can lack any significant reduction in shape and/or size, then termed a frondose inflorescence (Weberling 1989). In all the species of Diplusodon that possess simple inflorescences, they are of the frondose type. Among the species with synflorescences, they are frondose to frondose-bracteose in the majority of cases or bracteose, as in D. panniculatus, D. bradei, and D. ramosissimus.

In addition to the polytelic type of inflorescence, described as the basic pattern for Diplusodon, two species were found with an unusual form: D. ovatus Pohl (Fig. 15.3F) and D. panniculatus. The inflorescences of these species show that, unlike all other species of the genus and contrary to the definition of the group as polytelic (Weberling 1988), the principal axis of the inflorescence terminates in a flower, i.e., it is monotelic (Fig. 3.8E, F). This also applies to all the paraclades. In these inflorescences, the differentiation, development, and anthesis of the terminal flowers (TF) always precede those of the paraclades, whereas paraclades of the same branching order develop basipetally. Such an inflorescence is called a thyrsoid (a thyrse-like inflorescence with terminal flowers, according to Troll 1969; Briggs and Johnson 1979; Weberling 1988). The distal paraclades (“short paraclades,” cf. Troll 1965; Weberling et al. 1993) of such inflorescences are cymose-dichasial, so that the whole structure is a thyrse-like inflorescence with a terminal flower, hence a “thyrsoid” (Troll 1969; Briggs and Johnson 1979; Weberling 1988, 1989). Such cymose short paraclades are initially triads but are able to produce further lateral flowers in the axils of the two prophylls of each new axis (Fig. 3.8D–F).

It has been proposed elsewhere (Sell 1969, 1976; Weberling 1988) that the polytelic type of inflorescence has been derived more than once from the monotelic type during the evolution of the angiosperms. It has been hypothesized to have occurred by loss of the terminal flower and specialization of the paraclades from the monotelic system into solitary lateral flowers or lateral cymes that make up the elements of the florescence in the polytelic system. The monotelic synflorescence is a character that seems to have evolved at least four times within Lythraceae (Graham et al. 2005). Secondary monotelic synflorescences are reported, representing one or more additional transitions from polytely to monotely (Cavalcanti and Rua 2008; Inglis and Cavalcanti 2018), challenging the general view of polytelic inflorescences as derived from monotelic ones (Weberling 1985, 1989).

Most Lythraceae genera, including Diplusodon, have polytelic synflorescences (Weberling 1988; Graham et al. 1993), which represents the plesiomorphic condition of the family (Graham et al. 2005). Six genera, Duabanga Buch.-Ham., Sonneratia L.f., Punica L., Lawsonia L., Lagerstroemia, and Galpinia N.E.Br., have monotelic inflorescences, a condition that seems to have evolved at least four times within the family (Graham et al. 2005). Secondary monotelic synflorescences are here reported, which represent one or more additional transitions from polytely to monotely. Such a transition has been hypothesized to be the outcome of a rather complex evolutionary process, involving the specialization of distal paraclades (homogenization) together with the onset of acropetal flowering (racemization) and the loss of the terminal flower (truncation; Sell 1969, 1976). Thus, the regain of monotely has been implicitly considered by classical morphologists to be a very improbable evolutionary event. Nevertheless, current evidence from developmental genetics suggests the transition from monotely to polytely and vice versa is mediated by a simple regulatory genetic system (Kellogg 2000; Reinhardt and Kuhlemeier 2002; Angenent et al. 2005), so that a single mutation in the proper regulatory gene could suffice to explain the multiple reversions to monotely in Lythraceae (Cavalcanti and Rua 2008).

In the partial florescences of Diplusodon bradei, the very short axes produce flowers continuously, resulting in dense glomerules. In D. bradei, D. lanceolatus Pohl, and D. ulei Koehne, for example, accessory branches (AcB), sensu Weberling (1988), develop in the axils of the bracts, in the same position where the paraclades originate (Fig. 3.8D). Accessory branches were considered by Weberling (1988) to be very common in Lythraceae and were identified in this study for the first time in Diplusodon. According to Weberling (1988), the accessory branches increase the number of flowers in the inflorescence and can be from the first to the “n” order. In D. bradei and D. lanceolatus, they are up to the third order and can form solitary flowers or dichasia whose triads continue to develop flowers in the axil of their prophylls, repeating the same pattern found in the paraclades. A higher degree of this branch formation can also be seen at the base of the synflorescence and can occur within the paraclades and even in the adjacent accessory branches.

The types of inflorescences present in Diplusodon can be organized according to the terminology of Weberling (1988, 1989), to account for the change from the monotelic to the polytelic pattern, through the loss of the terminal flower, specialization of the paraclades, and the trend toward reduction of the number of flowers. Thus, the following inflorescence types have been observed in Diplusodon:

• 1 – Thyrsoid: monotelic synflorescence (with terminal flower) whose branches (long paraclades) bear short cymose branches (short paraclades) or triads (Fig. 3.8E, F)

• 2 – Thyrse: polytelic synflorescences (without terminal flower) whose branches (long paraclades) bear short cymose branches (short paraclades) or triads (Fig. 3.8D)

• 3 – Pleiobotrya, triplobotrya, or diplobotrya: polytelic synflorescences (without terminal flower) whose polytelic branches (florescences) bear solitary lateral flowers (Fig. 3.8B, C)

• 4 – Botrya: simple inflorescences (unbranched) with only lateral flowers (Fig. 3.8A)

3.5.2 Evolution and Diversification

In the phylogeny presented to Diplusodon (Cavalcanti and Rua 2008; Inglis and Cavalcanti 2018), some hypotheses can be offered regarding inflorescence evolution and diversification within Diplusodon. The plesiomorphic inflorescence pattern, i.e., frondose or frondose-bracteose compound racemes, seems to have been highly conserved. Alternative patterns, such as thyrses and thyrsoids, are restricted to a few species.

Changes involving meristem-identity genes (Coen and Nugent 1994; Angenent et al. 2005) are probably responsible for several striking features in inflorescences of some Diplusodon species, such as the switch to monotely, the production of high-order lateral cymes, and the proliferation, i.e., the return to vegetative growth, of the apical meristem of the racemes. On the other hand, paedomorphic flowering seems to be the outcome of a change in flowering-time genes (Coen and Nugent 1994; Angenent et al. 2005).

Another set of modifications involves variations in internode elongation and in relative development of subtending leaves. At the time a shoot apical meristem (SAM) is induced to flowering, some internodes, leaf primordia, and axillary meristems are already differentiated, whereas the meristematic apex remains undifferentiated. Both preformed and neoformed (after induction) internodes can either elongate or remain short. Differences in the distribution pattern of elongated and short internodes are largely responsible for striking variations in inflorescence appearance yet without deep structural modifications. The same is true for the hypopodium/epipodium elongation pattern. The plesiomorphic condition of elongated internodes has been repeatedly lost during inflorescence evolution of Diplusodon, and reduced (strictly, non-elongated) internodes are widespread within the genus. In the most radical cases, highly congested inflorescences arose (Cavalcanti and Rua 2008).

Development of leaf primordia can be more or less altered after flowering induction, generally leading to a gradual reduction of inflorescence leaves: the more incipient the development of a leaf primordium at induction, the less developed the resulting mature leaf. It seems to be the case of most species of Diplusodon that a gradual reduction from foliage leaves to bracts can be observed along the synflorescence main axis. The occurrence of strictly bracteose synflorescences is apomorphic within Diplusodon. In such synflorescences, the transition between foliage leaves and bracts is abrupt. Whether it results from a sudden suppression of pre-existing leaf primordia or from the fact that the flowering region is entirely produced from previously undifferentiated meristematic tissues is yet to be explored (Cavalcanti and Rua 2008).

3.6 Bracts, Prophylls, and Peduncles

In the frondose inflorescences of Diplusodon, bracts are found that are very similar morphologically to vegetative leaves, although they are sometimes smaller. The reduction in size is greater and gradual in species with frondose-bracteose inflorescences and abruptly evident in those with bracteose inflorescences whose bracts are sharply distinct from foliage leaves. The bracts can subtend both solitary flowers, in botrya, such as those of D. imbricatus Pohl, or in dichasial cymes, as in the thyrsoids of D. panniculatus (Fig. 3.8F).

The prophylls in Diplusodon are always opposite, and in the majority of cases they are distinct in form from the foliage leaves and bracts. They are variable in form and indumentum (Fig. 3.9), showing the same diversity as foliage leaves and bracts. The relationship of prophyll length to floral tube length is used as a taxonomic character in Diplusodon and represented by the following variants: prophylls reaching the apex of the floral tube (Fig. 3.9C, G, I, L); prophylls surpassing the apex of the floral tube (Fig. 3.9A, E); prophylls reaching to the middle of the floral tube (Fig. 3.9B, J); and prophylls reaching the base or less than the base of the floral tube (Fig. 3.9F, H).

Fig. 3.9

Flowers in Diplusodon. A. D. paraisoensis (Cavalcanti et al. 390, CEN). B. D. helianthemifolius var. helianthemifolius (Longhi-Wagner et al. [CFCR] 9325, SPF). C. D. glaucescens (Scatena et al. [CFCR] 10477, SPF). D. D. speciosus (Cavalcanti et al. 86, SPF). E. D. rotundifolius (Menezes et al. [CFCR] 10600, SPF). F. D. nitidus (Hatschbach & Nicolack 53031, MBM). G. D. sessiliflorus (Philcox & Onishi 4782, UB). H. D. panniculatus (Romaniuc & Sajo 388, CEN). I. D. minasensis (Cavalcanti et al. 220, SPF). J. D. imbricatus (Pereira-Silva et al. 10648, CEN). K. D. cordifolius (Irwin et al. 24807, UB). L. D. punctatus var. punctatus (Cavalcanti et al. 1015, CEN)

The pedicel is regarded in this study as composed of the epipodium, the axis between the flower and the last node of the axis where the flower terminates; the anthopodium (of Briggs and Johnson 1979) together with the hypopodium, the proximal axis that precedes the epipodium (the peduncle of Briggs and Johnson 1979).

In Diplusodon, the peduncle is variable in size and can be articulated, that is, composed of hypopodium, mesopodium, and epipodium, or it may be composed only of the hypopodium. Two prophylls always occur on each peduncle. To evaluate the specializations of the inflorescence in Diplusodon, the tendency for reduction seen at the level of the peduncles needs to be analyzed further.

In the dichasial lateral cymes, peduncles can be seen whose prophyllar buds are able to further produce two flowers. Departing from this pattern, and according to Weberling (1988), the probable sequence of evolutionary events in Myrtales inflorescences involves a reduction from dichasium to cymes of triads, leading to a simple raceme, and finally to a spike, as found in the paraclades of Diplusodon lanceolatus. In this species the flowers are sessile, totally lacking a pedicel (lacking differentiated hypopodium and epipodium). This last pattern is further reduced in simple botrya, without branching in the inflorescence other than production of floral axes.

In addition to the pattern of dichasial lateral cymes, the occurrence of two axillary peduncles initiated from the same node was also observed in Diplusodon ulei and D. ciliatus (T.B.Cavalc.) T.B.Cavalc. This suggests that a reduction in the inflorescence of Diplusodon from a more complex pattern to simple botrya and spikes has taken place.