Tradescantia subg. Austrotradescantia (D.R.Hunt) M.Pell., PhytoKeys 89: 47. 2017.
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|Tradescantia subg. Austrotradescantia (D.R.Hunt) M.Pell., PhytoKeys 89: 47. 2017.|
Tradescantia sect. Austrotradescantia D.R.Hunt, Kew Bull. 35(2): 440. 1980.
Tradescantia fluminensis Vell.
Herbs chamaephytes, base definite or indefinite, perennial, frequently succulent, terrestrial, rupicolous or epiphytes. Roots thin, fibrous. Stems prostrate with ascending apex or erect, herbaceous to succulent, rarely fibrous, little to densely branched, rooting at the basal nodes or at the distal ones when they touch the substrate. Leaves sessile to subpetiolate; distichously or spirally-alternate, evenly distributed along the stem, rarely congested in a rosette; sheaths closed; ptyxis involute or convolute; blades flat to falcate and/or complicate, base asymmetrical, midvein conspicuous, rarely inconspicuous, adaxially impressed, abaxially prominent, rounded, secondary veins conspicuous or inconspicuous. Synflorescences terminal or axillary in the distal portion of the stems, composed of a solitary main florescence, 1-4 per leaf axis. Inflorescences (main florescences) consisting of a pedunculate double-cincinni fused back to back, sometimes composed of 1 –3(– 5) cincinni; inflorescence bract hyaline, tubular, inconspicuous; peduncle bracts absent; supernumerary bracts rarely present; cincinni bracts leaf-like, rarely spathaceous, differing from the leaves mostly only in size, similar or unequal to each other, saccate or not, free from each other; cincinni sessile, contracted, opposite to subopposite; bracteoles inconspicuous, imbricate, linear-triangular to triangular, hyaline. Flowers bisexual, actinomorphic, flat (not forming a floral tube); pedicel gibbous at apex, upright at anthesis and pre-anthesis, deflexed at post-anthesis; sepals equal, free, chartaceous, ovate, dorsally keeled or not, apex acute; petals sessile, equal, free, elliptic to ovate to broadly ovate, flat or plicate, base cuneate to obtuse, margin glabrous, apex acute; stamens 6, arranged in two series, equal, filaments free from the petals, straight at anthesis and post-anthesis, white, rarely pink, basally densely bearded with moniliform hairs, hairs as long as the stamens, white, anthers basifixed, rimose, connective rhomboid, yellow, anther sacs ellipsoid, yellow, pollen yellow; ovary subglobose to globose, white, glabrous, locules 2-ovulate, style straight at anthesis and post-anthesis, white, obconical at base, conical at the apex, stigma punctate, pistil longer than or the same length as the stamens. Capsules subglobose to globose, light to medium brown when mature, glabrous, loculicidal, 3-valved, sometimes apiculate due to persistent style base. Seeds 1-2 per locule, ellipsoid to narrowly trigonal, ventrally flattened, cleft or not towards the embryotega, testa costate to rugose with ridges radiating from the embryotega; embryotega dorsal, relatively inconspicuous, without a prominent apicule, generally covered by a cream farina; hilum linear, on a weak ridge.
Habitat, distribution and ecology.
Tradescantia subg. Austrotradescantia is distributed from southern Bolivia, Paraguay, Argentina, Southeastern to Southern Brazil and Uruguay (Fig. 1). Its species can be found growing understorey in moist and shady forests, open fields, rocky outcrops and are especially common in disturbed areas ( Pellegrini 2015, 2017; Pellegrini et al. 2016).
Conservation and invasiveness.
Tradescantia subg. Austrotradescantia as a whole seems to be in need of little conservationist attention. Most species possess wide native distribution, with only T. atlantica , T. chrysophylla , T. hertweckii and T. seubertiana possessing narrower distributions and thus meriting some conservationist attention. Out of the 13 species accepted in the present study, one (i.e. T. fluminensis ) is already known to represent a troublesome weed worldwide, while the other species have never been considered invasive or to possess an invasive potential. Nonetheless, after careful examination of herbarium specimens collected outside the subgenus native distribution range, I came to the conclusion that several records of T. fluminensis as a weed actually represent misidentified specimens of T. mundula or, more rarely, specimens of T. cymbispatha and T. crassula . Tradescantia cerinthoides is also widely cultivated worldwide, especially its pink and lilac-flowered morphs. Despite my not having observed any unquestionable records that indicate that T. cerinthoides has escaped from cultivation, this species also possesses intense vegetative growth and thus a great potential to become an invasive species if not properly monitored. All invasive accessions done so far for T. fluminensis must urgently be redone in order to properly understand extension of this species’ invasion, discount records now known to represent other species from the subgenus and to appropriately access the threat of invasiveness of the other species of T. subg. Austrotradescantia which also possess records outside their native range.
The name of this subgenus means " Tradescantia from the South", making reference to its exclusively South American distribution.
Tradescantia subg. Austrotradescantia is a morphologically peculiar group in Tradescantia , being easily recognised by its generally distichously-alternate leaves, sepals ovate, generally all dorsally keeled; filaments basally densely bearded with long moniliform hairs; style obconic at base and conic at apex, stigma punctate with type D papillae; seeds with costate testa and relatively inconspicuous embryotega; small bimodal and numerous chromosomes (n = 10-numerous); and a unique chemical profile ( Pellegrini 2017). Two morphological groups are accepted for the subgenus, both supported by recent phylogenetic studies ( Pellegrini 2017). The T. fluminensis group is composed of generally more delicate plants, with prostrate stems with ascending apex, indefinite bases, subpetiolate leaves, cincinni bracts saccate at base and white petals. Nonetheless, this group also includes the T. tenella complex, which possesses erect stems, definite base, flowers that range from white to pink, seeds with rugose testa and hilum shorter than half the length of the seeds. The species in the T. fluminensis group occur almost exclusively in Tropical and Subtropical Rainforests but are also commonly found growing as weedy plants throughout their distribution range ( Pellegrini 2017). The T. crassula group is composed of succulent plants, with erect stems, complicate leaves, cincinni bracts not saccate at base, petals ranging from white to pink to lilac and seeds cleft towards the embryotega. These species are intimately related to the two southern domains of South America, characterised by open and/or drier vegetation formations: the Chaco (which is part of the Dry Diagonal) and the Pampa (which is mostly represented by grasslands). The species from the T. crassula group are morphologically very similar due to many overlapping morphological characters; with indumentum type and distribution in the sepals being the most useful character for separating its species.
Recommendations for field collectors.
As widely known, Commelinaceae is a group where flowers are generally poorly preserved in herbarium specimens, making it especially difficult to work with (Faden 1991). In T. subg. Austrotradescantia , taxonomy relies greatly on indumentum characters, with its type, distribution and colouration being especially important. Moreover, the indumentum in the pedicels and sepals seems to be constant within the same species and variable between different species. Thus, I recommend that field workers pay special attention to the plant’s indumentum, collect young and mature branches in order to correctly characterise the species (since features like the presence of a subpetiole and shape of the leaf-blades might vary during development), record the colouration of vegetative and reproductive organs and, whenever possible, attach photographs to the herbarium sheets. Images are of great aid and can help identify even the most incomplete and damaged specimens. Furthermore, whenever possible, spirit collections and live specimens for cultivation are welcome, since they enable the proper study of the delicate flowers of these plants and live specimens might help us understand their morphological variation and plasticity.
Roots, stems and growth forms.
As stated by Pellegrini (2017), most Tradescantia species are perennial herbs, all of them lacking rhizomes. In T. subg. Austrotradescantia , the roots are always thin and fibrous and never tuberised as in many species of the other four subgenera. In the mat-forming species, roots are produced throughout the stems, whenever they touch the substrate. In the species with erect stems, roots are restricted to the basal-most nodes of the plants. The stems can vary in posture, from prostrate (generally with ascending apex) to erect, while the branching pattern ranges from unbranched to little branched at the base or branched to densely branched in the upper half. The leaf-opposed line of uniseriate hairs is generally observable in most species, except T. seubertiana which is completely glabrous. Nonetheless, it is not constant in any of the remaining species, being either present or absent, depending on the specimen or even on the maturation and/or position of the stem (i.e. younger shoots tend to produce this leaf-opposed line of uniseriate hairs, which is generally lost with age). Finally, in species densely covered by indumentum, such as T. cerinthoides , T. chrysophylla , T. cymbispatha and T. mundula , the leaf-opposed line of uniseriate hairs is generally absent or, if present, is very hard to differentiate from the dense surrounding indumentum.
The indumentum in T. subg. Austrotradescantia is consistently composed of uniseriate hairs, ranging from eglandular to glandular (Fig. 2 C–F). Special attention should be also given to the prickle-hairs that are generally 2-celled and present at the margins of the leaf-sheaths and blades (Fig. 2B, E), but consistently absent only in T. seubertiana (Fig. 2A). Glandular hairs can be found in the vegetative organs of T. cerinthoides but are consistently found in the pedicels of most species and the sepals of many (Fig. 2F). As aforementioned, indumentum morphology is key for species delimitation in T. subg. Austrotradescantia , being easily observed most of the time. Nonetheless, some species are especially susceptible to the excessive heat of some plant driers, which can lead to an artificial loss of hairs. Two good examples are T. cymbispatha and T. mundula , where the first has been observed to sometimes lose its characteristic dense strigose indumentum in some excessively dried specimens from Argentina and Southern Brazil, while the second presents delicate sepal hairs that commonly fall dur ing the drying process. However, due to the almost unique combination of morphological characters (e.g. sessile and succulent leaves, with generally elliptic blades and sepals lacking dorsal keels and evenly velutine), T. cymbispatha can be easily identified even when the vegetative hairs are almost completely lost during the excessive drying process. In T. mundula , this partial loss of sepal hairs can lead to confusion in the identification of dried specimens, making it even more similar to T. fluminensis s.s.
The leaves in T. subg. Austrotradescantia can range from distichously- to spirally-alternate, sometimes in the same species (e.g. most species belonging to T. crassula group), from sessile to distinctively subpetiolate (Fig. 3), but always presenting an asymmetric base (Fig. 3). The blades tend to decrease in overall size towards the apex of the stem, while subpetioles decrease in length. Alternatively, the basal-most leaves tend to possess wider bases when compared to the leaves towards the apex of the stem. Subpetioles are restricted to some species in the T. fluminensis group, with sessile leaves being plesiomorphic in Commelinaceae ( Pellegrini 2017). The apex of the leaf-blades is generally acute but can also range from acute to caudate or obtuse (Fig. 3). The base of the leaf-blades can range from cuneate to obtuse to cordate or from truncate to amplexicaulous (Fig. 3). Finally, the ptyxis can either be convolute or involute, with convolute leaves being restricted to the T. crassula group and involute leaves restricted to the T. fluminensis group.
Striped leaves have long been observed, described and utilised for species delimitation in the family. In some species (e.g. T. soconuscana Matuda and T. zebrina Heynh. ex Bosse and their relatives), it is a marking feature that greatly aids their recognition ( Pellegrini 2017). According to my anatomical observations and ongoing studies on the matter, the silver stripes in Commelinaceae seem to consist of aerenchymatous tissue that might help understorey plants increase the amount of light they are able to absorb, by directing light through reflection to the inside of the chlorophyllate parenchyma (unpublished data). Nonetheless, this feature has been observed not to be constant in most taxa of the family, being environmentally controlled in at least Buforrestia C.B.Clarke, Dichorisandra J.C.Mikan, Floscopa Lour., Plowmanianthus Faden & C.R.Hardy and Siderasis Raf. emend. M.Pell. & Faden (pers. observ.). In Tradescantia , this feature seems to be phylogenetically related (at least to some degree), since it is only known to occur in T. subg. Campelia ( Pellegrini 2017). In T. subg. Austrotradescantia , a myriad of cultivars of several species are known for their variegated leaves (e.g. T. cerinthoides , T. decora , T. fluminensis and T. mundula ; Fig. 4), but also found in members of different subgenera [e.g. T. spathacea Sw., T. zanonia (L.) Sw. and T. zebrina - T. subg. Campelia -, T. pallida (Rose) D.R.Hunt and T. sillamontana Matuda - T. subg. Setcreasea -, but it is unknown to me to occur in T. subg. Mandonia and T. subg. Tradescantia ; pers. observ.]. However, this variegation does not seem to be homologous to the silver stripes commonly observed in other members of the family. These stripes do not seem to be produced by the concentration of aerenchymatous tissue, but actually seem to be caused by the loss of pigmentation in the leaves. They actually look similar to the symptoms caused by some strains of the tulip breaking virus (family Potyviridae ) in the perianth of Liliales ( McKay and Warner 1933; McWhorter 1938) and more precisely to symptoms caused the commelina yellow mottle virus ( Badnavirus spp., family Caulimoviridae ), being generally malefic to the infected plants ( Hull 2007; Valverde et al. 2012). These white stripes (sometimes also yellow or pink to vinaceous, due to the presence of secondary pigments; Fig. 4) are not commonly observed in natural populations, being almost exclusively recorded in cultivated plants (pers. observ.). During cultivation in the greenhouses of the Jardim Botânico do Rio de Janeiro, some specimens not originally striped, were observed to acquire such features. The appearance of white to yellow stripes in the leaves occurred shortly after a great aphid and mealybug infestation struck the live collection. Specimens from different genera, such as Commelina and Floscopa also acquired such stripes. After some months, the affected specimens either withered and died or survived and lost the stripes (pers. observ.). This pattern is coherent with the one observed in the transmission and spread of viruses from families Potyviridae , Caulimoviridae and other plant infecting families that cause mosaic and breaking patterns in plants ( McKay and Warner 1933; McWhorter 1938; Andret-Link and Fuchs 2005; Hull 2007; Valverde et al. 2012). Thus, in the present study, striped specimens of T. subg. Austrotradescantia are disregarded, being considered merely as sick plants or artificially selected morphotypes of no taxonomic relevance.
The inflorescence architecture in T. subg. Austrotradescantia follows the double-cincinni pattern, as described by Panigo et al. (2011) and indicated by Pellegrini (2017) as characteristic to Callisia s.l., Tradescantia and Tripogandra s.l. The cincinni bracts are always frondose, being leaf-like in most species but spathaceous in T. decora and T. umbraculifera (Fig. 5C, D, I). The base of the cincinni bracts can be saccate or not, with saccate bracts being synapomorphic to the T. fluminensis group ( Pellegrini 2017). Supernumerary bracts are rare in T. subg. Austrotradescantia , being exclusively recorded for T. decora . The cincinni are always sessile, contracted and fused back to back. Due to great reduction in the main florescence, the cincinni seem opposite in most specimens. Nonetheless, in some specimens, a malformation in the inflorescence can cause the internodes between the cincinni to elongate, thus producing subopposite cincinni (Fig. 5A; Pellegrini 2017). Most inflorescences are composed of two cincinni, as the double-cincinni architecture would suggest. Nonetheless, some exceptions are recorded, with T. crassula producing perfect double-cincinni, but also commonly producing axillary inflorescences composed of a solitary cincinnus (Fig. 5B; Pellegrini 2017). Furthermore, T. decora is the only species in T. subg. Austrotradescantia to regularly present main florescences with more than two cincinni, generally producing main florescences with 2 –3(– 5) cincinni (Fig. 5C, D; Pellegrini 2017).
The flowers in T. subg. Austrotradescantia are always flat (i.e. not forming a floral tube), with flowers being held in an upright position at pre-anthesis and anthesis and later acquiring a deflexed position at post-anthesis and fruiting (Fig. 6F; Pellegrini 2017). As with the other species of Tradescantia , the species of T. subg. Austrotradescantia possess scentless flowers ( Pellegrini 2017) and, like all species of Commelinaceae , completely lack nectaries of all kinds (Faden 1992). A marking floral conservatism can be easily observed in T. subg. Austrotradescantia , with gross floral morphology presenting little taxonomic relevance for species delimitation in the subgenus ( Pellegrini 2017). Some taxonomic relevance can be given to the shape of the floral buds (that can be of great help in differentiating closely related taxa) and floral diameter (which can also help differentiate some species).
Aside from the pubescence, little variation is observed in the sepals of T. subg. Austrotradescantia . The sepals are always equal, free, chartaceous, ovate, with acute apex. They can range from medium to dark green or from purple to vinaceous to dark vinaceous and can be dorsally keeled or not (Fig. 6 A–C; Pellegrini 2017). Measures can be of some help, but they do commonly overlap between closely related species.
The deliquescent petals are also quite homogeneous in T. subg. Austrotradescantia , being always sessile (i.e. without a claw, like some species of T. subg. Campelia , T. subg. Mandonia and T. subg. Setcreasea and all species of T. subg. Tradescantia ; Pellegrini 2017), equal, free, elliptic to ovate to broadly ovate, with cuneate to obtuse base, glabrous margin and acute apex. Little colour variation is observed, especially when compared with T. subg. Campelia and T. subg. Tradescantia . All species possess predominantly white petals, with the exception of T. seubertiana , which always possesses pink petals. Nonetheless, specimens also presenting petals in different hues of pink or lilac can sometimes be observed in T. cerinthoides , T. cymbispatha , T. decora , T. tenella and T. tucumanensis . Finally, petals in T. subg. Austrotradescantia can either be flat or plicate. The plicate petals are caused by a fold along the petals’ midvein and are exclusively found in T. atlantica M.Pell. sp. nov. and T. fluminensis s.s. (Fig. 6 D–F).
As shown by Pellegrini (2017), all species of T. subg. Austrotradescantia possess filaments densely bearded at the base with moniliform hairs, these hairs being as long as the stamens or at least the filaments; anthers basifixed, with expanded, rhomboid and yellow connectives, divergent and elliptic anther sacs and yellow pollen grains in vivo (Fig. 6F). This pattern is exclusive to T. subg. Austrotradescantia ( Pellegrini 2017), but of almost no relevance in differentiating the species within the subgenus.
The gynoecium in T. subg. Austrotradescantia presents a highly conservative morphology, being always white and glabrous, the ovary ranging from subglobose to globose, the locules 2-ovulate, ovules anatropous with axial placentation, the style being always straight at anthesis and post-anthesis, obconical at base, conical at the apex and culminating in a reduced and punctate stigma ( Pellegrini 2017). Finally, the pistil, as a whole, can either be much longer than or approximately the same length as the stamens.
Capsules and seeds.
Fruit morphology is extremely conservative in Tradescantia , being characterised by light to medium brown, thin-walled, loculicidal capsules, subglobose to globose in shape, externally glabrous and smooth and sometimes apiculate due to the persistent base of the style. The capsules are always 3-valved (Fig. 7A), with the only known exception in the genus being T. orchidophylla Rose & Hemsl., which seems to exclusively present a 2-locular gynoecium and, consequently, producing 2-valved capsules. Each locule can produce up to two seeds, which will directly influence their shape. The seeds range from ellipsoid to narrowly trigonal, commonly with a more truncate side if two seeds are produced in the same locule, ventrally flattened, cleft or not towards the embryotega (with cleft seeds being synapomorphic to the T. crassula group; Fig. 7B) and the testa can be either costate (Fig. 7B, C) or rugose, with ridges or pits radiating from the embryotega (i.e. rugose testa being exclusive to the T. tenella species complex; Fig. 7D). The embryotega is dorsal and relatively inconspicuous, without a prominent apicule, being generally covered by a cream farinae (Fig. 7 B–D). The hilum is linear, located on a mild ridge and can vary in length depending on the species group: (1) longer than ½ the length the seed in the T. crassula group (Fig. 7B); (2) ca. equal to ½ the length of the seed in most species of the T. fluminensis group (Fig. 7C); and (3) shorter than ½ the length of the seed in the T. tenella species complex (Fig. 7D).
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