Raveniopsis microphyllus K.Wurdack, sp. nov., a new species known only from a single peak in the Pakaraima Mtns. of Guyana, is described and illustrated. This white-flowered shrub adds to the many narrow-endemic Guiana Shield species in the genus, and is unique in bearing small, trifoliate, sclerophyllous leaves. Leaf anatomy and surface micromorphology of the new species were examined to document its montane adaptations. The multiple trichome types of the leaves and flowers of new species were characterized, and the systematics value of the considerable foliar trichome variation in Raveniopsis is discussed.

Guiana Shield, Guyana, Kamakusa Mtn., leaf anatomy, Raveniopsis , Rutaceae , trichomes

The Rutaceae of the Guiana Shield region of northern South America contains about 60 species (nearly half endemic) in 20 genera (Kallunki 2005). Of those genera, three (Apocaulon R.S. Cowan, Decagonocarpus Engl., Rutaneblina Steyerm. & Luteyn) are small endemic groups and a fourth, Raveniopsis Gleason, is near endemic. Raveniopsis, the focus of this study, appears to be a classic Guiana Shield radiation where of its 19 currently recognized species, 16 are narrow montane endemics known from one or a few sandstone mountains (i.e., 13 are known from a single tepui including four species endemic to Sierra de la Neblina and three to Auyán-tepui) in Venezuela (Amazonas, Bolívar), Brazil (northern Amazonas), and Guyana (Mazaruni-Potaro, described herein). In addition, one species is more widespread and extends to lower elevations across this region, and two species are low-elevation disjuncts further south in Brazil (southeastern Amazonas, Rondônia).

Raveniopsis are attractive shrubs, mostly less than 2 m tall (up to 5 m in R. stelligera [R.S. Cowan] R.S. Cowan), with a diversity of leaf, trichome, and floral morphologies. Their leaves can be simple or trifoliate, and can be thin or clearly adapted for montane life in being thickened and well-covered in diverse types of trichomes. Among the species are two strikingly different floral syndromes that presumably reflect pollinator differences, and include: (1) short-tubed white to sometimes pinkish-tinged, or (2) longer-tubed red to orange corollas. The genus is characterized by a combination of pentamerous flowers, subequal and mostly free sepals that are longer than wide, androecia of two fertile anthers with basal appendages and three staminodes, zygomorphic tubular corollas with five lobes, and 5-parted apocarpous gynoecia producing follicular fruit.

Many of those diagnostic features also serve to group Raveniopsis with the “Angostura Alliance” (Galipeeae subtribe Galipeinae), which includes about 25 genera and 130 species restricted to the neotropics (Kubitzki et al. 2011; Bruniera et al. 2015). Many of the genera are small (13 have 1–2 species), and Raveniopsis is the second largest genus after Conchocarpus J.C. Mikan (53 species, including Almeidea St.-Hil.; Bruniera et al. 2015). Molecular phylogenetic studies have so far indicated that most of the limited Angostura Alliance taxon sampling (10 genera) form a core clade, and that taxonomic adjustments are needed to improve subtribal and generic circumscriptions (Groppo et al. 2008, 2012; Bruniera et al. 2015). Based on floral characters, Raveniopsis appears to be closely related to Ravenia and a further suggestion has been made that perhaps both genera should be combined (Kubitzki et al. 2011). Raveniopsis has not been sampled for any published molecular phylogenetic study, although preliminary evidence indicates that the genera should be kept separate (K. Wurdack unpublished).

While new exploration of montane areas of the Guiana Shield is expected to yield novelties, it was surprising that a very distinct new species of Raveniopsis was among collections made during recent botanical exploration of Kamakusa Mtn. in Guyana. The significance of this discovery was not known at the time of collection, although its identity was puzzled over by expedition members including the author. Kamakusa Mtn. is the highpoint of the Merume Mtns., a subrange within the Pakaraima Mtns., and forms part of an escarpment bordering the lowland (<100 m) rainforests of the lower Mazaruni River. This escarpment defines the eastern edge of a broad upland region (mostly >1000 m) within the Guiana Shield that continues westward into adjacent Venezuela and there includes part of the Gran Sabana and numerous scattered tepuis. While the summit of Kamakusa Mtn. apparently had not been botanically explored prior to 2012, an expedition in June–July 1960 lead by Stephen Tillett on behalf of The New York Botanical Garden traversed its lower slopes and made many type collections of taxa in diverse families (Wurdack et al. 2013). This richness in novelties suggests a higher degree of local endemism than observed in other mountains in the vicinity except Mt. Ayanganna, although many of those taxa remain poorly collected or studied.

Scanning electron microscopy (SEM) was with a Zeiss EVO MA15 SEM at 10–12 kV after directly mounting dried herbarium specimen fragments, and sputter coating the samples with 25 nm of Au/Pd. For leaf anatomy, a rehydrated fragment was paraffin-embedded, sectioned at 10 μm, stained with toluidine blue O, and imaged with a Zeiss Universal Compound Microscope. The pollen was only examined and measured with SEM due to the few grains available. Trichome morphology and terminology follows Webster et al. (1996) that treats the great diversity of indument types in Croton L. (Euphorbiaceae), which like Raveniopsis has considerable variation in branched forms.

In species keys (i.e., Cowan 1960; Steyermark 1980; Kallunki 2005) Raveniopsis microphyllus would quickly group with the three other compound-leaved species, R. cowaniana Steyerm. & Luteyn, R. stelligera (R.S. Cowan) R.S. Cowan (including R. liesneri Steyerm.), and R. trifoliata R.S. Cowan. However, those taxa are very different from R. microphyllus in much larger leaflet size, different types of trichomes, red-orange tubular corollas, and biogeography. They occur in Venezuela at different elevations on Sierra de la Neblina where R. cowaniana and R. trifoliata are endemic, and slightly broader-ranged R. stelligera also extends to Cerro Duida and Cerro Yutajé. Compound leaves aside, Raveniopsis microphyllus appears most morphologically similar to R. breweri Steyerm. in small, ovate lamina, white flowers, and trichome type (see below). Raveniopsis microphyllus has trifoliate leaves with very small leaflets (5–8.8 × 3–5.3 mm) that are adaxially pubescent when young (becoming glabrescent), a unique form of small rosulate trichomes, single (to few) flowered subsessile inflorescences, small sepals (largest 3 × 1 mm), small corollas, and hirsute anthers. In contrast, R. breweri has larger (22–32 × 10–16 mm), simple leaves that are adaxially glabrous except along the midvein, larger flattened rosulate trichomes, pedunculate 3–7 flowered racemose inflorescences, larger sepals (largest 8 × 2 mm), larger corollas, and glabrous anthers. In R. breweri the suggestion (fig. 1 in Steyermark 1980) that two staminodes are laterally conjoined to the stamens was not confirmed here, although the type collection was not examined. My observations of R. breweri (Huber & Medina 8529, US; Prance & Huber 28254, US) are that all three staminodes are free, although distally loosely confluent due to tangled trichomes. In addition, the two anthers are laterally coherent as has been found in some species of Raveniopsis and other Angostura Alliance genera (personal observation; Kallunki 1991; El Ottra et al. 2013), but which does not appear to be the case in R. microphyllus where the anthers are free in bud and at dehiscence. Raveniopsis ruellioides, the only other species in Guyana, differs in larger, simple, pellucid-punctate leaves with exclusively simple trichomes, and long pedunculate inflorescences with red flowers and foliaceous sepals.

The leaves of Raveniopsis microphyllus appear well adapted for a montane environment due to their small size, considerable vestiture, thick cuticle, and coriaceous nature. Pellucid dots (oil-containing secretory cavities), which are a characteristic feature of Rutaceae leaves, are not visible in R. microphyllus, nor do they appear differentiated anatomically. They are surficially visible in some thinner-leaved species (e.g., R. necopinata Kallunki, R. ruellioides, and R. stelligera) but are not similarly obvious in other coriaceous species. I also did not note them elsewhere in R. microphyllus (i.e., floral parts during wet dissections), although some darkened surficial cells on the anthers may be secretory cells. Unfortunately at the time of collection no attention was paid to any presence of citrus or resinous odor that would indicate essential oils. Leaf anatomy (Fig. 2E, F) shows a dorsiventral structure and a thick cuticle (Fig. 2G; 20–23 μm at the thinnest point mid-cell and thicker where adjacent cells meet, as measured via SEM) on the large-celled epidermis of the glabrescent adaxial side. The epidermis of the abaxial side is small-celled and lacks the cuticle thickening. Stomata are confined to the abaxial side where they are densely packed under the canopy of stalked trichomes (Fig. 3D) and absent along the slightly raised secondary vein courses. The mesophyll is poorly differentiated into palisade and spongy layers. Elongate styloid crystals are sparsely present in the mesophyll and mostly oriented parallel to major veins. The crystals are square in transverse section (Fig. 2C) and have obliquely faceted, needle-like ends (Fig. 2D).

Figure 2. 

Morphology and anatomy of Raveniopsis microphyllus. A Life photo of flowering branch showing strongly zygomorphic, white corolla B Pollen with mesocolpium-centered equatorial view of whole grain C Transverse section of leaflet edge under polarized light, showing bright styloid crystal (at arrow) D In-situ styloid crystals near secondary vein E Transverse section of entire leaflet (tiled image) F Transverse section in closer view showing thick adaxial cuticle, and abaxial trichome with multicelled stalk G Adaxial epidermis showing thick cuticle above epidermal cells. (Source: Tripp 3191, US).

The indument on Raveniopsis is very diverse in form (i.e., variations on glandular, simple, dendritic, multiradiate, and rosulate) both among species and often within an individual plant in a location-dependent manner. The interspecific trichome diversity is taxonomically useful (e.g., applied in the species key of Steyermark (1980)) and may be phylogentically informative, although care is needed to observe homologous plant structures when making fine comparisons. Raveniopsis microphyllus bears five different location-specific trichome forms, with some minor intergradation, that can be grouped into three basic types including: (1) simple, (2) rosulate, and (3) multiradiate. While rosulate can be considered a subtype of multiradiate (see Webster et al. 1996), the distinction is useful here for descriptive purposes. The simple trichomes are confined to inside the flower, and can be of two forms as either stiff and straight (strigose) on the anthers, or crinkled (pilose) on the filaments and adjacent inner corolla wall. The rosulate trichomes (Fig. 3A–B, D) are tufts with numerous soft, often twisted radii (arms) and are well developed on the abaxial laminar surface and along stems and petioles. The multiradiate trichomes (Fig. 3A, C) while similar to the rosulate form differ in having fewer and stiffer radii and are often further differentiated into a modified form with an erect longer central radius (porrect-multiradiate); they occur on the adaxial side and margins of the lamina, and outer surfaces of the flower. Both rosulate and multiradiate trichomes are stipitate with a short, fragile (allowing the trichomes to easily detach), non-vascularized multicellular stalk, which in cross section is a rosette of 6–9 cells surrounding a central cell (Fig. 3E). The trichomes and their stalks have differential staining relative to the site of epidermal attachment (Fig. 2F). After the trichomes weather off the adaxial side, the remnants of the stalks are visible as rosette outlines (Fig. 3C). The species of Raveniopsis most closely approaching R. microphyllus in laminar trichome morphology are R. breweri, R. peduncularis Pittier & Lasser (but not “appearing lepidote”, fide Kallunki 2005), and R. jauaensis Steyerm., that were grouped as “stellate-squamose” in the species key of Steyermark (1980). They share similar tufted rosulate appearance with R. microphyllus but are larger and tend to be flattened (i.e., squamose), due to loss or shortening of central radii (Fig. 2F, G). These differences are more pronounced in R. peduncularis and R. jauaensis, which are further distinguished from R. breweri in bearing red flowers on elongate dischasial inflorescences.

Figure 3. 

Trichome and leaf surface comparisons of Raveniopsis microphyllus (A–E) and R. breweri (F–G). A Abaxial side of leaflet showing trichomes transitioning from rosulate to porrect-multiradiate with central long radii at margin (sampled from proximal margin of lateral leaflet) B Back of rosulate trichome showing central multicellular attachment stalk, and curled free arms C Adaxial side of young leaflet showing multiradiate trichomes, and rosette bases of trichomes that have weathered off D Abaxial side of leaflet with most trichomes removed to show dense stomata and stalk bases of rosulate trichomes; note fungal hyphae on epidermis E Close-up of stomata and stalk base of rosulate trichome F Raveniopsis breweri abaxial trichome G Back of Raveniopsis breweri abaxial trichome, showing central multicellular attachment stalk. (Abbreviations: s = stalk. Source: R. microphyllus, A–ETripp 3191, US; R. breweri, F–GPrance & Huber 28254, US).

The reticulate pollen of Raveniopsis has been shown to have aperture (3, 5, 6- aperturate) and size variation (Morton and Kallunki 1993). Although I had little pollen of R. microphyllus available, it appears reticulate, 5-colporate, prolate-spheroidal, and 45–50 × 43–48 μm in size (polar:equatorial ratio 1.05; n = 3, via SEM) (Fig. 2B). The pollen of R. microphyllus is very similar to other species in details of its reticulate sculpture, and falls in between the two reported size classes (Morton and Kallunki 1993).

The historical biogeography of Raveniopsis is likely complex and interesting given the variety of distributional patterns represented by species that are widespread, isolated endemics, sympatric endemics, or Amazonian disjuncts. Moreover, the sympatric species are morphologically diverse and (mostly) not obvious local species complexes. Raveniopsis microphyllus is the easternmost member of the 16 tepui species, and separated by 230–250 km from potential Venezuelan close relatives on Auyán-tepui (3 spp.) and Macizo del Chimantá (2 spp.). Raveniopsis breweri, which is the most morphologically similar species to R. microphyllus, is among the Auyán-tepui endemics. Raveniopsis ruellioides is a variable species (especially in leaf shape) that is broadly distributed across the Guiana Shield in southern Venezuela and eastward into the Pakaraima Mtns. It tolerates a broad elevation range (300–2600 m), although is mostly montane (1500+ m) and usually reaches lower elevations along watercourses. In the Pakaraima Mtns. it geographically most closely approaches R. microphyllus on Mt. Ayanganna, but as previously noted is morphologically very different and likely distantly related. Raveniopsis sericea R.S. Cowan was reported from Guyana (Kallunki 2005; Funk et al. 2007) but this record appears to be incorrect as further documentation could not be found (MO, NY, US, Biological Diversity of the Guiana Shield specimen database; J. Kallunki, personal communication). Otherwise that high altitude (1800–2500 m) species is endemic to the Macizo del Chimantá in Bolívar, Venezuela.

I thank Alice Tangerini (US) for the botanical illustration, Stanley Yankowski (US) for the leaf anatomy and associated images, the reviewers (Milton Groppo and José Rubens Pirani) for their helpful comments, the NMNH Scanning Electron Microscopy Lab, and MO, NY, and US for relevant collections. For fieldwork, I thank the other members of the 2012 Kamakusa Expedition including Aleks Radosavljevic, Karen Redden, Erin Tripp, and our Amerindian team led by James Ralph. The expedition was supported by the National Geographic Society / Waitt Grants Program (Karen Redden, PI), and the Biological Diversity of the Guiana Shield Program (BDG) of the Smithsonian Institution.