Croton rizzinii is similar to Croton celtidifolius Baill., but differs from the latter by the yellowish latex (vs. ochraceous to reddish latex), branches with appressed to stipitate stellate-porrect, stipitate-fasciculate and appressed-multiradiate trichomes (vs. appressed to stipitate stellate, stellate-porrect, appressed dendritic and dendritic-porrect trichomes), stipules (10–)12–14 mm long, laciniate-glandular (vs. 2–6 mm long, entire, eglandular), bracts with two inconspicuous glands (colleters) at the base (vs. eglandular bracts), staminate flowers with 64–76 stamens (vs. 50–60), campanulate pistillate flowers, sepals ca. 6–9.5 mm long, valvate to slightly imbricate (vs. rotate, ca. 2.5–5 mm long, valvate).

BRAZIL. Rio de Janeiro: Teresópolis, Parque Nacional da Serra dos Órgãos, BR-495, rodovia Teresópolis-Itaipava, em borda de mata de encosta, 22°24'40.8"S, 43°02'08.5"W, alt. 1414 m, 28 Jan 2017, fl, S.Q. Farias & J.L. Silva 205 (holotype: R!; isotype: RB!).

Trees ca. 5–10 m tall, monoecious; young branches with a dense to sparse indumentum of appressed to stipitate stellate-porrect, stipitate-fasciculate and appressed-multiradiate trichomes, cinereous, yellowish, or pale ferrugineous; latex yellowish. Leaves alternate, simple; lamina 7–22 × 2.4–12.2 cm, discolorous, membranaceous, ovate to cordate, apex acute, long-acuminate to caudate-acuminate, base rounded, obtuse to cordate, adaxial surface sparsely pubescent with indumentum of appressed to short-stipitate fasciculate trichomes, more concentrated along the veins, abaxial surface densely pubescent with indumentum of stipitate stellate-porrect to stipitate-multiradiate trichomes; venation brochidodromous, trinerved at the base to palminerved; margin inconspicuously dentate, with ovoid to cylindrical glands (colleters) at the tip of each tooth; petioles 1.9–13 cm long, with dense indumentum of appressed to stipitate stellate-porrect, stipitate-fasciculate and appressed-multiradiate trichomes; nectary glands 2, acropetiolar, patelliform, shortly stipitate to stipitate, on the abaxial side; stipules (10–)12–14 mm long, linear-lanceolate, cream to yellowish-green, margin laciniate-glandular, sometimes with a glandular tip, abaxial surface with sparse indumentum of appressed stellate-porrect to multiradiate trichomes. Inflorescences terminal, erect, 6–29 cm long, axis with dense indumentum of appressed to stipitate stellate-porrect trichomes; proximal cymules bisexual, spaced along the axis; cymule bracts variable in size and shape, 4–9 × 1.6–2 mm, narrowly oblong, linear-lanceolate or lanceolate, abaxial surface with appressed stellate-porrect trichomes, with 2 sessile, minute, basal glands (colleters), margin entire to irregularly dissected with stipitate-fasciculate trichomes. Staminate flowers rotate, 7–11 mm long; pedicel 2.5–6 mm long, with indumentum of appressed stellate-porrect trichomes; sepals 5, valvate, connate at the base, entire, equal, 3.5–4 × 2–2.5 mm, ovate, apex acute, adaxial surface with simple trichomes at the base and along the margin; abaxial surface with appressed stellate-porrect trichomes; petals 5, 2.2–3.5 × 0.9–1.2 mm, distinct, oblanceolate, apex acute, adaxial surface with simple trichomes at the base, abaxial surface with simple trichomes along the margin; stamens 64–76, filaments 2.5–4.5 mm long, with simple trichomes, disc 5-segmented, receptacle villose with dense simple trichomes. Pistillate flowers campanulate, 6.5–10.1 mm long, sessile to subsessile; sepals 5, 6–9.5 × 4–5 mm, valvate to slightly imbricate, connate at the base, entire, unequal, ovate to broadly ovate, apex acute to abruptly acute with glandular tip (colleter), adaxial surface with indumentum of short-stipitate stellate trichomes at the distal portion, abaxial surface with indumentum of appressed stellate-porrect and appressed-multiradiate trichomes; maculate glands sometimes present on some of the sepals; petals each reduced to a filament with an apical gland (colleter); ovary ca. 3.4 mm in diam., densely covered with pale ferrugineous rosulate trichomes; styles 3, 4-fid to multifid (12–18 terminal tips), connate at the base, with appressed stellate-porrect trichomes; disc 5-segmented. Capsules 7.5–10 × 4.8–5.4 mm, subglobose, densely covered with pale ferrugineous rosulate trichomes; sepals and columella persistent; columella 6 mm long. Seeds 4.1–5.4 × 3–3.3 mm, oblongoid, brown, ribbed; caruncle 1 × 2.5 mm, cream, transversely oblong.

Figure 1. 

Croton rizzinii. A Adult individual in hillside forest B young flowering branch C mature flowering branch D detail of a mature flowering branch E detail of laciniate-glandular stipules F young branch showing yellowish latex G detail of leaf showing acropetiolar glands H–K stages of development of pistillate flowers H top view of a young pistillate flower I young pistillate flower showing a maculate gland on the distal portion of the sepal J young pistillate flower K mature pistillate flower L fruit M inflorescence showing pistillate and staminate flowers N inflorescence showing fruits. (Photos by S.Q. Farias).

Figure 2. 

Images from herbarium specimens of Croton rizzinii. A Flowering branch B inflorescence C indumentum on the adaxial surface of the lamina D indumentum on the abaxial surface of the lamina E detail of leaf showing the two acropetiolar glands F stipules G detail of a bract of a cymule with glands (colleters) at the base H mature pistillate flower I detail of the styles connate at the base J pistillate flower with ovary removed, showing disc and a gland at the base of one of the sepals K seed (ventral surface). (A, B S.Q. Farias 241 C, D, G S.Q. Farias & J.L. Silva 239 E, F, K S.Q. Farias & J.L. Silva 234; H, I, J S.Q. Farias & J.L. Silva 205, holotype).

BRAZIL – Rio de Janeiro • A. Souza et al. 1576 (R); Teresópolis [Guapimirim], Serra dos Órgãos, rodovia Rio-Teresópolis, próximo ao rio Soberbo; 22°29'23.56"S, 43°00'27.71"W; alt. 505 m; 28 Nov. 1986; fl • A. Souza et al. 1575 (R); Petrópolis, Parque Nacional da Serra dos Órgãos, BR-495, rodovia Teresópolis-Itaipava; 28 Nov. 1986; fl • S.Q. Farias & J.L. Silva 239 (R); ibid, km 9; 22°24'38.5"S, 43°02'24.2"W; alt. 1416 m; 28 Apr. 2018; fl • R. Barnes s.n. (R 186364, FUEL34028, INPA 212592, IPA 61764, MBM 275097, RB 377649, SP 361735); Teresópolis; 30 Jun. 1995; fl • R. Barnes s.n. (R 185344); Serra dos Órgãos, próximo ao abrigo 3; 22°26'54.32"S, 42°59'0.21"W; alt. 1000 m; 25 Jan. 1995; fl • S.Q. Farias & J.L. Silva 200 (R); BR-495, rodovia Teresópolis-Itaipava; 22°24'40.8"S, 43°02'08.5"W; alt. 1414 m; 15 Jan. 2017; fl, fr • S.Q. Farias & J.L. Silva 234 (R, RB); ibid; 2 Apr. 2018; fl, fr • S.Q. Farias 241 (R, RB); ibid; 2 Mar. 2019; fl, fr.

Croton rizzinii is only known from the Serra dos Órgãos National Park, with records in the municipalities of Guapimirim, Petrópolis and Teresópolis (Rio de Janeiro) (Figure 3). It grows in montane ombrophilous dense forest, between 500 and 1500 m elevation and in disturbed areas like forest edges and roadsides.

Figure 3. 

Map of Rio de Janeiro State showing the distribution of Croton rizzinii (black circles). The inset map of Brazil on the left provides the context with the state of Rio de Janeiro highlighted in grey. ES = Espírito Santo, MG = Minas Gerais, RJ = Rio de Janeiro, SP = São Paulo.

The new species has been collected in flower in January, March, April, June, November and December. The flowering peak is in December and January, with fruits between January and April.

The specific epithet is given in homage to Carlos Toledo Rizzini, a Brazilian botanist, for his dedication to the study of the flora of the Serra dos Órgãos mountain range.

Croton rizzinii is known from a few collections in three municipalities of the PARNASO. It has an estimated Extent of Occurrence (EOO) of 19,653 km². Although the species occurs within a conservation unit, it suffers considerably from anthropic pressure caused by tourism, urbanisation, agriculture and grazing. Based on our data, Croton rizzinii could be assessed either as Critically Endangered (CR B1b[iii]) or Data Deficient (DD). The latter category is based on the lack of sufficient information regarding the distribution and size of populations.

Croton rizzinii can be recognised in the field by its yellowish latex, laciniate-glandular stipules that are cream to green-yellowish in colour, bracts with two inconspicuous glands (colleters) at the base, campanulate pistillate flowers with sepals covering the entire ovary and the presence of maculate glands on the adaxial surface. We are assigning the species to Croton sect. Cyclostigma (Van Ee et al. 2011) due to its arborescent habit, yellowish latex, stellate trichomes, triplinerved to palmate venation, acropetiolar glands, basal bisexual cymules, pistillate flowers with vestigial petals and staminate flowers with numerous stamens. However, further testing using molecular characters would be desirable to confirm its phylogenetic position. The finding of a new species in this section in a relatively well explored area is surprising and shows that botanical exploration continues to be relevant in highly diverse Neotropical areas and especially in biodiversity hotspots (Myers et al. 2000, Joppa et al. 2011) regardless of their current level of floristic inventory and the taxonomic knowledge of the focal group (Riina et al. 2018).

Croton rizzinii can be distinguished from other Brazilian Cyclostigma species by its laciniate-glandular stipules, bracts with glands (colleters) at the base, sepals of the pistillate flowers with glands (colleters) at the apex and, sometimes, macular glands on the adaxial surface. Another character distinguishing it from its most similar species in the ARF is the size of seeds, which are the smallest in size amongst them (Table 1). Amongst the ARF Cyclostigma species, Croton rizzinii appears to be most similar to C. celtidifolius (Santos et al. 2017; Caruzo and Cordeiro 2007). Both species occur sympatrically and syntopically and can be confused by their similar ovate to cordate and pubescent leaves, trinerved at the base to palminerved venation, short-stipitate to stipitate petiolar glands on the abaxial side of the petiole and subglobose fruits. However, they can be readily distinguished by several characters, mainly by those related to latex colour, type of trichomes on branches, number of acropetiolar glands, stipules, bracteoles, stamen number and pistillate flowers (see Table 1 and identification key). Due to their resemblance, specimens of C. rizzinii have often misidentified as C. celtidifolius in herbaria.

Croton rizzinii is also similar to C. vulnerarius Baill. and C. alchorneicarpus Croizat, mostly regarding the young pistillate flowers. Croton vulnerarius and C. rizzinii occur sympatrically within PARNASO, but so far, they have not been found in the same locality. These species share an arborescent habit, pubescent leaves, conspicuous stipules, sessile to subsessile campanulate pistillate flowers, valvate to slightly imbricate sepals and styles connate at the base. Nevertheless, they can be distinguished by several characters (see Table 1 and identification key). In relation to C. alchorneicarpus, both species present ovate to cordate leaves, trinerved at the base to palminerved venation, conspicuous stipules, sessile to subsessile pistillate flowers with styles connate at the base, rotate staminate flowers and subglobose fruits. However, they too can be separated by several vegetative and reproductive characters (see Table 1 and identification key).

Several collections (Cordeiro 3056, 3057, Riina and Caruzo 1526, 1529) from ARF areas of São Paulo and Rio de Janeiro states show characteristics intermediate between Croton alchorneicarpus and C. rizzinii. The overall aspect of the plant and the floral morphology are more similar to C. alchorneicarpus, but the stipules are more similar to those of C. rizzinii (laciniate-glandular). Further studies and additional sampling of Croton trees from the ARF are needed to determine the identity of these specimens.

Croton lagoensis Müll. Arg. is also similar to C. rizzinii, but it occurs in deciduous forest and in transitional areas between the Cerrado and the ARF in the state of Minas Gerais (Santos et al. 2017). Both species have ovate to cordate and pubescent leaves, two acropetiolar glands and campanulate pistillate flowers with sepals covering the entire ovary. However, C. rizzinii differs from C. lagoensis mainly by its arborescent habit and other features listed in Table 1 and in the identification key.

The description of C. rizzinii adds to the number of species with laciniate stipules in Croton and in section Cyclostigma in particular. Laciniate stipules are found in seven species in Cyclostigma (C. charaguensis Standl., C. churutensis Riina & Cornejo, C. medusae Müll. Arg., C. perspeciosus Croizat, C. purdiei Müll. Arg., C. rizzinii and C. speciosus Müll. Arg.), but they can be present in other sections of Croton (sect. Adenophylli Griseb., sect. Barhamia (Klotzsch) Baill., sect. Medea (Klotzsch) Pax, amongst others) (Van Ee et al. 2011). However, given the inconsistencies found in Croton taxonomic treatments regarding the terms used to describe stipules, we suggest further studies to standardise this terminology across the genus.