Euphorbia rubrostriata, Drake, 1903

Haevermans, Thomas & Hetterscheid, Wilbert L. A., 2021, Taxonomic changes and new species in Malagasy Euphorbia (Euphorbiaceae), Phytotaxa 492 (1), pp. 1-61 : 51-52

Key 2:—Species of the informal Euphorbia rubrostriata group

1. Plant ca. 10 cm high, developing numerous mostly ground-dwelling branches ......................................... Euphorbia mangokyensis

- Plant 0.3–6 m high, never with ground-dwelling branches...............................................................................................................2.

2. Branches all or almost all angulate....................................................................................................................................................3.

- Branches all or almost all rounded ....................................................................................................................................................4.

3. Plant up to 70 cm high, base tuberous, developing few main stems, hairs on the leaves and petioles very long (3 mm), inflorescences, 1–3 times dichotomous, carrying 2–8 cyathia................................................................................................. Euphorbia hofstaetteri

- Plant 50–100 (–150) cm high, largely or entirely plagiotropic, base not tuberous, developing several to many stems, hairs on the leaves short (1 mm), inflorescences on the branch tips, 1–4(–5) times dichotomous, 1.5–10 cm long, carrying 2–16(–32) cyathia. ........................................................................................................................................................................ Euphorbia xanthadenia

4. Spines usually complex, with 2–5 accessory, small spines on the main spine base............................ Euphorbia pseudodidiereoides

- Spines usually simple, or with 1(–2) accessory spine(s) on the basiscopic side of the main spine base ..........................................5.

5. Plant erect, fastigiate, very large, to 6 m high, main stem to 10 cm diam., glabrous in all its parts except the pubescent ovary, cyathophylls very large, the spreading part 1.5–2 cm wide, red with yellow veins............................................. Euphorbia gigantea

- Plant mostly 0.3–2 m high, plagiotropic, or erect but non-fastigiate, base 1–5 cm diam., at least leaves abaxially, partly pubescent, spreading part of the cyathophylls at most 1 cm wide., yellow, yellow with red margin, minutely red-punctuated, or red with yellow veins...................................................................................................................................................................................................6.

6. Plant usually single-stemmed, leaves narrowly elongate-elliptic to lanceolate ............................................. Euphorbia rubrostriata

- Plant 3- or multi-stemmed, leaves (narrowly) elliptic, obovate or orbicular ....................................................................................7.

7. Leaves narrowly elliptic, acute, glabrous on both sides.................................................................................... Euphorbia retrospina

- Leaves elliptic, oblong, obovate, oval or orbicular, at least pubescent at the base of the abaxial surface or on both surfaces ........8.

8. Plant plagiotropic, to ca. 50 cm high, roots with swollen, succulent base, yellow, leaves both surfaces densely, velvety pubescent, adaxial surface distinctly glaucous grey-green.............................................................................................. Euphorbia tsihombensis

- Plant erect, 0.5–1.5 m high, roots fibrous, not yellow, adaxial leaf surface mid green or grey-green, glabrous, or slightly pubescent at the base, or pubescent all over, hairs either minute or 1–2 mm long ............................................................................................9.

9. Leaves coriaceous, obovate, sessile or nearly so, adaxial surface grey-green, both surfaces pubescent with long or very short hairs .................................................................................................................................................................................................10.

- Leaves thin, petiolate, adaxial surface mid green, glabrous or pubescent near the base.................................................................11.

10. Leaves (except the youngest) on both sides with minute hairs (ca. 0.2 mm long), cyathophylls 6 × 8 mm, abaxial surface pubescent, flat, not keeled, upper part spreading with rounded top and small apiculum, style trichotomous from the middle or higher............. ....................................................................................................................................................................... Euphorbia itampolensis

- Leaves on both sides densely pubescent, hairs easily visible, to 2 mm long, cyathophylls erect, 6 × 5 mm, both surfaces pubescent, abaxial surface keeled, upper part acute with acute apiculum, style trichotomous from the base............... Euphorbia mahafalensis

11. Terminal parts of young branches 5 mm in diam. or more, spines on actively growing parts, with a broad base (3 mm), from 1/3 rd from the base tapering to the top, leaves on the branch tips 2–4 cm long, inflorescence 1–4 cm long, short or long pedunculate, common peduncle curved or nearly straight, 1–2-times dichotomously branched, carrying 2–4 cyathia, cyathophylls obliquely outward, not fully spreading, 4–5 mm broad, adaxially all yellow or with a narrow reddish margin, abaxially all yellow, or yellow flushed red ........................................................................................................................................................ Euphorbia rigidispina

- Terminal parts of young branches 2 mm diam., spines on actively growing parts, with a narrow base (1 mm), tapering from base to top, leaves on the branch tips 1–2 cm long, inflorescence 1–1.5 cm long, short pedunculate, common peduncle curved, 1–2- times dichotomously branched, carrying 2–4 cyathia, cyathophylls adaxial surface greenish, yellow or pale dirty yellowish with or without reddish between the veins, abaxial surface pale yellow or red with yellow veins...................... Euphorbia graciliramulosa