Euphorbia L.
A genus with about 2087 species distributed throughout the tropical, subtropical and temperate zones, mainly in America and Africa. In North Africa, the genus is represented by 85 species, of which a dozen can be considered purely shrubby, 5 are cactiform and the rest suffruticose (with more or less lignified stems, at least at the base) or herbaceous.
Among the species that are not very woody but with erect stems that almost reach 1 m in height (or even exceed it), 6 of them can be highlighted. The most robust are E. paniculata Desf. and E. clementei Boiss. Both with erect stems, little ramified, up to 1(1.5) m tall and verrucose cocci. Among them they differ because the first has 2-4 pleocasial rays and the second has 5-7 rays. Widely distributed by the Mediterranean parts of Morocco, Algeria and Tunisia. Other non-woody species of good size, with upright stems that almost reach 1 m in height (or even exceed it) are E. characias L. in the Mediterranean parts of Morocco and Libya, and others restricted to specific habitats, from the sand dunes and coastal rocky areas (E. paralias L.), the high mountains (E. nicaeensis All.) and in the sand dunes of the Sahara (E. guyoniana Boiss. & Reut).
Regarding the conservation status, the 6 aforementioned species have a wide distribution, and are not considered threatened. In the IUCN Red List of Species, E. paniculata it has been assessed as Least Concern (LC) globally (de Bélair, 2010), while the other 5 are currently not evaluated. In the Livre Rouge de la flore vasculaire du Maroc (Fennane, 2021) E. paniculata, E. clementei and E. guyoniana have been considered in the Data Deficient (DD) categorie, E. characias and E. nicaeensis as Least Concern (LC) and E. paralias as Near Threatened (NT).
According to Benadí, J. et. al. (1997) in Flora Iberica, VIII: 210-285, the primary inflorescence cannot be considered an umbel (as it was traditionally described) because it is not a simple racemose inflorescence but a “pleiochasium” with different levels of complexity of cymose structures.
Regarding the structures that serve as mechanical defense, whose precise characterization is very important in cactiform species, the rule of using the term “spines” has been followed to refer to firm, slender and pointed structures, which represent leaves or modified stipules. Sometimes they can merge to give a single structure from paired modifications, such would be the case of a pair of stipules that generate a solitary spine. On the other hand, the term “prickles” is used to refer to pointed structures resulting from growth of the epidermis or the bark.
The cactiform species should not be confused with the species of Caralluma, Apteranthes and related genera (Apocynaceae), which has some very robust species in North Africa, reaching 1 m in height. The species in Euphorbia are spiny, with white latex and tiny flowers (2.5-6.5 mm in diameter); the cactiform species in Apocynaceae do not have spines, their latex is colorless, and their flowers are larger (6-30 mm).
There used to be a magazine devoted exclusively to this genus: The Euphorbia Journal. Strawberry Press. Mill Valley, California. Another useful source, especially for infrageneric taxonomy, is the portal: https://app.tolkin.org/projects/72/taxa
Key to species
1 Plants cactiform, succulent, spiny 2
1 Plants shrubby, woody or only young branches succulent, unarmed or with spines born on woody stems 6
2 Leaves well developed at the top of the stem Euphorbia sudanica
2 Leaves reduced to tubercles throughout the stem 3
3 Stems with triplets of pointed structures close together: pairs of identical well-developed prickles (1.5-3.5 mm), together with longer solitary spines (12-27 mm) Euphorbia triaculeata
3 Stems with pairs of pointed structures together: pairs of spines, without prickles (or if present then deciduous) 4
4 Spines less than or equal to 1,5 mm long Euphorbia polyacantha
4 Spines longer than 1.5 mm 5
5 Stems and branches with 5-10 angles Euphorbia officinarum
5 Stems and branches tetragonal Euphorbia resinifera
6 Spiny plants Euphorbia cuneata
6 Unarmed plants 7
7 Plant with an ephedroid appearance: very branchy, with very thin stems (0.5-1 cm), few leaves, soon deciduous and small (less than 30 × 7 mm) linear-lanceolate on young succulent twigs Euphorbia nubica
7 Plant without ephedroid appearance, broader stems, larger leaves (if equal or smaller, then they are oblong-obovate and arise from woody twigs) 8
8 Nectaries with appendages that have the shape of a croissant or horns 9
8 Nectaries without appendages 10
9 Stems clearly woody, bark reticulated with clear scars left by the leaves when they fall, young branches reddish Euphorbia dendroides
9 Stems and branches thickened, bark whitish-pruinose, smooth (occasionally somewhat fissured at the base), young branches greenish Euphorbia regis-jubae
10 Fruit glabrous or hairy, little or not at all warty 11
10 Fruit warty 15
11 Shrub up to 3(4) m in height, leaves up to 150 mm long Euphorbia nereidum
11 Shrub up to 1.5(2) m in height, leaves up to 70(80) mm long 12
12 Fruit 3.5-5.5 × 4-5.5 mm, longly pedicellate (3-5 mm) Euphorbia squamigera
12 Fruit 7-9 × 8-11 mm, shortly pedicellate (0.5-1.5 mm) 13
13 Leaves obovate-oblong. Styles welded up to a quarter of their length, or less. Capsules pubescent Euphorbia adenensis
13 Leaves lanceolate or linear. Half-length welded styles. Capsules glabrous, rarely glabrescent 14
14 Stem leaves lanceolate to linear-lanceolate, 14.1-44.9(50) × 2.9–10(12)mm; mucron 0.3–0.6 mm long or non-mucron. Ovaries and capsules slightly pubescent, sometimes glabrescent Euphorbia balsamifera
14 Stem leaves linear, (32.2)40.5–77.5(100) × 3.1–7.4 mm; always mucronate, with mucron (0.1)1–1.5(2) mm long. Ovary and capsule densely pubescent, never glabrous Euphorbia sepium
15 Leaves linear lanceolate to ovate-lanceolate, acute or acuminate, fruit 4-5.5 mm Euphorbia bivonae
15 Leaves oblong-ovate, rounded to obtuse at the apex, fruit 3.5-4 mm Euphorbia hierosolymitana
Updated by: F.J. Pérez García, J.F. Mota Poveda & J. Charco.