[2.15] Viola sect. Sclerosium

Viola sect. Sclerosium W. Becker in Nat. Pflanzenfam., ed. 2 [Engler & Prantl], 21: 374. 1925.

—Lectotype (designated here): Viola cinerea Boiss.

=Viola [sect. Nomimium; unranked] §.2. Cinereae Boiss., Fl. Orient. 1: 451. 1867, p. p. (excl. V. spathulata) ≡ Viola [unranked] (”Gruppe”) Cinereae Boiss. em. W. Becker in Beih. Bot. Centralbl., Abt. 2, 36: 36. 1918

Description.—Annual herbs or perennial subshrubs, glabrous or densely short-pubescent. Axes morphologically differentiated in aerial stems and short axillary branches bearing cleistogamous flowers. Stipules small, lanceolate. Lamina ovate to lanceolate, remotely denticulate, petiolate. Corolla pink with a green throat. Spur short and thick. Style slender and cylindrical or slightly clavate, crested; crest a pair of apical or subapical lateral ear-like processes. Simultaneous production of chasmogamous in upper leaf axils and cleistogamous flowers on short branches in lower leaf axils. Allotetraploid (CHAM + MELVIO). Secondary base chromosome number x’ = 11.

Diagnostic characters.—Style with a pair of apical or subapical lateral ear-like processes. Base chromosome number x = 11.

Ploidy and accepted chromosome counts.—4x, 8x; 2n = 22 (V. stocksii).

Age.—Crown node 3.5–10 Ma [150].

Included species.—7. 

Violabehboudiana Rech. f. & Esfand., V. cinerea Boiss., V. erythraea (Fiori) Chiov., V. etbaica Schweinf., V. kouliana Bhellum & Magotra, V. somalensis Engl., V. stocksii Boiss.

Distribution.—Northeastern Africa to southwestern Asia (Figure 29). Disjunctly distributed in the monsoon region on both sides of the Red Sea, Sokotra and the Arabic coast of the Indian Ocean, southern Iran, most of Pakistan, and northwestern India.

Discussion.—Variation patterns within sect. Sclerosium are poorly understood. It contains closely related races that are difficult to delimit but differ in distribution, life history traits (annual or perennial), pubescence, and style shape. Nine allopatric taxa have been described [1,150,295,296] but most authorities have interpreted the variation as more or less continuous and have retained only one or two variable species [79,297]. However, a detailed study of the Iranian taxa [91,150] revealed three morphologically discrete species and allopolyploid relationships among them (V. stocksii 4x; V. cinerea 8x; V. behboudiana 8x), which may suggest more taxa warrant recognition within the section. Section Sclerosium may have started to diversify in Late Miocene 3.5–10 Ma ago [150]. The young age corroborates the low morphological differentiation among taxa. The crown group age coincides with the initiation (or intensification) of the Indian monsoon system, caused by the uplift of the Himalayas and the East African mountain plateaus [298,299]. The precipitation brought by the monsoon plays an important role for the flora in this otherwise arid region.

Section Sclerosium is vegetatively somewhat similar to sect. Xylinosium (especially Viola scorpiuroides) but the sections are distantly related, allopatric, they differ in several important characters, and any similarity must be interpreted as parallel adaptation to arid environments.