Plectranthus
Description
Herbs or undershrubs.
Leaves opposite.
Flowers usually small, in lax or dense, 6-many-flowered cymes or verticillasters which are arranged in terminal and axillary spurious spikes, racemes or panicles.
Stamens 4, declinate;
Distribution
Africa present, Asia-Tropical, tropical and subtropical regions of the Old World present
About 200 spp. in the tropical and subtropical regions of the Old World, many in Africa; throughout Malesia.
Taxonomy
There is no unanimity of opinion about generic delimitation in the Coleus-Plectranthus complex, there being, for Malesia, at least four names concerned. MORTON and LAUNERT are being inclined to recognize only few genera or only one in this complex, but CODD and BLAKE suggest there are more.
About Ceratanthus there is not much trouble; its distinctly spurred corolla and the insertion of the stamens (the posterior pair near the base of the corolla tube, the anterior pair near the mouth) are two good characters for its distinction.
As to Coleus it has been held that this was distinct by the fused bases of the stamens. As MORTON pointed out — and both LAUNERT and CODD agree — this is an unreliable character in some species, and in degree even variable in the type species of Coleus, C. amboinicus. I must maintain, however, that in that species there is always a fusion. However, in closely related species such as Plectranthus apoensis, P. congestus, etc. which share the calyx characters, the stamens are free. And thus Coleus — as to the type species — cannot be divorced from Plectranthus.
There remains then the question whether there are sharply defined taxa within Plectranthus (incl. Coleus), for which only two characters could be used, viz the structure of the calyx and of the inflorescence, and also whether the bracts are differentiated from the leaves or not, and whether the peduncles are developed or not. Apart from our view that the latter characters are not particularly important, calyx and inflorescence characters cannot be correlated.
As to the calyx there are three main types which are quite distinct in extreme form, viz:
In checking these characters with the species, it appears, however, that these extremes are connected by intermediate structures. In Coleus scutellarioides sensu str. the lateral lobes are stunted and rounded, but in C. galeatus and C. sparsiflorus they are triangular, pointed and somewhat larger, though still smaller than the other teeth. Plectranthus congestus belongs to the amboinicus type, but the lateral lobes are very wide and almost rounded. In the Papuan specimens of Coleus scutellarioides the lateral teeth are half as long as those of the lower lip and obliquely truncate. In C. galeatus the acute-triangular teeth are almost as long as the upper lip, the lower lip being longest. In Plectranthus teysmannii and P.javanicus the teeth are almost equal, but in P. apoensis, congestus, and parviflorus the upper lip is wider than the others.
The only conclusion can be that in this complex the calyx structure is variable and one is unable to sharply define taxa within it, and the inflorescence structures (namely whether of stalked cymes or in verticillasters) are not always correlated with the calyx characters, which of course defeats distinction of more than one genus.
About Ceratanthus there is not much trouble; its distinctly spurred corolla and the insertion of the stamens (the posterior pair near the base of the corolla tube, the anterior pair near the mouth) are two good characters for its distinction.
As to Coleus it has been held that this was distinct by the fused bases of the stamens. As MORTON pointed out — and both LAUNERT and CODD agree — this is an unreliable character in some species, and in degree even variable in the type species of Coleus, C. amboinicus. I must maintain, however, that in that species there is always a fusion. However, in closely related species such as Plectranthus apoensis, P. congestus, etc. which share the calyx characters, the stamens are free. And thus Coleus — as to the type species — cannot be divorced from Plectranthus.
There remains then the question whether there are sharply defined taxa within Plectranthus (incl. Coleus), for which only two characters could be used, viz the structure of the calyx and of the inflorescence, and also whether the bracts are differentiated from the leaves or not, and whether the peduncles are developed or not. Apart from our view that the latter characters are not particularly important, calyx and inflorescence characters cannot be correlated.
As to the calyx there are three main types which are quite distinct in extreme form, viz:
- Solenostemon SCHUMACHER & THONN. em. MORTON (Coleus sect. Solenostemon (SCHUMACHER) BTH., Coleus sect. Solenostemoides BRIQ.): Calyx distinctly 2-lipped, upper lip large reflexed, lower lip 2-fid to various degree, with narrow segments, lateral teeth very short and rounded.
- Coleus amboinicus group: Upper lip as in Solenostemon, but all four other lobes equally large, narrow pointed.
- Plectranthus sensu str. (Isodon (BTH.) KUDO; Plectranthus sect. Isodon BTH.; Plectranthus subg. Isodon (BTH.) BRIQ.; Elsholtzia sect. Rabdosia Bl.; Rabdosia (Bl.) HASSK.): all 5 calyx segments about equal, fairly short in proportion to the tube.
In checking these characters with the species, it appears, however, that these extremes are connected by intermediate structures. In Coleus scutellarioides sensu str. the lateral lobes are stunted and rounded, but in C. galeatus and C. sparsiflorus they are triangular, pointed and somewhat larger, though still smaller than the other teeth. Plectranthus congestus belongs to the amboinicus type, but the lateral lobes are very wide and almost rounded. In the Papuan specimens of Coleus scutellarioides the lateral teeth are half as long as those of the lower lip and obliquely truncate. In C. galeatus the acute-triangular teeth are almost as long as the upper lip, the lower lip being longest. In Plectranthus teysmannii and P.javanicus the teeth are almost equal, but in P. apoensis, congestus, and parviflorus the upper lip is wider than the others.
The only conclusion can be that in this complex the calyx structure is variable and one is unable to sharply define taxa within it, and the inflorescence structures (namely whether of stalked cymes or in verticillasters) are not always correlated with the calyx characters, which of course defeats distinction of more than one genus.
Cytology
DE WET () published an account of chromosome numbers of South African species, but there are obviously at least three base numbers and from these no evidence can be produced for sustaining generic distinction.
Citation
BRIQ. 1897 – In: E. & P., Nat. Pfl. Fam. 4: 359
CODD 1968 – In: Taxon: 239
BTH. 1876 – In: B. & H., Gen. Pl. 2: 1176
Bl. 1826: Bijdr.: 835
KENG 1969 – In: Gard. Bull. Sing.: 48
CODD 1975 – In: Bothalia: 436
R.BR. 1810: Prod.: 505
sensu lat. L'HERIT. 1975 – In: Bothalia: 372
BULL. & KILLICK 1957 – In: Taxon: 239
S. T. BLAKE 1971 – In: Contr. Queensl. Herb.: 4
J. K. MORTON 1962 – In: J. Linn. Soc. Lond. Bot.: 231
BRIQ. 1897 – In: E. & P., Nat. Pfl. Fam. 4: 352
CODD 1971 – In: Mitt. Bot. Mûnchen: 245
BTH. 1876 – In: B. & H., Gen. Pl. 2: 1175
BTH. 1876 – In: B. & H., Gen. Pl. 2: 1175
S. T. BLAKE 1971 – In: Contr. Queensl. Herb.: 1
KARA 1972 – In: J. Jap. Bot.: 193
BRIQ. 1897 – In: E. & P., Nat. Pfl. Fam. 4: 359
CODD 1971 – In: Mitt. Bot. Mûnchen: 249
J. K. MORTON 1962 – In: J. Linn. Soc. Lond. Bot.: 251
KENG 1969 – In: Gard. Bull. Sing.: 141
LAUNERT 1968: p. 295. – In: Mitt. Bot. Mûnchen: pl. 1-3
S. T. BLAKE 1971 – In: Contr. Queensl. Herb.: 6