Further research on lemon scented thyme

In my 2005 article in Plant Heritage I revealed that in 1811 Thymus citriodorus was described as a species and not a hybrid of T. vulgaris and T. pulegioides as generally referred to in literature for many years.  I have subsequently done further research into the literature on T. citriodorus and am very grateful to Dr. Brent Elliott for his assistance in sourcing the publications referred to by Schreber and Persoon in the Lindley Library.  I am also grateful to Dr. Alan Paton, Assistant Keeper of Herbarium at Royal Botanic Gardens, Kew, for his help in interpreting this literature.

The IPNI (International Plant Names Index) lists T. citriodorus as a subspecies of T. serpyllum: Thymus serpyllum L. subsp. citriodorus Pers. Syn. Pl. (Persoon) 2(1): 130. (1806).   Alternatively it is also quoted as varietas (Thymus serpyllum var. citriodorum).   However at the beginning of the 19th Century neither status was yet in use.  Persoon (1806) originally published the epithet citriodorus.  Within T. serpyllum, Persoon (1806) detailed two elements: α lanuginosus and β citriodorus.  Under the epithet citriodorus he recognises two varieties: major and minor.  As citriodorus is divided into varieties by Persoon and as it is placed under a species (T. serpyllum), botanists have given the epithet citriodorus subspecific rank and this interpretation is followed here.  Under the var. major of subsp. citriodorus, Persoon (1806) refers to T. exserens Ehrh. and under the var. minor he refers to T. includens Ehrh.  The type of the name citriodorus must therefore be the type of either T. exserens Ehrh. ex Link (1822), or T. includens Ehrh. ex Rchb.(1831).  IPNI lists these names as synonyms of T. serpyllum.  However Reichenbach in Flora Germanica lists T. exserens and T. includens as synonyms of one element of T. chamaedrys, a synonym of T. pulegioides.  This indicates that the use of the epithet serpyllum by IPNI should be regarded as referring to the common name Serpyllum (T. pulegioides), rather than T. serpyllum (creeping thyme).

Persoon's description in Synopsis plantarum refers to older publications which describe lemon scented thymes; Vahl's Florae Danicae (1799), Vaillant's Botanicon Parisiense (1743) and Bonelli's Hortus Romanus (1775).  Hortus Romanus refers to Caspar Bauhin's Pinax Theatri Botanici (1623), Tournefort's Institutiones Rei Herbariae (1719) and Tabernaemontanus's Eicones Plantarum (1590).  Lemon scented thymes in these publications are usually given the name Serpyllum foliis citri odore.  Although Persoon makes no reference to any herbarium specimen, I understand that Persoon material of this taxon is held in Paris (P.), but I have not yet been able to see a copy of it.  There is a microfiche in the library at Royal Botanic Gardens, Kew, but it is not indexed and so far it has not been possible to locate it within the microfiche.

Schreber subsequently treated this thyme as a species: Thymus citriodorus (Persoon) Schreb. in Schweigg. and Koerte, Fl. Erlang. ii, 17 (1811).  He also refers to August Bachmann's (Rivinus) Introductio generalis  in rem herbarium (1690) and Volckamer's Flora Noribergensis (1700).  In Riv. mon. there is a description and illustration of Serpyllum citratum, which shows the growth pattern typical of T. pulegioides.  Volckamer refers to Parkinson's Theatrum Botanicum (1640).  There is an illustration of Serpillum vulgare minus (ordinary mother of thyme) in Theatrum Botanicum, which shows the typical growth pattern of T. serpyllum; creeping stems with roots at the leaf nodes and flowering stems with spherical heads of flowers.  There is also an illustration of Serpillum citratum (lemmon thyme) and this shows the typical growth pattern of T. pulegioides; a single group of roots and the typical upright branched inflorescence of the species, with flowers in terminal clusters and verticillasters below.

In view of Persoon's references to older publications where the lemon thymes described and illustrated have the characteristics of T. pulegioides, in my opinion it is more likely that he is actually describing T. pulegioides and not T. citriodorus (misapplied), available in the nursery trade.  It should also be borne in mind that the common name for T. pulegioides in Germany is Serpyllum and that creeping thyme is known as Quendel and this could have led to later confusion between the species.  I have been in contact with Munich Botanic Garden and they have informed me that their specimen described as T. serpyllum var. citriodorus from the herbarium Schreberianum, was identified by Jalas in 1972 as T. pulegioides L.  It would appear that neither Jalas nor Morales has typified lemon scented thyme more recently.

In the early 20th Century further confusion regarding species of lemon scented thymes arose because nurserymen who introduced them, wrongly placed them in the species T. citriodorus, because they smelt of lemon.  Inexplicably botanists regarded T. × citriodorus and its cultivars, as cultivars within a hybrid T. × citriodorus, between T. pulegioides and T. vulgaris, because they had the characteristics of T. pulegioides.  In 1924 in Feddes Repertorium, Ronniger refers to T. × citriodorus in Britain.  Stearn, in 1951 in the RHS Dictionary of Gardening and Bean in 1980, in Trees and Shrubs Hardy in the British Isles, both describe cultivars of T. × citriodorus.  Ronniger considered that the only examples of T. × citriodorus available in Britain were cultivated garden plants.  At that time the cultivars 'Aureus' and 'Golden Dwarf' were both available from nurseries, but it should be noted however that these lemon scented thymes are actually cultivars of T. pulegioides and not T. × citriodorus hort.  The Golden Thyme DNA Study demonstrated that these lemon scented thymes should be regarded as T. pulegioides and that T. citriodorus hort., T. pulegioides and T. vulgaris are distinct taxa.

This confusion clearly demonstrates the importance, when determining correct attribution and appropriate status, of using several methods of delineation.  This involves assessing the general characteristics of the sample, keying it out using a botanical key and comparing its DNA with a known base line of species and then using all three results to come to a final conclusion.

References

Persoon, C.H. (1807). Synopsis Plantarum, 2(1): 130-131.  C.F. Cramer/J.G. Cotta, Paris & Tubingen.

1405. THYMUS. Cal. bilabiati faux villis clausa.  Cor. lab. superius planum, emarginatum.
SERPYLLUM. Cal. campanulatus aut ovatus, bilabiatus: laciniis 3-superioribus latioribus; inferioribus ut plurimum subulatis et ciliatis.
1. serpyllum, flor. capitatis, caulibus decumbentibus, fol. planis obtusis basi ciliatis. L. Flor. dan. t. 1165. Vaill. bot. t. 3a. f. 9. Flor. β. Sabb. hort. 3. t. 69.  Hab. in aridis apricis.
2. lanuginosus, flor. capitatis, caulib. repentibus hirsutis, fol. obtusis villosis. Willd. Hab. in rupibus,
aridis.
β. Citriodorum, fol. ovatis glabris odore Melissae officinalis.  Dantur insuper Var. major, staminib. exsertis: Th. exserens: Ehrh. et minor, stam. in cor. latentibus: Th. includens. Ehrh.

[translation]
1405. THYMUS. Cal. bilabiate. Throat closed with hairs.  Cor. upper lip flat, emarginate.
SERPYLLUM Cal. campanulate or oval, bilabiate: with 3 upper broader flaps; the lower awl-shaped and fringed with hairs for the most part.
1. serpyllum, flowers in clusters, with prostrate stems and flat obtuse leaves fringed with hairs.  Lives in sunny dry places.
2. lanuginosus, flowers in clusters, with hairy creeping stems, and hairy obtuse leaves.  Lives in dry rocky places.
β. Citriodorum, with oval glabrous leaves, and an odour of Melissa officinalis.  The [following] vars are given above: var. major, with protruding stamens: Th. exserens Ehrh. and minor, with stamens concealed in corolla: Th. includens Ehrh.

Schreber, J.C.D. (1811) In Schweigger, A.F., and F. Koerte, Flora Erlangensis pp 16-19.  J. J. Palm, Erlangen.

3. citriodorus, Citronen - Quendel.
Floribus verticillatus, caulibus erectiusculis subpubescentibus, foliis lanceolatis ovatisque subciliatis, corollis calyce longe ciliato brevioribus, staminibus inclusis.  Schreb. Serpyllum citratum.
Riv. mon. t.41. f.3. s. fl. Th. citriodorus Pers.
Fl. Jul. Aug. in hortis; an der Doser brücke.  Volkamer.
Labium superius calycis trifidum, mucronatum; inferius eo paullo longius.
 

Bauhin, C. (1623) Pinax Theatri Botanici, p 220. Ludwig König, Basel.

Serpyllum
V. Serpyllum foliis citri odore.
Serpyllum alterum citratum, Matthiol. citrumolens, Gesn. hort. Thal. citratum, Tab. Eyst.
Serpyllum sylv. 1. citratum, Cast.
Serpyllum referens planta, odore citri, Caes.
Serpyllum pannonicum 1. Clus. pan. & hist.
Thymum latifolium, Ger.ico.

Bauhin, J. (1650-51) Historia Plantarum Universalis, 3: 270.  Publisher unnamed, Yverdon.

....1. & Serpyll. Citrii odore. H. Lusit. Serpyllum citratum. C. Bauh. phyt. Serpillum 4. siue foliis Citri odore, & Serpyll. citratum Tabernamontani.
In Pannonia mixtim cum vulgari prodit, cum co etiam florens lunio.  Ego prope Dornach Serpillum Citrii odore mense Augusto obseruaui.

Bonelli, G. (1775) Hortus Romanus, 3: pl. 68-69.  Bouchard & Gravier, Rome.

Tab 68. Thymus vulgaris, folio latiore
Tab 69.  Serpillum vulgare maius, flore purpureo
1 Serpillum foliis Citri odore. C. B. Pin. I.B.H. 197. Serpillum Citri odore. I.B. 3. Part. 2. 270. Serpillum Citratum. Tabern. Icon. 360. Ital. Serpillo Cedrato.

Parkinson, J. (1640) Theatrum Botanicum, pp 6-9. Thomas Cotes, London.

11. Serpillum Citratum. Lemmon Tyme.
The Lemmon Tyme is somewhat like the common wild kinde in the manner of growing, but standeth more upright, bushing thicke with branches, the leaves are small and of a darke greene colour like them but have the sent of a Pomecitron or lemmon: the flowers are more white then in the common sorts.

Reichenbach, L. (1830) Flora Germanica Excursoria, 1: 312-313.  Karl Knobloch, Leipzig.

2120.  Th. Chamaedrys Fr.  adscendens diffusus glabrescens, ramis bifariam pubescentibus, floribus capitatis verticillastisque, foliis ovatis petiolatis.  Rchb. pl. crit. IX. -
α. calycibus spadiceis, capitatis aut subverticillatis, huc: Th. citriodorus Schreb. (non Hortul. qui Th. elatus Schrad.)  Th. Serpyllum Auct. Hayne Arzng. XI. 1. fig. med.  Pro stam. mox inclusis mox exsertis: Th. includens et Th. exserens Ehrh. - Th. pulegioides R. ramis tetraquetris crassiusculis rigidulis bifariam albo-pubescentibus, verticillis numerosis multifloris, demum omnibus remotis.  Verticillis paucis: Vaill. par. t. 32. f.7.8? - Th. subcitratus Schreb. praecedentis forma gracilior, verticillis paucifloris, imo superioribus comosis. -
β. calycibus viridibus, capitatus: Th. montanus W.K. t. 71. Serpyll. pannon. I. Clus. 359. - verticillis inferioribus remotiusculis, ramis foliisque pilosis: Th. decumbens Brnh. Vaill. par. t. 32. f. 6. - verticillis omnibus demum remotis, ramis filiformibus foliisque glabriusculis: Th. silvestris Schreb. Vaill. par. t. 32. f.9. Hayne 1.c. fig. dextr. Etiam hic ut reliqui vel includens vel exserens.  Th. glabratus Schult. - An Feldrainen, Haiden, Waldrändern, auf Hügeln, Bergen. - Juli. Aug. 

Rivinus, A.Q. (1690) Introductio generalis in rem herbarium, 11.  Plate 41. Christoph Günther, Leipzig.

Serpylli variae sunt species, nempe aliae repentes, aliae magis erectae, quamius a serpendo dictum videatur serpyllum.  Flores si spectas, deprehenduntur ii nunc majores, nunc minores, modo purpurascentes, modo candidi.  Si folia, sunt quaedam rotundiora, quaedam oblonga magis & angusta.  Planta universa communiter glabra est, occurrit tamen & penitus hirsuta in montibus.  Praeterea in odore quoque sua est diversitas ac potissimum cunctis probatur citratrum, poma scilicet citri vel melissam redolens.

Ronniger, K. (1924) Beiträge zur Kenntnis der Gattung Thymus L.  Feddes Repertorium, 20: 321-336.

11. T. pulegioides × vulgaris (in England nur Kulturpflanze).

Royal Horticultural Society.  (1951) RHS Dictionary of Gardening, 4: 2108-2111, Oxford University Press/RHS, London.

Tabernaemontanus, J.C. (1590) Eicones Plantarum, Pl. 360.  Nicolaus Bassaeus, Frankfurt

Serpillum citratum/Citronen Quendel
(Serpillum foliis citri odore.  C.B. Pin 220 - handwritten in RHS copy)

Tournefort, J.P. (1719) Institutiones Rei Herbariae, 197. Typographia Regia, Paris.

Serpillum foliis Citri odore C.B. Pin 220. Serpillum Citrii odore J.B. 3. Part. 2. 270. Serpillum Citratum Tabern. Icon. 360.

Vaillant, S. (1727) Botanicon Parisiense, p 184. J. & H. Verbeek, Leiden.

5. Serpillum foliis Citri odore C.B. Pin.

Volckamer, J.G. (1700) Flora Noribergensis, 315 (381).  Michaellianas, Nuremberg.

SERPILLUM Citratum Park. Serpillum Citri odore JB. Serpillum Citri odore JB. Serpillum Citratum Matthiol. colitur in viridar. nostr. Pernn. Propagantur & multiplicantur singulae hae Serpilli species ramulis avulsis & in terram depactis. Hinter der Doser Brucken. ....