Thymus. Identification of the three native British Species. A search in Breckland.
Three species recognised in Britain:-
1) Thymus drucei ( also called polytrichus) Wild Thyme
2) Thymus serpyllum Breckland Thyme
3) Thymus pulegiodes Large Thyme
This division is largely based on the work by C.D. Pigott and is backed up the chromosome count.
https://nph.onlinelibrary.wiley.com/doi/pdf/10.1111/j.1469-8137.1954.tb05255.x
T. drucei 2n=52-56
T. serpyllum 2n=24
T. pulegiodes 2n=28
The problem as recognised by Mr Pigott is that in the field the variation of T. drucei is such that its features can overlap with the other two species. This creates major problems. Following are a few photos documenting the difficulties in identification of these species.
Several features used by Mr Pigott ( e.g. density of stomata, mean length of stomata guard cells etc) to distinguish species are not useful in the field and even they have overlap. So what features are useful in the field? Stem hair distribution is the main one but they are only possibly reliable just below the inflorescence. The stem hairs of the runners seem less reliable and those on the other parts not very reliable at all. Stem hairs within the inflorescence can be different to those below, at least in T. pulegiodes.
Initial results suggest that T. puglegiodes can be told apart from the other two, but separation of T. serpyllum from drucei is very very difficult.
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| Simplified text book stem hairs best observed just below inflorescence. |
Following are a few photos taken in an attempt to separate and identify these three species. Very much a learning curve with much confusion and doubt. Identification relies on very subtle features, see following Pigott statement:-
"In both T. pulegioides and most races of T. drucei, the leaves are generally twisted on their petioles so that they lie almost horizontally, while in British plants of T. serpyllum the young runners tend to curve upwards and the narrow leaves are borne in a very characteristic upright position."
The above sounds good but in practice serpyllum and drucei are both variable and overlap in growth features with both often having runners that terminate with an upward pointing shoot. More often drucei has a runner tip extension with just flat opposite side leaves without the development of side shoots but this is not always the case.
1) Starting with what should be the easiest species to identify, Large Thyme T. puglegiodes.
Main features:- Tufted rather than forming dense mats with stem often erect except where heavy grazing. Leaves large (9(-17) x 4 mm.), ovate with distinct petioles, glabrous on the surfaces, slightly folded. Foliage strongly thyme-scented. Lack of ground hugging runners is a key feature.
The first site in Cambridgeshire visited was Linton where two populations are known, one in a housing estate and the other at a disused train station. It is likely that these are come from planted origin. They did exhibit the required features. The plants were large, robust and despite the last day of October still in flower.
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| T. pulegiodes, Linton, 31st Oct 22 |
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| T. pulegoides. Dead flower stem showing stem hairs. |
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| Left T. pulegoides , right T. drucei. |
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| T. puglegoides from Linton housing estate. Top of leaf. |
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| T. pulegiodes. Underside of leaf. Veins not always as visible as in this plant. |
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| T. pulegiodes close up of stem hairs. Linton. |
A second site for T. pulegiodes was visited but this proved much harder to be certain what species we were looking at. Grimes Graves is a well known site for T. pulegiodes but also has records for T. drucei. Grimes Graves has short turf which excludes Thyme from most areas but ant hills and bumps provide suitable habitat. The plants here were smaller. Fotunately some were still in flower despite the late date of 8th November.
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| T. pulegoides, Grimes Graves. Flowering stem. 8th Nov 2023 |
The upright and tall inflorescence plus the stem showing the hairs in lines just at the corners of the square stem all look good for T. pulegiodes. The much smaller leaves were not quite what we were expecting. In this photo they are showing the folded feature. The plants also when crushed exhibited a strong aromatic scent although not everybody can smell this.
At the time I was not aware of the slight folding of the leaves but this photo shows that feature quite well. Not an easy feature to distinguish from drucei which can also sometimes have a slight folded shape.
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| T. pulegoides at Grimes Graves. |
Photo above shows the leaves are slightly folded, have sparse well spaced large oil glands and lack the long hairs (bristles) although just a few on the leaf margin occur (see top right).
In conclusion, T. pulegoides has a more upright form than either drucei or serpyllum which are more mat forming and have runners that root at internodes. The flowering inflorescence is tall and rises above the foliage. The foliage is strongly aromatic if you are sensitive to it. The stem hairs are in vertical lines at the corners of the square stem but you can get a few on the flat sides as well. The leaves are described as elliptical to ovate with folding. The midrib is often visible on the top surface as a pale line and is the folding axis, especially at the base of the leaf. Leaves are typically 8mm long and 4mm wide but length can vary (6-12mm) and are 'usually no bristles' except maybe one or two on the pedicle. Actually they are not hairless as the margins have tiny curved white hairs which are also found on the upper surface especially near the margin. These were seen under microscope at X20-40. All three species can have these curved margin hairs.
The plants found at Lakenheath Warren had leaves down at 5mm long and just a couple of long bristles at the base and quite long petioles but this was in early December and on late growth. Even in December the aromatic smell was very strong unlike nearby drucei.
The problem in identification comes from extreme versions of drucei which share the ascending shoots, have large leaves (up to 11.5mm long) and have two densely hairy sides parted into two ranks on the stem. According to Pigott this variety is found in south-east and east England on Chalk and in France. I have not seen an example of drucei which is that extreme.
Finally the distribution of pulegoides is much more limited than drucei as it has a much more south-east Britain pattern, often following the chalk. See latest BSBI maps.
Finally it is suggested in Pigott that pulegoides has very few or no stomata on the upper surface and this expanded in "The Vegetative Key to the British Flora" to say it has none. This is not true as both the Linton and a population at Lakenheath Warren have been shown to have a lot of stomata on the upper surface. In fact all three species shows many stomata on the upper surface although Pigott did find the density of stomata did vary. I don't think this will be a useful field feature and my results do not agree with the values reported by Pigott. Stomata density did vary across the leaf and maybe the density of stomata is on average, less in pulegoides but this needs checking at more sites.
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| T. pulegoides upper leaf showing stomata. |
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| T. puglegoides upper leaf surface with oil gland and stomata. |
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| T. serpyllum from Icklingham Plains upper leaf surface. |
Widely distributed from the north of Scotland, Ireland and much of England T. drucei is mat forming with dense rounded heads of pink flowers rising from the mat. It is faintly aromatic.
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| T. drucei Foxhole Heath. |
The mat forming habit is not so apparent when plants are growing through other dense vegetation which forces a more upright growth. This was one of the sources of confusion. The inflorescence is short which fits drucei. The leaves have quite a few of the very long hairs(bristles) that often occurs in drucei. Many of the leaves are flat although some show some folding. (Foxhole Heath is a known site for the rare Breckland Thyme, T. serpyllum.)
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| T. drucei. Foxhole Heath. |
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| T. drucei. Foxhole Heath. Many bristles on leaf margins. |
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| T. drucei. Devils Dyke. Hairs all round the stem! |
Above shows more variation of stem hairs with the upper internode showing two faces one having less hairs which conforms to the expected. The stem shows quite a square section but this is hard to show in photographs and in practice you have to rotate the stem to see the subtle square sides. The next internode down shows almost even hairs on all four faces and white decurved hairs that approaches what you might expect in serpyllum. Leaf bristles are few, another reason why these plants might be thought to be serpyllum.
The Devils Ditch plants have long been subject to debate about their identity but it seems probable that they are T. drucei which have stem features and small leaves of serpyllum due to the tough conditions at the top of the chalk bank. Unfortunately no flowering stems were present during this visit but Alan Leslie provided me a pressed sample from June 2022 which shows that at this site stem hairs that are long and distributed evenly all round the stem. Pigott does suggest that this is rare but does occur. A feature of all three species is that the hairs get longer as you get nearer the inflorescence.
These Devils Ditch plants do appear to be confused in some features, maybe it is not possible to split serpyllum from drucei on physical features.
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| T. drucei from Devils Ditch, Cambridgeshire. Pressed example. |
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| T. drucei Deadman's Grave. 6th December 2022 |
The leaves are thick and fleshy and have oil glands. Those with sacks of oil still present are shown in following photo. Many more glands have lost their sacks and are just large holes in the leaf surface.
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| T. drucei leaf showing oil glands with sacks of oil still present. |
The leaf surface has the large oil glands and also a lots of conical pits which are also oil glands missing the oil which was held in a thin sack. There are also white structures which are too large to be stomata and appear to be some form of protrusion, maybe tiny hair like structures. All three species have these oil glands.
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| T. drucei?. Deadman's Grave. 6th Dec. 2022. Mat like with runner. |
Above photo showing a probable T. drucei forming mat of leaves. A runner is heading off right and will develop roots. Note that within the matt of leaves the new shots are pointing up which is supposed to be a feature of serpyllum making this a hard plant to identify. Non inflorescence stem hairs suggested this was drucei however the growth pattern looks more like serpyllum. The key difference does appear to be the growth pattern seen in the runner heading off to the right but this is a plant that shows just how difficult it is to separate between these two species.
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| Growth pattern of T. drucei runner with mainly flat side leaves and limited shoot clusters. |
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| T. serpyllum runner with more shoot clusters and less side leaves. |
3) Thymus serpyllum. Breckland Thyme.
This species proved the hardest to identify. Its range is limited to a few Breckland sites only. Since its features are so similar to T. drucei with overlap in many, a candidate must have several features:-
a) Very small leaves with just a few (0-3(4) long hairs (bristles) on each side near the leaf base, not including those on the petiole. Many T. drucei also have just a few similar hairs but other have lots of hairs. Sometimes T. drucei can have very small leaves so there is overlap. Sometimes drucei has more rounded leaves but the table of ratio of length to width given in Pigott underestimates the variation in my opinion. New leaves have no bristles and these seem to grow as the leaf develops.
b) Serpyllum has stems with rounded corners without the edge found on many T. drucei. This is very hard to see and it is best to look at several sections on the stem. It is best on a flowering stem below the flowers but in November/ December flowers were few and although non-flowering stems were possibly more variable, it seemed still possible to see a difference in many plants but there is serious overlap.
c) The stem hairs on serpyllum are mostly evenly distributed round the stem. In both T. drucei and T. serpyllum they are white and downcurved. The stem hairs tend to be similar and not as variable as in drucei. In both species there are different hair lengths but you need a microscope to see that. The length of these down curved hairs did vary depending on the site. The problem with this feature is that some drucei, for example those at the Devils Ditch have hairs all-round the stem. The other problem is that serpyllum occasionally can have some parts of stem showing drucei like features. As previously stated stem hairs are best looked at on the flowering stems but although less reliable, the runner stem hairs often give some indication subject to more variation.
d) The leaves form little upright 'shoots' whereas in drucei the leaves are flatter. This slight structural difference is considered by Pigott to be consistent in the Breckland populations. This is probably the most reliable feature to distinguish between these two species but is very subtle. Drucei tends to have more flatter leaves on each side of the runners especially the terminal ones but serpyllum can also occasionally have some too. Occasionally serpyllum can show the really vertical stems as shown in Mr Pigott's drawing but so far these drawings seem to overstate the difference.
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| T. serpyllum Icklingham Plains 14Feb2023 |
e) Neither T. serpyllum or T. drucei have much aromatic smell even when crushed, just a faint chemical smell although this was only tested in the winter.
Taking structure first which Pigott regarded as important.
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| T. serpyllum Deadman's Grave. 6th Dec 2022 |
The runner stem has upright little shoots of opposite leaves. Drucei has them as well in the mat but there is a subtle difference, best seen in the runner shoots.
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| T. serpyllum Deadman's Grave. 6th Dec 2022 |
Part of the mat showing the little upright shoots of serpyllum.
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| T. serpyllum. Thetford Warren Lodge 22 Nov 2022 |
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| T. serpyllum Icklington Triangle 14th Nov 2022 Little tufts or 'shoots' of leaves on upright stems. |
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| T. drucei. Flowers and mat of leaves slightly flatter . Foxhole Heath 14Nov2022 |
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| T. drucei Red Lodge 24 Nov 2022 Lots of bristles. |
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| T. drucei Red Lodge 24 Nov 2022 Almost no bristles |
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| T. serpyllum or drucei? Rampart Field 24 Nov 2022 |
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| T. serpyllum? Rampart field. Tiny new leaves on new shoots. Older leaves narrow. 22Feb23 |
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| T. serpyllum. Thetford Warren Lodge |
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| T. serpyllum Thetford Warren Lodge , close up of stem. |
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| T. drucei Red Lodge, close up of stem |
" mean leaf length 5.2 mm. + 0.54), narrowly spathulate (mean leaf length/breadth ratio 3.56±0.59), and glabrous, except for the basal marginal bristles"
I think these values refer to full developed leaves. Quite where the leaf ends and the petiole starts is another tricky area but it looks like the petiole which is short is included in his measurements. The Pigott paper also covers various races of drucei which have leaves from 3mm to 12mm long so drucei could have even smaller leaves than serpyllum in some cases.
Conclusion.
Separation of serpyllum from drucei is not easy. There does appear to be a population of plants at sites known to contain serpyllum that do have a certain 'jizz'. These plants are typically grow in open sandy areas with no competition but can spread to areas with more vegetation. Most plants are well established and cover about a square foot having enlarged through runners. These plants always have small leaves with just a few bristles on older leaves. Stem hairs on runners are often spread all round the stem but on occasion can be more like drucei with two sides having more hairs. The 'jizz' factor is as Pigott suggests due to a more upright growth pattern and is probably the only way to distinguish between serpyllum and drucei. It is hard to illustrate using photos actually what is the difference is, as the growth patterns are variable but putting the difference into words is even harder. The following from Mr Pigott does not quite describe the difference.
"In both T. pulegioides and most races of T. drucei, the leaves are generally twisted on their petioles so that they lie almost horizontally, while in British plants of T. serpyllum the young runners tend to curve upwards and the narrow leaves are borne in a very characteristic upright position."
Some like those at Rampart Field are examples of plants that seem intermediate and are particularly difficult. It would seem that vein pattern of the leaf has too much variation to be useful. Also the number and size of stomata is also too variable to separate these two species.
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| Thymus serpyllum, Rampart Field, 4th April 2023 |
More research during the summer showed those plants considered to be serpyllum flower about four to six weeks later than those considered to be drucei. The serpyllum were flowering in mid July.
