Wednesday 17 January 2024

Thymus. Is identification possible?

 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.

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. 

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. 

T. pulegiodes, Linton, 31st Oct 22

This photo shows the overall plant is more erect, not forming dense mats being more bush like. The inflorescence is tall and held high on a vertical stem. It would appear that looking at the pink flowers will not help in separating these three species. 
The arrangement of hairs on the flowering stem is normally regarded as the key separator in identification, however there are problems. This photo above actually shows the best place to look, that is just below the inflorescence. Within the inflorescence the hairs just present on the angles of the square stem change to become spread all round the stem. Stem hairs on just leafy shoots are less reliable being more variable according to the literature.
T. pulegoides. Dead flower stem showing stem hairs.

The key feature is that the stem hairs are concentrated at the corners of the square stem. Actually the stem is rectangular and better described as four-angled. The faces of the stem can have some fine hairs between the rows at the edge and you can just see these in the photo at the lower internode.  
Within the inflorescence more hairs are away from the edges and towards the top the stem can appear round and have hairs all round.

Left T. pulegoides , right T. drucei.

Crushing some leaves produced a strong aromatic smell. The leaves were bigger  and broader than some T. drucei collected from a nearby site.  The problem with size based on the papers by Mr Pigott is that some T. drucei in E and SE England can also have large leaves, also other sites had pulegiodes with much smaller leaves. A more interesting detail is his comments is that T. pulegiodes has slightly folded leaves. This can be seen in the photo above where it is noticeable at the base of the leaves. This is a subtle feature and both the other species can exhibit slightly folded leaves but worth checking. Occasionally T. drucei can also have a folded leaf.

Other features are the lack of long hairs (bristles) on the leaves, although the particular sample above of Drucei is also lacking hairs(bristles), part of its variation, some lack hairs (bristles) while other are very hairy. Actually it is just possible to see the leaf margin has tiny curved hairs and this seems to be a feature found in some of all three species. It does appear that tiny hairs also are on the leaf surface near the margin but are too small to see without a microscope.

T. puglegoides from Linton housing estate. Top of leaf.

The above photo shows the leaves with the large oil glands scattered over the surface. They normally hold a yellow ball of liquid which reflects the light. They are much larger than the stomata which are not visible at x15. All three species have these glands. The visible part of the stem also shows the four sided stem with hairs just at the corners which is expected for T. puglegiodes

T. pulegiodes. Underside of leaf. Veins not always as visible as in this plant.

A feature of this photo above is the very long white hairs ( bristles) which are coming from the petiole. These very long hairs are a feature of all three species and can be highly variable in extent especially in T. drucei.
There are very few bristles on this plant which would appear to be typical of pulegoides. Short curved hairs line the stem angles which again is a good sign for T. pulugoides. The underside of the leaves have prominent oil glands and veins. There are three lateral veins per side showing on these leaves which are quite easy to see. This may be a function of the large size of these leaves as with T. drucei with smaller leaves I have only seen two veins per side. Mr Pigott has illustrations showing three veins per side for T. drucei so this is probably not a useful feature. 
T. pulegiodes close up of stem hairs. Linton.

Just to show that on some plants some hairs are not just limited to the angles and spread onto the sides. The stem hairs are white and down curved.

We were happy that the two Linton populations conformed to T. pulegiodes.

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.  

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. 

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.

T. pulegoides upper leaf showing stomata.

T. puglegoides upper leaf surface with oil gland and stomata.

The above photo shows similar density of stomata to other samples of Breckland serpyllum and drucei.
Just below the pointer is an oil gland. The guard cells of the stomata are slightly shorter at 20.5 micrometer than the other two species. Serpyllum from Icklingham Plains had stomata guard cells at 23.7 micrometer similar to nearby drucei. There does appear to be some variation in stomata size across a single leaf so not sure this is a useful feature.

T. serpyllum from Icklingham Plains
upper leaf surface.

Density of stomata does vary across the leaf surface but the results have shown similar density of stomata on all three species which in complete contrast to the result tabled by Pigott where puglegoides was shown to have far fewer. This needs further investigation by it does appear that pulegoides can have a very low density on some plants but others show similar density to drucei

2) Thymus drucei also called Thymus polytrichus. Wild Thyme.

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.

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.)

T. drucei. Foxhole Heath. 

Another shot of a similar plant at Foxhole Heath. Same short inflorescence. Leaves are really quite small. Still have the long margin hairs (bristles) and the photo below shows the number is too high for serpyllum

T. drucei. Foxhole Heath. Many bristles on leaf margins.

The above photo shows that the stem hairs are not quite as simple as you might think based on the simplified drawing above but does basically conform. The middle section shows that the stem has a square section and the right face has almost no hairs. The other face shows lots of very short hairs. The lowest section of the stem is a bit out of focus but has few hairs.

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. 

T. drucei from Devils Ditch, Cambridgeshire. Pressed example.

T. drucei  Deadman's Grave. 6th December 2022

Above plant has quite rounded leaves and they also have lots of the long hairs (bristles) on the margin and some on the upper surface. The stem is quite square with some short and straight hairs plus some very short hairs on the top surface and a few very short hairs on the surface facing us.

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.

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.

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.

Growth pattern of T. drucei runner with mainly flat side leaves and limited shoot clusters.

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.

T. serpyllum Icklingham Plains 14Feb2023
Above photo shows a very good example of the more vertical shoots of serpyllum on a runner. In this case the side leaves have gone brown and also it is unusual in only single shoots are rising from each internode when normally there would be a symmetrical pair. 

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.

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.

T. serpyllum Deadman's Grave. 6th Dec 2022

Part of the mat showing the little upright shoots of serpyllum.

T. serpyllum. Thetford Warren Lodge 22 Nov 2022

T. serpyllum Icklington Triangle 14th Nov 2022 Little tufts or 'shoots' of leaves on upright stems.

T. drucei. Flowers and mat of leaves slightly flatter . Foxhole Heath 14Nov2022

T. drucei Red Lodge 24 Nov 2022 Lots of bristles.

T. drucei Red Lodge 24 Nov 2022 Almost no bristles
The majority of drucei leaves are held flat but some shoots are upright, see top right of above photo. 

T. serpyllum or drucei?  Rampart Field 24 Nov 2022
Plants at Rampart Field were particularly difficult. Some had very small narrow leaves but others like the example above had leaves which might fit better with drucei. Runner stem hairs were often evenly distributed round the stem. Probably serpyllum.
T. serpyllum? Rampart field. Tiny new leaves on new shoots. Older leaves narrow. 22Feb23

T. serpyllum. Thetford Warren Lodge
The best place to see the difference in structure is on the runners that run out on the soil at the edge of the plant. They have long internodes which show the stem section and hairs well. Stems within the mat often have very short internodes making it hard to see features.

T. serpyllum Thetford Warren Lodge , close up of stem.

T. drucei Red Lodge,  close up of stem

The T. drucei stem in this example is quite rounded and lacks the difference in hair density with similar hairs all round. However the density of the hairs is not as even as in the stem above showing a T. serpyllum. The variation in T. drucei is a problem.

Leaves.  Pigott claims that the Breckland sites have consistently small leaves:-

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.


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. 

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. 

It would seem that some drucei with hairs all round the stem are often not really separable from serpyllum and features overlap. The subtle difference in growth pattern did separate plants but again some drucei showed a similar growth pattern. Runners were taken and potted up which developed in the spring and the majority of those thought to be serpyllum flowered 4-6 weeks after the drucei so it seems we got most correct. There still remained a few plants that puzzled and one might wonder if these two are really separate species or that some mixing is going on. The results from the potted on runners suggest these are two separate species that can often be separated but a few plants are intermediate and are possibly either hybrids or possibly just extreme examples of variation. 

The situation in Euro-Asia is where Thymus is separated into 215 separate species clearly needs genetic analysis to clarify this number. This is proving very difficult as several attempts using DNA barcoding have failed to determine species boundaries.

Examples of research from Bulgaria and Italy are :-

Peter Leonard
December 2023


1 comment:

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