The following is a summary of a short presentation given at the 2016 SBRS Autumn Get-together.
(1) Polypodium vulgare/P. interjectum
Both species are probably quite frequent in Sussex and can present considerable difficulties for both beginner and experienced botanist alike. The only certain way of telling them apart is by microscopic examination of the sporangia, and specifically by determining the average number of thick-walled annulus cells, with P. vulgare having 10-14 on average and P. interjectum 7-9. The excellent illustrations in Stace (2010) and the Plant Crib show the differences very clearly and are an excellent aid. There do seem to be differences in habitat preference, with the former tending to occur on more acidic substrates and the latter on more calcareous ones. As calcareous habitats are probably more widespread in Sussex, encompassing the chalk downs, the lime-based mortar of walls, immature sand dunes and various clay formations, it might have been predicted that P. interjectum would be the more frequent of the two, however our post-2000 records do not necessarily reflect this. There are also well-known differences in frond shape, with P. vulgare being narrowly lanceolate to linear and P. interjectum being more ovate. It must be emphasised again that the identity cannot be inferred from habitat or frond shape either alone or in combination, and that the microscopic characters described above and expanded upon in Stace are the only truly reliable ones.
(2) Polypodium x mantoniae
This is the hybrid between Polypodium vulgare and P. interjectum. It is the commonest Polypody hybrid nationally, though there is at present only one confirmed record of it for Sussex. It is intermediate in both macro- and microscopic characters, having for example an average of 9-10 thickened annulus cells. It often, though not always, exhibits hybrid vigour, with large robust fronds forming extensive monoclonal patches. However robustness and intermediacy of characters are not enough to clinch the identity-sterility must be demonstrated. This hybrid is either completely sterile or very nearly so and any spores it does produce will be shrivelled and empty. With this in mind it would be a good idea to acquaint yourself with the appearance of healthy, viable spores from sporulating P. vulgare and P. interjectum
(3) Calystegia x lucana
This is the hybrid between the native Calystegia sepium and the alien introduction C. silvatica. Although far from being unknown in Sussex, this hybrid will almost certainly have been greatly under-recorded. In a survey carried out in the London area in the mid-1960s, it was found that 3% of the white-flowered Calystegia plants examined were referable to C. x lucana. It might now be the dominant taxon in that area. According to Stace’s Hybrid Flora (2015), the hybrid “shows a complete range between the two species, probably due to frequent backcrossings”, from which it can be inferred that C. x lucana is fertile. The Plant Crib identifies a number of abnormalities that are particularly prevalent in the hybrid. They include misshapen bracteoles, the possession of three bracteoles rather than the normal two, and the possibility of a second corolla tucked into the axils of the bracteoles. The characters separating C. sepium and C. silvatica are well covered in Stace and other references so I will not repeat them here, but as with the species, when trying to identify the hybrid it is necessary to look at both floral and leaf characters. As propagules such as rhizome fragments can be spread by human activity, the hybrid could very well occur in the absence of one or both parent species. This might be inconvenient in one way, preventing direct comparison of hybrid and parent material, but it does mean that the chances of our meeting with the former are increased considerably.
I now want to consider two infraspecific Calystegia taxa which in theory would not be difficult to name. The first is C. sepium subsp. roseata. This has never been recorded in Sussex and is more of a western plant, but given the apparent eastward migration of such traditionally western species as Erodium moschatum and Polycarpon tetraphyllum in recent years, as well as its presence in the neighbouring vice-counties of south Hampshire and the Isle of Wight, it might be waiting out there somewhere for those who seek it. As a native it is strictly coastal with its favoured habitat described in the Flora of Hampshire (1996) as “brackish marshes,” which might include maritime ditches, hedgerows and reed beds. It has corollas that are predominantly pink (with narrow white stripes) and is hairy, particularly on the young shoot tips and around the leaf sinuses.
Calystegia sepium subsp. sepium forma colorata also has predominantly pink corollas but is glabrous. It might not be as exacting in its habitat requirements, though ironically the only Sussex record so far is for a coastal site at East Head.
(4) Veronica hederifolia
The great majority of Veronica hederifolia records now coming in are for the segregates, but with the odd one still for the aggregate. Whilst it is essential to use at least three or four characters to make a determination, it is not necessary to wait for fruits. The two subspecies have different chromosome numbers and are quite distinct in many, albeit sometimes subtle ways.They also tend to grow in quite different habitats. The Plant Crib is the most useful reference in this regard as it gives so many differences, with the floral characters of anther size and style length being among the most reliable. The greater effort being made to record V. hederifolia to subspecific level has revealed that susp. lucorum is a good deal more frequent than was originally thought. This is perhaps not really surprising as it seems to be more shade-tolerant than subsp. hederifolia.
Brewis, A., Bowman, P. & Rose, F. (1996). The Flora of Hampshire. Harley Books, Colchester.
Rich, T.C.G. & Jermy, C. (1998). Plant Crib. BSBI, London.
Stace, C. A. (2010). New Flora of the British Isles. 3rd edition. CUP, Cambridge.
Stace, C. A., Preston, C. D. & Pearman, D. A. (2015). Hybrid Flora of the British Isles. BSBI, London.