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Four species of Punctelia occur in Britain. Until very recent years, only two of these (P. subrudecta and P. jeckeri) were regularly found in central England.
P. subrudecta and P. jeckeri:
P. subrudecta and P. jeckeri:
P. subrudecta develops punctiform soralia mainly on the lobe surface. P. jeckeri develops abundant frilly soralia along internal lobe edges. The distinction isn't absolute, more one of degree.
Another useful character is the presence of slight pruinosity near lobe edges in P. jeckeri, not seen in P. subrudecta.
These images are P. jeckeri showing light dusting of pruina, its cause being crystalline matter produced on upper cortex.
These images are P. jeckeri showing light dusting of pruina, its cause being crystalline matter produced on upper cortex.
As the generic name suggests, members of Punctelia generally have punctiform soralia, a good lead-in character. As with most natural history, it is difficult to draw up an absolute list of characters defining the members of a genus.
Punctelia borreri is spreading rapidly but its separation from P. subrudecta causes problems. The keys separate the two by 'underside black' in the former, 'underside pale brown or partly white' in the latter.
However, this is the underside of P. subrudecta!
However, this is the underside of P. subrudecta!
It is not uncommon for P. subrudecta to have a zone of quite dark, almost black-ink washed colour near the margin. It is important to examine the underside in central portions and consider the colour transition in general.
P. borr. then P. sub. in central part.
P. borr. then P. sub. in central part.
A (reliable) character of P. borreri is the light pruinosity widespread on the upper surface, never seen in P. subrudecta. This character is not mentioned in the British literature.
P. borreri:
P. borreri:
P. borreri has rather larger lobes, so that they tend to become wavy under their own weight. P. subrudecta lobes are more rounded and hold their shape (a descriptive, not necessarily absolute distinction).
Dobson suggests that P. borreri is pinkish-red while P. subrudecta is carmine-red. This distinction cannot be reliably detected. Thin layer chromatography or micro-crystallization would be required to distinguish between gyrophoric and lecanoric acids (both C+ red).
I can now reliably identify P. borreri but there were some pitfalls along the way, not helped by slightly misleading information in the literature. It will be interesting to chart its spread.
A word of warning. Parmelia sulcata can sometimes form punctiform soralia and look horribly like Punct. borreri. These forms of P. sulcata are sometimes assumed to be Punctelia on sight, the underside is seen to be black, and an incorrect record of P. borreri is created.
P. sulc:
P. sulc:
