A local botanist, Phill Brown, found this strange looking thistle about 2 or 3 miles from where we live
You can see that it is quite a robust plant, with large, fairly flat leaves that are deeply lobed.
The inflorescence is similar to melancholy thistle Cirsium heterophyllum, but perhaps a bit narrower than normal. The top of the stem has no spines and only weak ridges or wings, which also almost fits with Cirsium heterophyllum. It would have no ridges on the stem at all. This is a character not found in most other British thistles.
The word 'heterophyllum' means variable leaves and Cirsium heterophyllum can have both unlobed leaves or lobed leaves like these. The dense cottony underside to the leaves is also a Cirsium heterophyllum characteristic.
However, the undulate (wavy) margin to the leaf with strong spines, does not fit with Cirsium heterophyllum and this photo shows a leaf resembling those from marsh thistle Cirsium palustre, but broader and more robust.
You can also see the undulate spiny margin on this more-or-less entire leaf from the same plant.
Additional characters that fit with Cirsium palustre more than Cirsium heterophyllum include: height (Cirsium heterophyllum is normally shorter); well-spaced leaves along the stem; long branches; and shoots all from a single rosette (Cirsium heterophyllum normally has lots of vegetative growth about the base). In addition, the plant was pretty infertile with only 8 normal-looking seeds from 3 flower heads.
Initially it was the overall 'odd' appearance of the plant, plus the combination of Cirsium heterophyllum type lower leaves and the degree and nature of the branching, that caught Phill's attention and caused him to suspect that he had found the (unfortunately named) hybrid between the two, Cirsium x wankelii. The combination of aditional characters from both Cirsium heterophyllum and Cirsium palustre listed above, helped Quentin Groom to confirm the identification. This is only the third record for this hybrid in England!
That process of identifying a hybrid by assessing all of its characters and comparing them to possible parent plants, is typical of how hybrids have to be identified. Hybrid identification probably qualifies as a 'dark art'. Part of the trouble is that you can rarely fit hybrids neatly in to keys to species, as they are too variable. This variability is often due to back-crossing (i.e. hybrid plants crossing again with one of the parent species. This means you can get hybrid plants that are very close morphologically to either parent and other hybrid plants at stages in between the two extremes. That is just too much variability to accomodate in a dichotomous key usually. Where there is not so much back-crossing, it is usually less of a problem and genuinely intermediate plants are more common. An exception to this involves Roses which have an unusual way of inheriting genes from their parents (split 20-80, instead of the normal 50-50).
Even without back-crossing, first generation hybrids are not necessarily intermediate in every character between the two parents. There are several reasons for this, e.g. some characters are associated with dominant and recessive genes, so the morphology in the hybrid plant will resemble the parent plant with the dominant gene entirely.
As they can be so tricky to identify, it is often useful to collect some more information about the plant. What other species from the genus are growing nearby? If both potential parents are growing right next to the hybrid that is reassuring, but not absolutely necessary or conversely, not absolute proof in itself.
Most hybrids are at least partially sterile and so at least some will not form viable seeds. Sterile seeds often look shrivelled and empty compared to viable ones. However sterility in hybrids can vary between 100% and 0%. For this character to be useful, you really need to have some knowledge and experience of what the normal range of sterility is for the particular hybrid that you are dealing with. This information is not included in popular books, so you need to have access to specialist texts, or to someone with a particular expertise in the group of plants you are dealing with.
Hybrids often have some sterility in their pollen. This can be checked easily under the microscope by putting some pollen in stain. The viable pollen is usually coloured and full, whereas the sterile pollen appears shrivelled and colourless. You simple count how many of each type you have and from that work out the percentage fertility. Again for the results to be meaningful, you need to have some experience with the particular hybrids.
On top of all of that, of course, sterility can be caused by environmental factors, so just because you have some sterility, does not mean necessarily that it is a hybrid!
This all sounds complicated, but it is really a simplified account of some of the factors you consider with hybrids. It is no wonder that most people interested in plants groan in despair at the mere mention of the word hybrid.