Identification of Actinodium

Actinodium is quite a unique genus of flowering plant. Endemic to southwestern Western Australia, the flower heads look just like a daisy, but the genus is actually in Myrtaceae. There is one described species, Actinodium cunninghamii, the 'Albany Daisy'. Most observations get identified as this species.

However, there is actually a second entity in this genus, the currently undescribed phrase name species Actinodium sp. Fitzgerald River (H.A. Froebe & R. Classen 810). The two are very similar morphologically, and they also have strongly overlapping ranges. Here is how the two are separated in KeyBase:

Leaves 2.5–5 mm long; inflorescence heads 8–25 mm in diameter; linear bracts of outer sterile flowers 3-5 mm long = cunninghamii

Leaves usually 3.5–5.5 mm long; inflorescence heads 20–45 mm in diameter; linear bracts of outer sterile flowers 5–11 mm long = sp. Fitzgerald River

So broadly, sp. Fitzgerald River is a bigger plant. The leaf length character is the least useful given the ranges for the two strongly overlap, but the other two characters are almost mutually exclusive: sp. Fitzgerald River has flower heads with a much broader diameter, and the bracts of the outer sterile flowers are much longer.

(just noting that there may be other differences between the two, eg something like colour, but I am not aware of these differences, the only information I could find separating the two online is this key, so that's what I'm operating off)

Unfortunately most iNat observations of Actinodium don't have something for scale in shot, so it's difficult to confidently assign an ID in some cases. Having said that, there are definitely some observations where you can tell that the flower heads are quite small or quite large, and observations with hands in them help too. I'm going to review the 100+ records on iNat and, where an observation cannot be reliably assigned to either species, downgrade it to genus. For observations that are clearly sp. Fitzgerald River, the best ID for now is also genus, but I'll also add an observation field to keep track of them.

UPDATE: having gone through a few now, there are definitely observations that seem, to me at least, fairly easy to assign to either of the entities even without a sense of scale.

For example, here's one that is a very obvious cunninghamii based on very small flower heads: https://www.inaturalist.org/observations/59563521

And then compare that to something like this where the flower heads seem to clearly be very large, a stark difference: https://www.inaturalist.org/observations/35162058

Not all are so clear cut, and of course I picked these specific examples as 'extremes'/very obvious cases of each entity and not all of them look like this, but there are certainly ones which I'm confident about assigning.

Posted on Φεβρουάριος 25, 2024 0339 ΠΜ by thebeachcomber