Differentiating Cirrhopetalum brevibrachiatum from C. makoyanum
First, let us get this out of the way: I do not consider these plants as bulbophyllums (bulbophyllines, yes), so I am going to refer to these plants all throughout this article as 'cirrhopetalums'. The genus Bulbophyllum has become an unwieldy repository of plants with very diverging features that I dare anyone who disagrees to define Bulbophyllum- in just one sentence. However, I have nothing against people who do. There are far better things to talk about. Such as teaching you how to distinguish these two species apart.
Cirrhopetalum brevibrachiatum and C. makoyanum are two frequently confused species particularly in the Philippines where their ranges overlap. Both have slender flowers in yellow (most of the time though not all the time, for C. brevibrachiatum) minutely spotted with red, and there are no distinguishable means to tell them apart by just looking at the vegetative growths. However, seeing the differences is easy. But you are going to need a magnifying glass. And a good nose. And razor-sharp judgment.
For these two species, you only have to pay attention to two organs: the column wings and the labellum.
The column wings of C. brevibrachiatum are broadly rounded below and are terminated by a minute apiculate, antrorse (forward-facing) tip called a 'stelidia', hence, the species name 'brevibrachiatum' which translates to 'short arms' (in red). Geez, I love descriptive names. The labellum is curved at a sharp angle that is made eminent by a pair of broadly triangular prominences, in blue. However, some specimens I have seen throughout the years have those prominences very vaguely expressed to none at all. The column wings and stelidia appear to be reliable, diagnostic features.
Cirrhopetalum makoyanum also has its column wings broadly rounded below but the tips are upwardly curved, rather like a sickle. There is none of that forward-pointing appendage seen on the previous species, though people with poor eyesight or weak perceptions may mistake the tips for stelidia. In addition, the labellum has a pair of pointed calli situated at the hypochile (the base of the labellum), but you are going to need a magnifying glass to properly observe that. But really, it's that simple. I mentioned above that you are going to need a good nose for these two: while the flowers of C. makoyanum are odorless, those of C. brevibrachiatum typically have a weak, fetid odor. Despair not if you can detect nothing, as most people can't, anyway.
At this juncture, I should also add that whereas there is overlap in floral coloration, some plants of C. brevibrachiatum can have flowers other than yellow. See these three examples below:
The flowers of C. makoyanum are more slender and longer than C. brevibrachiatum's, However, this feature is scarcely feasible when determining a plant's identity as there are also C. brevibrachiatum with lateral sepals of comparable appearance. For certainty, always refer to the column wing shapes and the labellum callus to properly identify your cirrhopetalums of any species.
Further remarks on C. brevibrachiatum
You may come across references to C. brevibrachiatum illustrating just one inflorescence, with yellow flowers with broad sepals. But without photos of the column and the labellum as seen from the side, it would always be best to be skeptical. There was a report of this species' occurrence on Sabah in Borneo but the "solely photographic" proof "based on extralimital material" shows one such specimen. Unfortunately, Kew has since followed this authority and currently lists this species as also occurring on Borneo, leading to other possible misidentifications.
The photos of the line drawings I supplied above were taken from Gunnar Seidenfaden's 'Notes on Cirrhopetalum' (1973), and the line drawings were referenced by him from L. v. V. 392 which have duplicates in Kew (K) and the Nationaal Herbarium Nederland in Leiden (L), as well as 'Kjellberg 2443' which is deposited at the Swedish Museum of Natural History in Stockholm (S). The exclamation marks you see there (!) means that the author, Mr. Seidenfaden, had personally seen these specimens. The holotype (Schlechter 20702) appears non-extant; Friedrich Rudolf Schlechter deposited his specimens at the Berlin Botanical Museum, which was destroyed in 1943 during the Second World War. But what I am trying to say here is this: given a choice between a 'brevibrachiatum' featured in a book and ones that were drawn from the type materials then I certainly and unblinkingly would rest my faith on the latter. And in case you did not notice it, both the L. v. V. and the Kjellberg specimens have narrow and slender lateral sepals.
At this point, I am trying to teach you something that is practically impossible to impart to would-be orchidologists but will try anyway: good judgment. For such judgment to be made, one must be able to gather his references from reputable sources, then weigh these against one another. Merely Googling for photos taken from dubious sites (those that were manned by non-researchers) will get you nowhere. True, original descriptions should not be construed as final, as type specimens are often NOT representative of a species. Think about it: out of a population of hundreds, if not thousands of plants with a range of variations, a botanical collector would collect whatever is nearest to him, or whatever was available at that point in time, or whatever to his eyes was the most splendid example of this hitherto new species. Human biases come into play, and in many cases influence our concept of what a particular species should look like. However, if one feels like resorting to the go-to alibi of "this is only a variation of this and that" when confronted by a very different plant, then that person should be able to generate a battery of proofs that would buttress his claim, proofs that- whether he likes it or not- should still be grounded on the type specimen and/or the original description because no matter how imperfect those might be, there is no other acceptable point of reference. Merely citing a reference with zero proofs such as illustrations or close-up photos of the relevant floral parts- even if it was authored by a universally regarded 'authority'- does not constitute a 'proof'. Do not believe everything that is said to you, even those that are coming from me. Instead, use your eyes and a hefty serving of good judgment.
Alright, lesson's over.