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The original plant described as this form was collected from Port Douglas (north of Cairns) in 1932.
It appears that the variety was described based on in it having tan to brown tips to the flowers (instead of yellow of the best known variety var. tattonianum) and the labellum (lip) including red as well as purple markings and sometimes on a cream/ yellowish/greenish base (base colour of var. tattonianum is always white). Shape is described as similar to var. tattonianum.
I have observed a number of plants in flower in the Macalister Ranges and Port Douglas area – the type locality of this variety. To me, they appear to be var. tattonianum in shape and form, with a strong infusion of colouration from var. canaliculatum to the north. Indeed to the southern Macalister Ranges many are close to standard yellow and white var. tattonianum. They do not appear to have any of the interesting colour forms from var. canaliculatum (oranges, reds, yellows) rather being varying shades of brown. A number of plants in the area are also basically typical var. tattonianum in colour, though often with more intense yellows and/or a little extra colouration (yellows, reds) in the lip (which is never seen in pure var. tattonianum). In all ways the plants I have observed in the type locally for var. nigrescens match an intergrade between the two aforementioned varieties and I see no features to support explanation as a separate variety.
I suspect that the original describer of this variety (a W. H. Nicholls) was familiar with plants of Dendrobium canaliculatum var. tattonianum but not of var. canaliculatum from further up Cape York Peninsula. When seeing the flowers of the Port Douglas plant, he concluded it a new variety, unaware of the form of the original type plant from Cooktown described over 182 years previously.
Var. nigrescens is used extensively in reference to Dendrobium canaliculatum in catalogues and web forums at present, particularly for plants that I refer to as var. canaliculatum. Possibly many consider a dark form of a species (as var. canaliculatum often is) as correctly referred to as ‘nigrescens’ (from ‘negro’ – black or dark). Serious (academic) authors (e.g. Clem and Jones (2002), Adams (2015)) do not support a distinct var. nigrescens.
Whilst a moot point I personally consider the Tea Tree Orchids from the Port Douglas area as predominantly var. tattonianum with colour influence from var. canaliculatum. The colouring on the flower tips stops abruptly before the white centres (unlike var. canaliculatum which ‘fades’ to the centre) and the flower shape and substance is var. tattonianum. This being a moot point as plants from this broader area are clearly some mix of var. tattonianum and var. canaliculatum and it is impossible to be sure of the true extent of influence of each variety from plant to plant. Hobbyists such as myself can look at a plant and make a guess based on a few morphological features however many features of the varieties are likely not apparent or obvious based on morphology alone. Were the DNA studies ever undertaken it may be some plants appearing var. tattonianum from the north of its range are primarily of var. canaliculatum genome and vice versa.
Dendrobium canaliculatum var. nigrescens