Since I wrote my OGD posting on the differences between Dendrobium constrictum and D. capituliflorum, nine people have sent me photos of "abberant" plants that they think are D. constrictum. In every case the illustrated plant has been D. capituliflorum, as far as I could tell from the photos.
If you think you've got a potential D. constrictum in your collection, please check it against Liz Dauncey's article "D. purpureum & it's allies" in Kew Bull. Vol 489 (3): 545-576 (1993). Personally, I would hesitate before applying the D. constrictum label to any plant. In almost every way, D. constrictum lies well within the range of variability seen in D. capituliflorum. Most people don't realise how variable D. capitulflorum can be, especially in habit. In the ever-wet regions along the north coast of PNG, the species grows big and can be fairly spindly. I recorded a plant on Karkar Island with stems 45 cm long but only 1 cm wide in the middle, and Liz Dauncey reported a giant with stems 80 cm long. However, on PNG's dry south coast, the plants are much shorter and stouter. One population in Varirata National Park are lithophytic, with stems typically 4-7 cm tall and up to 20 mm in diameter. These plants also exhibit a tendency to produce aerial shoots. Leaf colour in D. capituliflorum is also variable; there are all-green ones, green ones with purple undersides, and the greyish-green ones with purple undersides. you can often switch a plant from one leaf colour to another by playing around with the light and water regimes. On some D. constrictum plants the rhizomes throw up clusters of stems (just like D. capituliflorum), while others produce individual stems at 5-20 mm intervals along a creeping rhizome, in which case they are often distinctly biseriate, ie on alternating sides along the rhizome. The stems grow up to 10 cm x 1 cm (well within the D. capituliflorum range) and the leaf sheaths are often pale-spotted when fresh. D. capituliflorum inflorescences contain 15-80 flowers; D. constrictum has 25-40. D. capituliflorum flowers are usually 12-18 mm long, but Dauncey recorded one as small as 7 mm. D. constrictum flowers are 10-14 mm long. No significant differences there. The only reliable differences are that in D. constrictum the petals are much shorter than the dorsal sepal (only slightly more than half the length) and are relatively much broader than in D. capituliflorum. They have an obtuse, almost bitten-off apex. D. capituliflorum petals can have an erose apex, but they are always about the same length as the dorsal sepal and the tip is acute or even acuminate or apiculate. A further difference lies in the lip; D. constrictum has a lip that is distinctly constricted in the middle, and a broad, almost rectangular midlobe with an obtuse, eroded apex. D. capituliflorum has has much narrower lip that is only slightly constricted in the middle, and a long drawn-out acute tip. Like I said, I would hesitate before applying the D. constrictum label to any plant unless I had examined the floral segments REALLY carefully. Cheers, Peter O'Byrne Singapore _______________________________________________ the OrchidGuide Digest (OGD) orchids@orchidguide.com http://orchidguide.com/mailman/listinfo/orchids_orchidguide.com
