Confused species

There are quite a number of orchid species, which are closely related, but still distinct enough to be viewed as separate species in their own right. It is such a pity that this view does not seem to be taken by the majority of botanists and taxonomists, whose main aim is to “lump” similar species into just one, too broadlydefined, highly variable species. In our opinion, if the differences between two species are constant, then that alone is good enough reason to keep them as distinct species.

The most commonly confused species are Dendrobium bursigerum, which is only found in the Philippines, and the widely spread Dendrobium secundum, which is recorded from the Andaman Islands, Myanmar (formerly Burma), Thailand, Peninsular Malaysia, Laos, Kampuchea (formerly Cambodia) Vietnam, Singapore, the Riau Archipelago, Sumatra, Java, Krakatau, Flores, Borneo, and Sulawesi (formerly Celebes).

These two species are easily separable by the following features: With Dendrobium bursigerum, the flowers are arranged radially on the inflorescence, meaning the flowers face in many directions, whereas the blooms of Dendrobium secundum all face in one direction, hence the common name of the “Tooth-brush Orchid.” The flowers of Dendrobium bursigerum are smaller and do not open widely, whereas those of Dendrobium secundum are larger and open considerably more. The number of flowers on the inflorescence of Dendrobium bursigerum are many more, than the number of flowers on the inflorescence of Dendrobium secundum.

There is also another pair of Dendrobium species, which are also frequently confused: Dendrobium rhombeum, which is endemic to the Philippines, and Dendrobium heterocarpum, which has been found in Sri Lanka, India, Nepal, Bhutan Myanmar, China, Peninsular Malaysia, Thailand, Laos, Vietnam, Sumatra, Java, Borneo, and Sulawesi.

Again they are readily separated firstly by comparing the habit of the labellum. In Dendrobium rhombeum the labellum is flat, and only has a patch of downy hairs in the central portion. The labellum of Dendrobium heterocarpum is considerably different and the apex curves backwards and the surface is almost completely covered in short hairs.

From my observations of the plants of these two species the plant of Dendrobium rhombeum can get to two meters in length, whereas the plant of Dendrobium heterocarpum only reaches lengths of 60 cm, and tends to grow more upright.

Another pair of confused species are the widely distributed Eria javanica and the poorlyknown Eria sessilifolia, which is only known from a single, preserved herbarium specimen of unknown origin. Eria sessilifolia definitely occurs in the Philippines and has been found in the islands of the Visayan Sea where it grows on trees and also on limestone rocks. Eria javanica is very widely spread throughout Asia, being found in India, Laos, Myanmar (Burma), Thailand, Peninsular Malaysia, Indonesia, Sulawesi, and New Guinea.

The differences between the two species is quickly recognized when one looks at the pedicel and ovary (the part of the flower that attaches it to the inflorescence). On Eria javanica the surface of the pedicel and ovary is covered with short black hairs, which gives the surface an almost dirty appearance, and there are no ridges. The pedicel and ovary of Eria sessilifolia has a number of high ridges, which are wavy, with very few hairs. Another easily seen difference, between these two species, is that the plant of Eria javanica has a distinct rhizome between the pseudobulbs, whereas the pseudobulbs of Eria sessilifolia are clustered tightly together.